Reduction of Fuel Tank Flammability in Transport Category Airplanes

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 25, 91, 121, 125, and 129

[Docket No. FAA-2005-22997; Notice No. 05-14]

RIN 2120-A123

Reduction of Fuel Tank Flammability in Transport Category Airplanes

AGENCY:

Federal Aviation Administration (FAA), DOT.

ACTION:

Notice of proposed rulemaking (NPRM).

SUMMARY:

This NPRM proposes new rules that will require operators and manufacturers of transport-category airplanes to take steps that, in combination with other required actions, should greatly reduce the chances of a catastrophic fuel-tank explosion. The proposal follows seven years of intensive research by the FAA in collaboration with industry into promising technologies designed to make fuel tanks effectively inert, thus preventing electrical and other systems from igniting flammable vapors in the fuel tank ullage (vapor space). The result of that research is that fuel tank inerting, originally thought to be prohibitively expensive, can now be accomplished in a reasonably cost-effective fashion and protect the public from future calamities which, we have concluded, are otherwise virtually certain to occur. The new rules, if adopted, would not actually direct the adoption of specific inerting technology either by manufacturers or operators but would establish a performance-based set of requirements that do not specifically direct the use of fuel-inerting but rather set acceptable levels of flammability exposure in tanks most prone to explosion or require the installation of an ignition mitigation means in an affected fuel tank. Technology now provides a variety of commercially feasible methods to accomplish these vital safety objectives.

DATES:

Send your comments on or before March 23, 2006.

ADDRESSES:

You may send comments, identified by Docket No. FAA-2005-22997, using any of the following methods:

DOT Docket Web site: Go to

http://dms.dot.gov

and follow the instructions for sending your comments electronically.

Government-wide rulemaking Web site: Go to

http://www.regulations.gov

and follow the instructions for sending your comments electronically.

Mail: Docket Management Facility; U.S. Department of Transportation, 400 Seventh Street, SW., Nassif Building, Room PL-401, Washington, DC 20590-001.

Fax: 1-202-493-2251.

Hand Delivery: Room PL-401 on the plaza level of the Nassif Building, 400 Seventh Street, SW., Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

For more information on the rulemaking process, see the

SUPPLEMENTARY INFORMATION

section of this document.

Privacy:

We will post all comments we receive, without change, to

http://dms.dot.gov

, including any personal information you provide. For more information, see the Privacy Act discussion in the

SUPPLEMENTARY INFORMATION

section of this document.

Docket:

To read background documents or comments received, go to

http://dms.dot.gov

at any time or to Room PL-401 on the plaza level of the Nassif Building, 400 Seventh Street, SW., Washington, DC between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

FOR FURTHER INFORMATION CONTACT:

Michael E. Dostert, FAA, Propulsion/Mechanical Systems Branch (ANM-112), Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 227-2132, facsimile (425) 227-1320; e-mail:

mike.dostert@faa.gov.

SUPPLEMENTARY INFORMATION:

Comments Invited

The FAA invites interested persons to participate in this rulemaking by submitting written comments, data, or views. We also invite comments relating to the economic, environmental, energy, or federalism impacts that might result from adopting the proposals in this document. The most helpful comments reference a specific portion of the proposal, explain the reason for any recommended change, and include supporting data. We ask that you send us two copies of written comments.

We will file in the docket all comments we receive, as well as a report summarizing each substantive public contact with FAA personnel concerning this proposed rulemaking. The docket is available for public inspection before and after the comment closing date. If you wish to review the docket in person, go to the address in the

ADDRESSES

section of this preamble between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. You may also review the docket using the Internet at the web address in the

ADDRESSES

section. Comments that you may consider to be of a sensitive security nature should not be sent to the docket management system. Send those comments to the FAA, Office of Rulemaking, ARM-1, 800 Independence Avenue, SW., Washington, DC 20591.

Privacy Act:

Using the search function of our docket Web site, anyone can find and read the comments received into any of our dockets, including the name of the individual sending the comment (or signing the comment on behalf of an association, business, labor union, etc.). You may review DOT's complete Privacy Act Statement in the

Federal Register

published on April 11, 2000 (65 FR 19477-78) or you may visit

http://dms.dot.gov.

Before acting on this proposal, we will consider all comments we receive on or before the closing date for comments. We will consider comments filed late if it is possible to do so without incurring expense or delay. We may change this proposal in light of the comments we receive.

If you want the FAA to acknowledge receipt of your comments on this proposal, include with your comments a pre-addressed, stamped postcard on which the docket number appears. We will stamp the date on the postcard and mail it to you.

Availability of Rulemaking Documents

You can get an electronic copy using the Internet by:

(1) Searching the Department of Transportation's electronic Docket Management System (DMS) web page (

http://dms.dot.gov/search

);

(2) Visiting the Office of Rulemaking's web page at

http://www.faa.gov/avr/arm/index.cfm

; or

(3) Accessing the Government Printing Office's web page at

http://www.access.gpo.gov/su_docs/aces/aces140.html.

You can also get a copy by submitting a request to the Federal Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence Avenue, SW., Washington, DC 20591, or by calling (202) 267-9680. Make sure to identify the docket number, notice number, or amendment number of this rulemaking.

Table of Contents

I. Executive Summary

II. Background

A. The Need for Safety Improvements in Fuel Tank Systems

B. Fuel Properties

C. National Transportation Safety Board (NTSB) Recommendations

D. FAA Response

III. Proposed Requirements Relating to Fuel Tank Flammability

A. Overview of the Proposal

B. Ongoing Responsibility of Type Certificate Holders for Continued Airworthiness

C. Applicability

1. Manufacturers and Holders of Type Certificates, Supplemental Type Certificates and Field Approvals

2. Airplanes

3. Fuel Tanks

4. Airplane Operators

D. Proposed Requirements for Manufacturers and Holders of Type Certificates, Supplemental Type Certificates and Field Approvals

1. New Airplane Designs

2. Existing Airplane Designs

3. Auxiliary Fuel Tanks

4. Methods of Mitigating the Likelihood of a Fuel Tank Explosion

a. Flammability Analysis Using the Monte Carlo Method

b. Ignition Mitigation Means

c. Flammability Reduction Means

i. Accounting for System Reliability and Performance Issues

ii. Warm Day Fleet Flammability Exposure

iii. Reliability Reporting

iv. Reliability Indication and Maintenance Access

d. Service Instructions and Service Bulletins

e. Critical Design Configuration Control Limitations (CDCCL)

f. Compliance Planning

i. Compliance Plan for Flammability Exposure Analysis

ii. Compliance Plan for Design Changes and Service Instructions

iii. Compliance Plan for Auxiliary Fuel Tanks

g. Compliance Schedule

E. Proposed Requirements for Airplane Operators

1. Requirement to Install and Operate FRM, IMM or FIMM

2. Authority to Operate with an Inoperative FRM, IMM or FIMM

3. Compliance Schedule

F. Additional Provisions

1. Relationship of this Proposal to Aging Airplane Regulatory Initiatives

2. FAA Advisory Material

3. FAA Oversight Office

4. Workplace Safety Issues

IV. Rulemaking Analyses and Notices

V. The Proposed Amendment

I. Executive Summary

Fuel tank explosions have been a constant threat with serious aviation safety implications for many years. Since 1960, some 17 airplanes have been destroyed as the result of a fuel tank explosion.

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Four fatal airplane accidents have been caused by fuel tank explosions just since 1989. Two of the more recent accidents—one involving a Boeing Model 747 (TWA Flight 800) off Long Island, New York in 1996 and the other, a Boeing Model 727 accident (Avianca Flight 203) in Bogotá, Columbia in 1989—occurred during flight and led to catastrophic losses, including the deaths of 337 individuals. The two other recent explosions occurred on the ground but led to nine fatalities.

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Although it was determined that a terrorist's bomb had caused the explosion of the center tank in the Bogotá accident, the NTSB determined the “bomb explosion did not compromise the structural integrity of the airplane; however, the explosion punctured the [center wing tank] and ignited the fuel-air vapors in the ullage, resulting in destruction of the airplane.” Investigations of the other three accidents failed to identify the ignition source that caused the explosion. But in each instance the weather was warm, with an outside air temperature over 80 °F, the incident occurred during the initial (ground, takeoff or climb) phases of flight, and the explosion involved empty or nearly empty tanks that had been previously fueled. Additionally, investigators were able to conclude that the center wing fuel tank in all four airplanes contained flammable vapors in the ullage (that portion of the fuel tank not occupied by liquid fuel) when the fuel tanks exploded. While the proposed requirements are not intended to address terrorist initiated fuel tank explosions, a system designed to reduce the likelihood of a fuel tank fire, or mitigate the effects of a fire should one occur, would have prevented these four fuel tank explosions.

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None of the 17 explosions occurred on an airplane manufactured by Airbus, who, along with Boeing, would be most affected by this rulemaking. Although Airbus currently delivers more airplanes worldwide than Boeing, their cumulative fleet hours are still relatively small, at approximately 65 million (approximately 9% of total fleet hours for all transport category airplanes). Based on the FAA's projection of the likelihood of an explosion based on one accident every 60 million hours, there is a 40% chance that no Airbus accidents would have occurred to date.

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Philippine Airlines 737 accident in 1990 and the Thai Airlines accident in 2001.

A statistical evaluation of these accidents has led the FAA to project that nine more transport category airplanes will likely be destroyed by a fuel tank explosion in the next 50 years, unless remedial measures are taken. Although we cannot forecast precisely when these accidents would occur, computer modeling that has been an accurate predictor in the past indicates these events are virtually certain to occur. We believe at least eight of these explosions are preventable if we adopt a comprehensive safety regime to reduce both the incidence of ignition and the likelihood of an explosion following ignition. We have already taken steps through other regulatory actions to reduce the chances of ignition. Today's proposal attempts to address the risk of an explosion by reducing the likelihood that fuel tank vapors cause an explosion when an ignition source is introduced into the tank.

Since the introduction of turbine powered airplanes, the FAA has premised its fuel tank rules on the assumption that fuel tanks will always contain flammable vapors and thus the best way to prevent explosions is to eliminate ignition sources. Since 2001, we have imposed airworthiness requirements (including airworthiness directives or “ADs”) directed at the elimination of fuel tank ignition sources. Although these measures—particularly Special Federal Aviation Regulation 88 of 14 CFR part 21 (SFAR 88), which requires the detection and correction of potential system failures that can cause ignition—should prevent some of the nine forecast explosions, review of the current designs of airplanes in the transport category of all major manufacturers has shown that unanticipated failures and maintenance errors will continue to generate unexpected ignition sources. We have concluded we are unlikely ever to identify and eradicate all possible sources of ignition.

To ensure safety, therefore, we must also focus on the environment that permits combustion to occur in the first place. Technology now exists that can prevent ignition of flammable fuel vapors by reducing their oxygen concentration below the level that will support combustion. By thus making the vapors “inert,” we can significantly reduce the likelihood of an explosion when a fire source is introduced to the fuel tank. Prototype onboard fuel tank inerting systems have been successfully flight tested on Airbus A320, Boeing Model 747, and Model 737 airplanes. Boeing applied in 2002 for type certification of an inerting system for the Model 747 that it plans to install on all new production 747 aircraft.

Because the chances of a fuel tank explosion naturally correlate with the exposure of the tank to flammable vapors, the proposed requirements would mitigate the effects of such exposure or limit such exposure to acceptable levels by mandating the installation of either a Flammability Reduction Means (FRM) or an Ignition Mitigation Means (IMM). In either case, the technology would have to adhere to performance and reliability standards that would be set by the FAA and contained in Appendices K and L to Title 14 Code of Federal Regulations (CFR) part 25.

If adopted, this rulemaking would amend the existing airworthiness

standards contained in 14 CFR 25.981 so as to require all type certificate (TC) holders and their licensees to develop FRM or IMM for many large turbine powered transport category airplanes with high risk fuel tanks. We would also amend 14 CFR parts 91, 121, 125 and 129 so as to require operators of these airplanes to incorporate the approved FRM or IMM and to keep them operational. We estimate that approximately 3,800 Airbus and Boeing airplanes operated in the United States would be affected. Fuel tank system designs in several pending type-certification applications, including the Airbus A380 and the Boeing Model 7E7, would also have to meet the proposed requirements.

We acknowledge that the proposed requirements are costly and propose these steps only after spending several years, in cooperation with scientists and other experts from the affected industry, researching the most cost-effective ways to prevent fuel tank explosions. Those efforts have resulted in the development of fuel-inerting technology that is vastly cheaper than originally thought.

The loss of a single, fully loaded large passenger aircraft in flight, such as a Boeing Model 747 or Airbus A380, moreover, would result in death and destruction causing societal loss of at least $1.2 billion based on prior calamities, and we project that the new rule would prevent four accidents of some type (for analytical purposes we assume the accidents would involve “average” aircraft with “average” passenger loads) over 50 years. Such estimates of harm do not account for the intangible costs of a series of in-flight explosions (such as a loss of confidence in aviation) or the indirect costs (such as trip cancellations following these incidents).

Our philosophy is to address aviation safety threats whenever practicable solutions are found, especially when dealing with intractable and catastrophic risks like fuel tank explosions that are virtually certain to occur. Thus, now that solutions are reasonably cost-effective, the Administrator has tentatively determined that it is necessary for safety and in the public's best interest to adopt the requirements proposed today. This action is in response to an NTSB recommendation.

II. Background

A. The Need for Safety Improvements in Fuel Tank Systems

Fuel tank explosions continue to occur despite many safety improvements over the last 40 years aimed at removing ignition sources from fuel tanks. Experience tells us that even with the latest and most comprehensive initiative, SFAR 88, we cannot adequately protect the public from fuel tank explosions absent measures designed to lessen the exposure of vulnerable tanks to highly flammable jet fuel vapors. Fortunately, by taking such steps now to complement ignition-source reduction measures already taken, we are confident that fuel tank explosions in affected aircraft will be nearly eliminated.

For a variety of reasons, SFAR 88, though a significant advancement in safety, will never provide a complete safeguard against fuel tank explosions; thus our analysis has assumed that SFAR 88 will not reduce the possibility of a fuel explosion occurring by more than 50 percent. To be sure, SFAR 88 has resulted in several significant changes in fuel tank system design and maintenance, including (1) new features to prevent dry running of fuel pumps within the fuel tanks; (2) use of ground fault protection of fuel pump power supplies for pumps or wires exposed to the fuel tank ullage; (3) addition of electrical bonds on some components; (4) use of electrical energy limiters on wiring entering fuel tanks that are “normally emptied”

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and located within the fuselage contour; (5) electrical bond integrity checks; and (6) improved maintenance programs. These design improvements, however, do not and cannot address all sources of ignition (such as external ignition sources resulting from fire).

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The phrase “normally emptied” refers to fuel tanks that contain a substantial vapor space during a significant portion of the airplane operating time. Tanks that are designed to be normally emptied have been installed in various locations including the center wing structure, horizontal stabilizers, wings and cargo compartments. Fuel loading and usage management practices on certain airplane models use the auxiliary fuel tanks for controlling the center of gravity.

Past experience, moreover, shows that it is not possible to pinpoint and remove every ignition source from a large, complex transport aircraft. For example, the FAA is aware of one case where a manufacturer had conducted an exhaustive design review to identify possible sources of arcing within the fuel tank after a fuel tank exploded due to lightning. The manufacturer identified several possible sources of the arcing, and the FAA issued ADs to correct these deficiencies. The same airplane design was then evaluated as a result of SFAR 88, and additional sources of lightning-induced ignition were identified. In another instance, a TC holder submitted a safety analysis to the FAA claiming that certain airplane models met existing system safety requirements of § 25.1309 and thus that the likelihood of an ignition source developing was extremely improbable (one in a billion flight hours). When the requirements of the SFAR 88 safety review and unsafe condition criteria were applied, however, approximately 80 new unsafe conditions were found. These conditions will now be addressed by AD for those airplane models but, in retrospect, it was clear that the manufacturer's claims were erroneous.

The safety reviews have also identified the potential for system failures (or “failure modes”) that cannot be eliminated as possible ignition sources at reasonable cost. For example, use of ground fault protection for fuel pump power supplies will protect the fuel pumps from shorts to ground (such as one might find from lightning), but will not protect the fuel pumps from shorts between the three power wires to the pump, commonly referred to as “phase-to-phase shorts.” Currently there is no proven component available to address this failure mode. Combinations of failure modes are even more problematic. We could require installation of redundant bond paths to prevent the latent failure of a critical electrical bond, but doing so would be cost-prohibitive.

Finally, human error creates continuing risk. Each attempt to fix an electrical system presents the possibility of an inadvertent introduction of a new ignition source. Maintenance oversights, such as the failure to properly install electrical bonds or improper installation or overhaul of components, compound the possibility of an ignition source developing.

Carrier fuel carrying practices could impact the possibility of an explosion as well. If a carrier decides to carry only that fuel necessary to meet the FAA's fuel reserve requirements, the likelihood of an explosion is greater than if it carries excess fuel. This potential exists because more ignition sources within the fuel tank are exposed to the ullage and because the fuel has insulating properties which keeps the fuel tank cooler. Thus, “tankering”, or carrying excess fuel, could theoretically lower the risk of an explosion. Current fuel management practices, where excess fuel is carried only when cost beneficial to the carrier, are largely market driven because airlines try to minimize their fuel costs to the maximum extent possible. Both the FAA and industry explored mandatory refueling of center wing tanks after the NTSB suggested the FAA adopt an interim flammability reduction measure in 1996. We determined that the reduction in

flammability exposure would not be significant and would not address the warm day flammability risk. Thus, while either reducing or increasing the amount of fuel carried in the center wing tank could theoretically have some impact on the risk of an explosion, the FAA does not believe that current fuel carrying practices are likely either to change significantly or to have a measurable impact on the overall risk of an explosion. We seek comment on this position.

B. Fuel Properties

Three conditions must be present in a fuel tank to support combustion and a fuel-tank explosion: Fuel vapor in the right amount, enough oxygen, and an ignition source. As discussed earlier, our regulatory efforts since piston-powered aircraft evolved into the jet age have been focused almost exclusively on the last item, ignition sources. A basic assumption in this approach has been that the fuel tank would contain flammable vapors under a wide range of airplane operating conditions. The question is, what level of exposure is safe?

Jet fuel vapors are flammable only in certain temperature and pressure ranges. The flammability temperature range of such vapors varies with the type and properties of the fuel, the ambient pressure in the tank, and the amount of dissolved oxygen released from the fuel into the tank. The amount of dissolved oxygen in a tank will also vary depending on the amount of vibration and sloshing of the fuel that occurs within the tank. The temperature range in which a flammable fuel vapor will form can vary with different batches of fuel even for a specific fuel type, but the threshold temperature for flammability decreases as the airplane gains altitude because of the corresponding decrease of internal tank air pressure. Thus, the higher the airplane is flying, the lower the ambient temperature required for a fuel tank to explode when an ignition source introduced.

Jet A fuel is the most commonly used commercial jet fuel in the United States and is widely used in other parts of the world. At sea level and with no sloshing or vibration present, these fuels have flammability characteristics that make it unlikely that the fuel molecules present in the fuel vapor-air mixture will ignite when the temperature in the fuel tank is below approximately 100 °F. The vapor will ignite, however, once the fuel temperature reaches approximately 175 °F, because of the increased concentration of fuel molecules at higher temperatures. At an altitude of 30,000 feet, the flammability temperature range drops to approximately 60 to 120 °F.

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Use of Jet A or Jet A-1 fuel thus tends to limit the risk of high flammability to warmer days.

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Most transport category airplanes used in air carrier service are approved for operation at altitudes from sea level to 45,000 feet.

Jet B (JP-4) is another fuel approved for use on most commercial transport category airplanes, although it is no longer used as a primary fuel for commercial transports. The flammability range of Jet B (JP-4) is about 15 to 75 °F at sea level and 20 to 35 °F at 30,000 feet. Because the flammable temperature range of Jet B fuel is more within the range of typical air temperatures at those altitudes where the airplane is likely to be operated, airplane fuel tanks with Jet B fuel are flammable for a much larger portion of the flight.

C. National Transportation Safety Board (NTSB) Recommendations

The NTSB determined that the probable cause of the in-flight explosion on TWA Flight 800 was the ignition of the flammable fuel/air mixture in the center wing fuel tank. However, the source of ignition energy for the explosion could not be determined with certainty. The Board also faulted, as contributing to the accident, the FAA's design and certification approach to transport-category airplanes, as it (1) concentrated solely on precluding all ignition sources, and (2) allowed heat sources to be located beneath the center wing fuel tank.

In 1996, the NTSB issued recommendations to improve fuel tank safety. The NTSB recommended both eradicating ignition sources and reducing fuel tank flammability.

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In their accident report, the Board concluded that “a fuel tank design and certification philosophy that relies solely on the elimination of all ignition sources, while accepting the existence of fuel tank flammability, is fundamentally flawed because experience has demonstrated that all possible ignition sources cannot be determined and reliably eliminated.”

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NTSB recommendations provided on page 309 of NTSB Accident Report, “In-flight Breakup Over the Atlantic Ocean, TransWorld Airlines Flight 800 Boeing 747-131, N93119 Near East Moriches, New York, July 17, 1996, Report number NTSB/AAR-00/03, DCA96MA070, Adopted August 23, 2000.

D. FAA Response

The FAA conducted ignition-prevention safety reviews following the 1996 accident, which revealed many new single-component failure modes that could ignite fuel tanks. We continue to issue ADs that require design or maintenance actions to address these deficiencies. These safety reviews also identified combinations of failures that could result in an ignition source, but as these combinations were less likely to occur than single failures, we determined that it was not practical to address them in existing airplanes. The safety reviews also confirmed that unforeseen design and maintenance errors could create ignition sources.

Recognizing the need to focus on flammability rather than just ignition, on April 3, 1997, the FAA published a notice in the

Federal Register

seeking comments on the 1996 NTSB recommendations on flammability exposure (62 FR 16014). That notice reviewed the service history of transport category airplane fuel tanks and the challenges underlying fuel-tank flammability reduction. Public comment indicated that more information was needed before we could begin a rulemaking on this safety issue.

Given that control of flammable vapors was a new concept, we assigned two Aviation Rulemaking Advisory Committee (ARAC) working groups to study the issues and provide recommendations. (The ARAC consists of interested parties, including the public, and provides a process to advise us on the development of new regulations.) The first working group reviewed the practicality of requiring flammability reduction, evaluating many different flammability reduction methods. Upon the recommendation of the first working group, the second working group then focused exclusively on fuel tank inerting.

On January 23, 1998, we published a notice in the

Federal Register

that established the Fuel Tank Harmonization Working Group as part of ARAC (63 FR 3614). This group was asked to recommend regulations on fuel tank flammability for both newly certificated and existing airplanes. The working group looked at fuel tank explosions that occurred after Jet A fuel had replaced Jet B fuel as the predominant type used on transport airplanes. The group examined the performance of two types of fuel tanks: the center wing fuel tanks located within the fuselage contour, and wing fuel tanks. Fuel tanks located in an aluminum wing are typically unheated and cool quickly when the wing surfaces are exposed to colder air during flight. Conversely, the center wing fuel tanks in certain airplanes have equipment underneath the tank radiating heat; in addition, with no surfaces exposed to outside air, the tank

cools much more slowly than a wing fuel tank.

The working group concluded that the safety records of fuel tanks located in aluminum wings of airplanes fueled with Jet A type fuel were satisfactory. These tanks had an average flammability exposure (as calculated under a methodology contained in proposed Part 25, Appendix L) of approximately 2 to 6 percent. However, the group found that on some airplane fleets the center wing fuel tanks had an average flammability exposure ranging from 7 percent to a high of 30 percent, a dangerous level.

The working group then evaluated many possible means of reducing or removing the hazards associated with explosive vapors in fuel tanks, such as fuel tank inerting, fuel tank cooling, fuel property alteration, fire suppression systems and polyurethane foam treatments. The ARAC sent the working group's report to the FAA on July 23, 1998 (Docket No. FAA-1998-4183, viewable on the U.S. Department of Transportation electronic Document Management System at

http://dms.dot.gov

).

The working group report concluded that flammability reduction was practical for new airplane designs, but impractical for current production designs or retrofit in the current fleet of transport category airplanes. The report recommended that the FAA begin rulemaking to add a requirement to § 25.981, so that fuel tanks in new airplane designs would have an average flammability exposure of less than 7 percent. The report also recommended requiring by regulation that each newly designed airplane incorporate means to mitigate the effects of an ignition of fuel vapors, such that any damage caused would not prevent continued safe flight and landing. The report reviewed various technical solutions, including control of heat transmission into fuel tanks, use of inerting systems, or ignition mitigation means like polyurethane foam. The report concluded that the best solution was likely to be control of heat transmission and suggested that the most practical means of control were (1) relocation of the air-conditioning equipment away from the fuel tanks; (2) ventilation of the air-conditioning bay to limit heating and cool fuel tanks; or (3) insulation of the tanks from heat. Nevertheless, the ARAC also recommended that we continue to evaluate the cost-effectiveness of other means for reducing flammable vapors in the fuel tanks, such as ground-based inerting of fuel tanks.

Based in part on the ARAC recommendations, we issued a rule entitled “Transport Airplane Fuel Tank System Design, and Maintenance and Inspection Requirements” in the

Federal Register

on May 7, 2001 (66 FR 23085). The rule added current § 25.981(c) which requires minimization of fuel tank flammability exposure in new type designs without setting a specific safety standard. Section 25.981(c) thus states:

(c) The fuel tank installation must include either—

(1) Means to minimize the development of flammable vapors in the fuel tanks (in the context of this rule, “minimize” means to incorporate practicable design methods to reduce the likelihood of flammable vapors); or

(2) Means to mitigate the effects of an ignition of fuel vapors within fuel tanks such that no damage caused by an ignition will prevent continued safe flight and landing.

Higher flammability tanks are typically located in the center wing box, in the horizontal stabilizer where little surface area is exposed to outside air, or in the cargo compartment. Our intent, as discussed in that rule's preamble was to “require that [such] fuel tanks are not heated, and cool at a rate equivalent to that of a wing tank in the transport airplane being evaluated.” We noted that, “This may require incorporating design features to reduce flammability, for example cooling and ventilation means, or inerting for fuel tanks located in the center wing box, horizontal stabilizer, or auxiliary fuel tanks located in the cargo compartment.” (Our reference to a wing tank was to a conventional subsonic airplane with aluminum wing tanks.) We also stated, “At such time as the FAA has completed the necessary research and identified an appropriate definitive standard to address this issue, new rulemaking would be considered to revise the standard proposed in this rulemaking.”

We then issued two Advisory Circulars, AC 25.981-1B, “Fuel Tank Ignition Source Prevention Guidelines,” and AC 25.981-2, “Fuel Tank Flammability Minimization.” These ACs described acceptable means of showing compliance with § 25.981(c). AC 25.981-2 specifically discussed the use of fuel tank inerting as a method of compliance with the flammability exposure requirements. To “inert” a fuel tank, as defined in AC 25.981-2, the percentage of oxygen in a fuel tank's air should not exceed 10 percent. (Later research, discussed below, showed that containing oxygen concentrations to 12 percent or less would inert a fuel tank.)

After revising § 25.981, we began scientific research, hoping to gain a better understanding of the ignition properties of commercial aviation jet fuel vapors. We also explored new ideas for removing flammable fuel air mixtures from fuel tanks, as well as other methods for improving fuel tank safety. Initially, efforts to develop commercially viable ways to remove flammable fuel vapors from tanks failed. For example, to lower the danger of fuel tank explosions after post-crash ground fires, systems were considered that would “scrub” the vapor in the ullage—ventilating the tank with air so as to prevent the build-up of flammable concentrations of fuel vapor. At the time, we found these systems to be impractical because of their weight, complexity, unreliability, and undesirable secondary effects on the environment.

On the recommendation of the ARAC, we refocused our efforts on reducing fuel tank flammability through nitrogen inerting. Public comment on the 1997 notice had suggested inerting was possible through adoption of a hollow fiber membrane technology, which separates oxygen from nitrogen in the atmosphere. (Air is made up of about 78 percent nitrogen and 21 percent oxygen.) The hollow fiber membrane material uses the absorption difference between the nitrogen and oxygen molecules to separate nitrogen-enriched air from oxygen. The technology had been used for many years in non-aerospace applications, such as obtaining oxygen-enriched air for medical purposes and generating nitrogen-enriched air to preserve produce in transport. In airplane applications, nitrogen-enriched air could be produced when pressurized air is forced through a canister that contains the hollow fibers. The created nitrogen-enriched air is then directed, at appropriate concentrations, into the ullage of fuel tanks and displaces the normal fuel vapor/air mixture in the tank. Use of this technology allows nitrogen to be separated from the available pressurized air onboard the airplane, which eliminates the need to carry and store nitrogen in the airplane.

Initially, we found that airplanes in the current transport category fleet were not designed with optimized air sources for creating nitrogen-enriched air. As a result, early designs required installation of an air compressor, adding significant weight and cost. Aware of the earlier system's disadvantages, our researchers worked to address those issues. Earlier fuel tank inerting designs, primarily produced for military applications to prevent fuel tank

explosions from battle damage, assumed a fuel tank was “inert” with a maximum of 9 percent oxygen content in the ullage. Achieving this level of concentration was not needed for transport category airplanes, as our research determined that a maximum oxygen content of 12 percent would be sufficient to protect airplanes from less powerful ignition sources typical of airplane system failures and malfunctions at sea level. Thus, our testing excluded turbulent flow flame propagation, or external fuel tank events, such as explosives and hostile fire. (The FAA test results are available in an FAA Technical Note: “Limiting Oxygen Concentrations Required to Inert Jet Fuel Vapors Existing at Reduced Fuel Tank Pressures” (DOT/FAA/AR-TN02/79). See:

http://www.fire.tc.faa.gov/pdf/TN02-79.pdf.

)

Terrorist initiated accidents were also excluded from consideration in the earlier ARAC reports and the possible benefits in the regulatory evaluation within this notice. While the proposed FRM requirements are not intended to address terrorist initiated explosions, such as the Bogata 727 accident discussed earlier, inerting fuel tanks may provide other significant secondary safety benefits by addressing flammability exposure. Testing conducted by China Lake Naval Weapons Center

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showed that inerting a fuel tank to 12 percent oxygen offers a high degree of protection from a fuel tank explosion when 30-millimeter high explosive incendiary projectiles shot into fuel tanks. The FAA invites comments related to the potential additional security benefits that may be achieved by imposing FRM.

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The Effectiveness of Ullage Nitrogen-Inerting Systems Against 30-mm High-Explosive Incendiary Projectiles, China Lake Naval Weapons Center, J. Hardy Tyson and John F Barnes, May 1991.

Based on our research, we identified a simplified inerting system that, using existing airplane pressurized air sources, could limit a fuel tank to the 12 percent oxygen content level. This concept eliminated the need for an air compressor, thus reducing the size and complexity of the system. Our research determined that the method of distributing the nitrogen-enriched air to the fuel tank could also be simplified, which further reduced the system's weight and installation cost. We now estimate that a simplified inerting system adequate to protect the center wing tank on airplanes in the existing fleet should weigh from 100 to 250 pounds and cost from $140,000 to $225,000 to procure and install in existing airplanes, depending on fuel tank capacity. (More information on the costs of these systems is provided in the Preliminary Regulatory Evaluation.)

The FAA has openly shared with industry information on the simplified inerting system design ever since it was first developed in May 2002. This design concept was adopted by Boeing when applying for a series of type certification and production approvals to incorporate a fuel inerting system using nitrogen air enrichment in all currently produced Boeing model airplanes. Thus, on November 15, 2002, Boeing applied for a change to TC No. A20WE to modify Boeing Model 747 series airplanes to incorporate the system into its center wing fuel tanks. It has since applied for similar approvals for the Boeing Model 737 series, Boeing Model 757 series, Boeing Model 767 series, and Boeing Model 777 series airplanes. We published a request for and received public comments on a Notice of Proposed Special Conditions for flammability reduction on the Boeing Model 747 on December 9, 2003 (68 FR 68563). Final Special Conditions No. 25-285-SC was issued on January 24, 2005 (70 FR 7800; February 15, 2005).

III. Proposed Requirements Relating to Fuel Tank Flammability

We are proposing today a performance-based set of requirements that do not specifically direct the use of fuel inerting, but rather set acceptable levels of flammability exposure in tanks most prone to explosion or require the installation of an ignition mitigation means in an affected fuel tank. We also by separate notice propose to revise Advisory Circular 25.981-2 so as to describe several means of compliance with these requirements, including both flammability-reduction means, such as cooling, inerting using nitrogen or carbon dioxide, and ignition-mitigation means, such as use of polyurethane foam or explosion suppression systems. The revised AC sets out detailed parameters for such systems if used as a means of achieving the targeted safety standards.

The rule, if adopted, would require a retrofit of much of the existing fleet of large airplanes but would not necessarily affect all transport aircraft. We will require retrofit based on safety needs, using a fleet average flammability exposure limit of seven (7) percent, the level recommended by ARAC. We know that this level is routinely exceeded in tanks that are incidentally heated by nearby air conditioning equipment and in unpressurized auxiliary fuel tanks that are located in the cargo compartment and that do not significantly cool. The vast majority of large transport category airplanes operating in the U.S., including all Airbus models and most Boeing models, have center wing tanks that are above this level. We estimate that 3,800 airplanes with flammability exposure level above 7 percent would be retrofitted if this rule is adopted.

As is the case for new production airplanes, all airplanes currently equipped with a normally emptied or auxiliary fuel tanks that have a flammability level above 7 percent could not have center wing tanks that are flammable more than 3 percent on average and 3 percent on hot days. Lowering the flammability levels of these fuel tanks in the existing fleet and limiting the permissible level of flammability on new production airplanes would result in an overall reduction in the flammability potential of these airplanes of approximately 95 percent.

Some airplane models have center tanks with a fleet average flammability exposure level that does not exceed 7 percent, including to the best of our information the Lockheed L-1011, and Boeing MD-11, DC10, MD80, and Boeing Model 727, and Fokker F28 MK100. At this time we do not believe that these airplanes would need FRM or IMM for their center tanks, unless the certificate holder has also installed an auxiliary fuel tank that is found to be affected.

7

7

Auxiliary fuel tanks are installed subject to amended supplemental type certificates or field approvals. As such they are “aftermarket” installations not contemplated by the original manufacturer of the airplane. Auxiliary fuel tanks are installed to permit airplanes to fly for longer periods of time by increasing the amount of available fuel. While all auxiliary fuel tanks are normally emptied, some “normally emptied” tanks are included in the original type design, such as the center wing tank on the Boeing 747.

A. Overview of the Proposal

Our proposal would require manufacturers and operators of most large transport category airplanes to reduce the average flammability exposure in affected fleets to tolerable levels of risk. Fleet average flammability exposure represents the percent of flight time that fuel vapors in the ullage are flammable, calculated across a fleet of an airplane type operating over the range of actual or expected flights and based on a wide range of environmental conditions and fuel properties.

8

This

rulemaking is premised on our finding that fuel tanks whose fleet-wide average flammability exposure is more than 7 percent have a “high flammability exposure,” which we consider unduly dangerous. This finding, in turn, is based on the reports and findings of the ARAC and our own risk assessment of the current transport category airplane fleet.

8

The airplane flammability exposure evaluation time begins when the airplane is prepared for flight (which commences upon the start of preparing the airplane for flight by turning on the auxiliary power unit/ground power, starting the environmental control systems, or taking other steps that begin the

initial preparation of the airplane), continues through the actual flight and landing, and ends when all payload has been unloaded and all passengers and crew have disembarked.

Our proposal would modify current regulations in several important respects, affecting both manufacturers (TC holders and STC holders) and operators (air carriers). We would significantly expand the coverage of part 25 by making manufacturers generally responsible for the development of service information and safety improvements (including design changes) where needed to ensure the continued airworthiness of previously certificated airplanes. This proposal would apply to holders of existing TCs, holders of STCs, applicants for changes to existing TCs, and certain other airplane manufacturers. We are proposing to specify the new requirements for these entities in a new subpart I to part 25, although we may decide to relocate these requirements at the time the final rule is issued to simplify harmonization efforts.

As to fuel tank flammability specifically, manufacturers, including holders of listed airplane TCs and of auxiliary fuel tank STCs, would be required to conduct a flammability exposure analysis of their fuel tanks, unless they have already notified the FAA that they will utilize an ignition mitigation means instead. A new Appendix L to part 25 will regulate the conduct of these analyses.

9

As discussed later in this document, the Appendix contains the method for calculating overall and warm day fuel tank flammability exposure values needed to show that the affected aircraft tanks comply with proposed limitations on flammability exposure levels, described below.

9

Rather than relying on the analysis already conducted pursuant to SFAR 88 and then simply regulating those airplanes with a demonstrated exposure level of 7 percent or greater, today's proposal contemplates requiring a new exposure analysis. The existing analyses, while helpful in positing which airplanes are likely to be affected by a final rule, were derived from incomplete, and sometimes differing, assumptions. Appendix L would correct such inconsistencies by establishing a single methodology for calculating average flammability exposure.

Where the required analyses indicate that the fuel tank has an average flammability exposure level below 7 percent, no changes would be required. However, for the other fuel tanks, manufacturers would be required to develop design modifications to support a retrofit of the airplane. Under today's proposal, the average flammability exposure level of any affected wing tank would have to be reduced to no more than 7 percent. In addition, for any normally emptied fuel tank (including auxiliary fuel tanks) located in whole or in part in the fuselage, flammability exposure would have to be reduced to 3 percent, both for the overall fleet average and for operations on warm days.

For long-pending certification projects that have not received a type certificate from the FAA prior to the date of the final rule (where application was received by the FAA before June 6, 2001, the effective date of 14 CFR 25.981(c), applicants would be required to limit the flammability exposure of any wing tank to no more than 7 percent. Any of those applicants whose proposals include any normally emptied or auxiliary fuel tank with a flammability exposure level that exceeds 7 percent would also have to meet the same flammability exposure requirements proposed for retrofit (i.e., 3 percent), if any portion of the tank is located within the fuselage contour. Applicants for more recent certification projects (where application was received after June 6, 2001), and all applicants for a TC or STC submitted after the effective date of the final rule would need to meet the new requirements of that section set forth in today's proposal.

We would set more stringent safety levels for certain critically located fuel tanks in most new type designs, while maintaining the current, general standard under § 25.981 for all other fuel tanks. We expect that as a result of this rule the design of most normally emptied and auxiliary tanks located, in whole or in part, in the fuselage of transport-category airplanes would need to incorporate some form of FRM or IMM. Regulations in a new proposed Appendix K to Part 25 contain detailed specifications for all FRM, if they are used to meet the flammability exposure limitations. These additional requirements are designed to ensure the reliability of flammability-reduction means, reporting of performance metrics and warnings of possible hazards in and around fuel tanks. Specifications for IMM are detailed in the current AC-25.981-2 and are not generally discussed in this document.

Type certificate holders for specific airplane models with high flammability exposure fuel tanks would be required to develop design changes and service instructions to facilitate the adoption of IMM or FRM. Manufacturers of these airplanes would have to incorporate these design changes in airplanes produced in the future. In addition, these sections would require design approval holders (TC and STC holders) and applicants to develop airworthiness limitations to ensure that maintenance actions and future modifications do not increase flammability exposure above the limits in this proposal. These design approval holders would have to submit binding certification plans by a specified date, and these plans would be closely monitored by the holders' FAA oversight offices to ensure timely progress.

Lastly, the proposal requires affected operators to incorporate FRM or IMM where required for high-risk fuel tanks in their existing fleet of affected airplane models. Air carriers would also have to revise their maintenance and inspection programs to incorporate the airworthiness limitations developed under the other proposals. We also intend to establish strict retrofit deadlines, which are premised on prompt compliance by manufacturers with their certification plans.

Table 1 summarizes the proposed regulatory changes that relate to fuel tank flammability safety. This table does not summarize the proposed regulatory changes that are common between this proposal and other aging airplane initiatives. Those changes are discussed in detail later.

Table 1.—Summary of Proposed Rules

14 CFR

Description of proposal

Applies to

25.1, 25.2

Expand applicability to current holders of TCs, STCs, and certain manufacturers

Amend § 25.2 to make reference to the proposed subpart I.

Applicants for TCs, and changes to those TCs for transport category airplanes. Manufacturers of certain airplane models.

25.981

Revise paragraph (b) to specify limits on fuel tank flammability

Add paragraph (c) to restate current option of providing ignition mitigation means (IMM).

Applicants for future TCs and design changes to those certificates.

Add paragraph (d) to include airworthiness limitation items (ALI) for IMM or Flammability Reduction Means (FRM), and move the existing ignition prevention ALI requirements into this paragraph.

Subpart I 25.1801

Defines the intent of the subpart

TCs, and design changes to those TCs for transport category airplanes. Manufacturers of certain airplane models.

25.1815

Require flammability exposure analysis of all fuel tanks within 150 days after effective date. If below 7 percent no flammability reduction required. Compliance with § 25.981(d) to define ALI required

TC holders.

If above 7 percent and in fuselage and normally emptied, must develop service instructions to meet § 25.981(b), (c) and (d)

If above 7 percent and other tank type, must develop service instructions to incorporate IMM (meet § 25.981(c), or reduce flammability to 7 percent).

Large transport category passenger airplanes, with passenger capacity of 30 or more or a payload of 7500 lbs or more (original TC or later increase).

Specific compliance dates for each Boeing and Airbus airplane model. Other models within 24 months.

25.1817

Require flammability exposure analysis of all fuel tanks installed under STC within 12 months after effective date

Require impact assessment of fuel tanks installed by STCs, and (for pending and future applicants) other STCs affecting fuel tank flammability, on IMM or FRM developed by TC holder under § 25.1815 to determine if any ALI has been violated 6 months after FAA approval of ALI submitted by TC holders under § 25.1815 or before certification, whichever is later.

Auxiliary tank STC holders for large transport category passenger airplanes, with passenger capacity of 30 or more or a payload of 7500 lbs. or more (original TC or later increase).

Require development of service instructions to correct designs that compromise ALI defined by TC holder under § 25.1815 within 24 months. Require within 24 months after TC holder compliance with 25.1815 development of service instructions for a IMM or FRM for any tank with flammability above 7 percent, if located within the fuselage and normally emptied

Applicants for future STCs or amendments to TCs that affect fuel tank system or IMM/FRM.

25.1819

Requires IMM or FRM for any fuel tank on a passenger airplane with a flammability level that exceeds 7 percent. Fuel tanks located in the fuselage and normally emptied must meet § 25.981(b) level. Other fuel tanks must not exceed 7 percent

Pending certification projects.

Pre Amendment 102.

Requires compliance with § 25.981(c)

Post Amendment 102.

25.1821

Requires any affected airplanes produced after a certain date to incorporate IMM or FRM

Manufacturers of certain airplane models.

Appendix 25 K

Establishes performance, reliability and reporting requirements for flammability reduction means

Applicants for approval of flammability reduction means.

Appendix 25 L

Defines flammability analysis method and input parameters that must be used in the analysis

Any person required to perform flammability analysis.

91.1509, 121.917, 125.509, 129.117

Require retrofit of IMM or FRM into large airplanes with high flammability fuel tanks. Require large transport category airplanes manufactured after specific dates to have IMM or FRM in high flammability fuel tanks. Require incorporation of ALI into the maintenance program

U.S. certificate holders and foreign persons operating U.S.-registered large transport category passenger airplanes.

B. Ongoing Responsibility of Type Certificate Holders for Continued Airworthiness

Several recent safety regulations necessitated action by air carriers and other operators but did not require design approval holders to develop and provide the necessary data and documents to facilitate the operators' compliance. Operators are often dependent on action by a design approval holder before they can implement new safety rules. Ongoing difficulty reported by operators in attempting to meet these rules has convinced us that the corresponding design approval holder responsibilities may be warranted under certain

circumstances to enable operators to meet regulatory deadlines.

We intend to require type-certificate holders, manufacturers and others to take actions necessary to support the continued airworthiness of and to improve the safety of transport-category airplanes. Such actions include performing assessments, developing design changes, revising instructions for continued airworthiness (ICA), and making available necessary documentation to affected persons. We believe this requirement is necessary to facilitate compliance by air carriers with operating rules that in effect demand the use of new safety features.

To address this problem, we propose to amend subpart A of part 25 to expand its coverage and to add a new subpart I to establish requirements for current holders. As discussed in our final rule, “Fuel Tank Safety Compliance Extension and Aging Airplane Program Update” (69 FR 45936, July 30, 2004), this and related proposals would add provisions to a new subpart I requiring actions by design approval holders that will allow operators to comply with our rules.

Part 25 currently sets airworthiness standards for the issuance of TCs, and changes to those certificates, for transport category airplanes. It does not list the specific responsibilities of manufacturers to ensure continued airworthiness of these airplanes once the certificate is issued. Therefore, we propose to revise § 25.1 by adding paragraph (c) to make clear that part 25 creates such responsibilities for holders of existing and supplemental type certificates for transport category airplanes, and applicants for approval of design changes to those certificates; we are also adding paragraph (d) to require design changes and other service activities by manufacturers when needed. In order to ensure the effectiveness of these changes, we would also amend § 25.2 (“Special retroactive requirements”) so as to require adherence to a new Subpart I which may require design changes and other activities by type certificate holders.

This proposal would establish a new subpart I, Continued Airworthiness and Safety Improvements, where we would locate rules imposing ongoing responsibilities on design approval holders. In the past, this type of requirement took the form of a Special Federal Aviation Regulations (SFAR). SFARs are difficult to locate, because they are scattered throughout Title 14. Placing all these types of requirements in a single subpart of part 25, which contains the airworthiness standards for transport category airplanes, would provide ready access to critical rules.

In preliminary discussions with foreign aviation authorities, with whom we try to harmonize our safety rules, they have expressed concern about consolidating parallel requirements in their counterparts to part 25. They have suggested that it may be more appropriate to place them in part 21 or elsewhere. Therefore, we specifically request comments from the public, including foreign authorities, on the appropriate place for these airworthiness requirements for type certificate holders.

We reserve additional sections in this proposal to include other subparts we would expect to create with future aging airplane rules, several of which are under development. Some of these proposals include similar language establishing the general airworthiness responsibilities of manufacturers and thus include some overlapping provisions. Once any proposal establishing these broad responsibilities becomes a final rule, we will delete the duplicative requirements from the other proposals and retain only that language pertinent to any specific new safety regulations (such as fuel-tank flammability reduction).

Except in one respect (discussed below), however, the ongoing-airworthiness requirements in Subpart I would not by their terms reach applicants for TCs with respect to new projects for which application is made after the effective date of the proposed rule. This is unnecessary because, when we adopt a new requirement for TC holders, there will be a corresponding amendment to part 25 expressly making compliance with the new, or a similar safety standard a condition for receiving a TC in the future. For example, in this proposal, the new requirements of § 25.981(b), (c) and (d) regarding FRM and IMM will govern future applications.

For safety reasons, however, we are requiring that any application for a type design change, whenever filed, not degrade the level of safety already created by the TC holder's presumed compliance with the subpart I rule. Currently, when reviewing an application for such a change, we employ the governing standards stated in part 21, specifically § 21.101. That section generally requires compliance with standards in effect on the date of application but contains exceptions that may allow applicants to show compliance with earlier standards. For example, if a change is not considered “significant,” the applicant may be allowed to show compliance by pointing to standards that applied to the original TC. (

See

AC 21.101-1, “Establishing the Certification Basis of Changed Aeronautical Products,” a copy of which can be downloaded from

http://www.airweb.faa.gov/rgl

).

With the adoption of subpart I rules, we must ensure that safety improvements that result from TC holder compliance with these requirements are not undone by later modifications. Therefore, even when we determine under § 21.101 that an applicant need not comply with the latest airworthiness standards, it will be required to demonstrate that the change would not degrade the level of safety provided by the TC holder's compliance with the subpart I rule. In the context of today's proposal, for example, this will mean that an applicant for approval of a design change would have to show that it would not increase the fuel tank flammability above the limits defined in this proposal or adversely affect the FRM or IMM established by the TC holder.

C. Applicability

1. Manufacturers and Holders of Type Certificates, Supplemental Type Certificates and Field Approvals

Today's proposal, if adopted, will impose requirements on TC holders for all affected transport category airplanes as well as STC holders and operators who have field approvals for auxiliary fuel tank designs. Not all airplanes would require the installation of an FRM or IMM. Those requirements would be based on the initial average flammability exposure analysis discussed in detail later in this document. However, the TC, STC or field approval holder would be required to develop and provide limitations on the types of alterations and operations permitted for the airplane in order to retain the validity of that initial analysis.

Today's proposal, if adopted, would apply not only to domestic TC holders, but also to foreign TC holders. This rule would be different from most type certification programs for new TCs, where foreign applicants typically work with their responsible certification authority, and the FAA relies, to some degree, upon that authority's findings of compliance under bilateral airworthiness agreements. No other certification authority has yet adopted requirements addressing fuel tank flammability for existing TCs. While some authorities have indicated an interest in adopting some type of requirements for new airplane designs, they may not adopt requirements

applicable to existing TCs. Accordingly, the FAA will retain the authority to make all the necessary compliance determinations, and where appropriate may request certain compliance determinations by the appropriate foreign authorities using procedures developed under the bilateral agreements. The compliance planning provisions of this proposed rule are equally important for domestic and foreign TC holders and applicants, and we will work with the foreign authorities to ensure that their TC holders and applicants perform the planning necessary to comply with those requirements.

As discussed briefly above, the proposed rule would require holders of existing type certificates to incorporate FRM or IMM into all new production airplanes if the fleet average flammability exposure level exceeds permissible levels. In past rulemakings where the FAA has required production cut-in of safety improvements, we have adopted rules prohibiting operators of airplanes produced after a specified date from operating those airplanes unless they are equipped with the improvements. This approach is effective in ensuring that U.S. operators receive the benefits of these safety improvements. But these rules do not apply to foreign operators, unless they operate U.S.-registered airplanes.

By requiring FRM or IMM separately from the operational rules proposed in this notice, the proposed rule would improve the safety of the overall fleet of larger transport category airplanes. This requirement would also facilitate the secondary market for these airplanes. Even if a manufacturer initially sells an airplane to a foreign operator who may not be required to have the system, the operator may later sell or lease it to a U.S. operator. The U.S. operator would be able to simply place it into service, rather than having to install a system. Given the frequency of airplane transfers in today's global economy, we think having these systems installed during production will provide significant long-term efficiencies since no retrofit would be required, as well as providing immediate safety benefits.

2. Airplanes

If adopted, this rule would apply, with some exceptions discussed below, to transport category turbine-powered airplanes with a maximum type-certificated capacity of 30 or more passengers, or a maximum payload capacity of 7500 pounds or more resulting from the original certification of the airplane or later increase in capacity. This would result in the coverage of airplanes where the safety benefits and the public interest are the greatest.

We are proposing to apply this rule to airplanes for which a passenger capacity of 30 or more has been approved at any time. In the past, some designers and operators have obtained design change approval to slightly lower existing capacity to avoid applying requirements mandated only for airplanes over specified capacities. Today's proposal would remove this possible means of avoiding compliance. It is also possible that an airplane design could be originally certificated with a capacity slightly lower than the minimum specified in this section, but through later design changes, the capacity could be increased above this minimum. Today's proposal addresses both of these situations by proposing to regulate all airplanes that have been approved for carriage of 30 or more passengers, or 7500-pound or more payload, at any time.

We considered applying this proposal to all part 25 airplanes. This would have resulted in modifications to all fuel tanks located in the fuselage that are normally emptied. However, smaller airplanes generally do not have a significant number of high flammability exposure fuel tanks. Few of the smaller transport category airplanes in the current fleet have center wing tanks that are normally emptied. While some of the smaller airplanes have auxiliary or normally emptied fuel tanks located within the fuselage contour, many of these airplane types use differential fuel pressure to transfer the fuel from the fuel tanks. The increased pressure results in a reduction in the fuel tank flammability by keeping the fuel vapors at a level where ignition is unlikely. We have determined that the benefits of including these airplane types in this proposal are not sufficient to warrant the cost.

Certain vintage airplanes type certificated before 1958, the beginning of the jet age, would be excluded from the requirements of this proposal. They are listed in § 25.1815(j). There are no known reciprocating-powered transport category airplanes currently in scheduled passenger service. Compliance would not be required for these specific older airplanes, because their advanced age and small numbers would likely make compliance economically impractical. If the public knows of other airplanes that may present unique compliance challenges, the FAA is interested in receiving comments. These comments may result in additional airplane models being excluded from the requirements of this proposed rule.

The proposal does not extend to airplanes used in all-cargo operations. Our analysis of the costs of extending the proposal to include these airplanes does not appear to be justified by the associated benefits. The potential loss of life in a single accident is much smaller on all-cargo planes of the size contemplated by today's proposal than on comparably sized passenger planes. The undiscounted cargo airplane costs would be about $261 million, with a present value of $110 million, while the benefits would be less than $1 million. However, the FAA does believe there is a risk to all-cargo airplanes because they share the same design features as at-risk passenger airplanes. We typically do not base our certification standards for transport category airplanes on use. Rather, our general philosophy is to address the performance characteristics of these airplanes because we believe all occupants should be protected against those designs that present a risk of serious injury or death.

We have not evaluated the risk to all-cargo airplanes because they are derivatives of passenger airplanes. The risk may be lower for all-cargo operations than for passenger operations. For example, if the risk of a fuel tank explosion per operating hour is the same for all-cargo planes as for passenger airplanes, the projected number of accidents for these planes is significantly less than one (0.15) in the next 50 years. This is because the projected number of miles flown by a cargo plane over the next 50 years is only 23 million miles. The risk may also be lower for all cargo operations because many cargo operations are conducted at night when the flammability of the fuel tanks is lower because of lower ambient temperatures.

The 747 has both a passenger version and a freighter. The Monte Carlo analysis conducted for the 747 included both types of airplanes, and was weighted primarily toward the passenger airplane because they make up the majority of the 747 fleet. Thus, it should be possible to model the risk of a fuel tank explosion for cargo airplanes separate from passenger airplanes. We request flammability analyses on all-cargo airplanes and on the passenger versions of the same airplane model, as well as any underlying data.

We have provided a breakdown of the estimated costs and benefits associated with requiring all-cargo airplanes be equipped with a means of reducing flammability in the preliminary regulatory evaluation. We believe that

the cost associated with providing a means of flammability reduction on newly designed cargo airplanes may be sufficiently low that it could make sense for all airplanes manufactured under a TC or amended TC applied for after the effective date of the final to have either an FRM or IMM. We believe there will be only a minimal cost associated with equipping newly designed all-cargo airplanes with a means of flammability reduction since the passenger version of the same model will be designed with such a system.

We request comment on whether, given the costs involved, the design rules, the production cut-in rules, or the operating rules, if adopted, should be applied to all-cargo airplanes.

Even with the categories of airplanes excluded that are discussed above, we recognize that this proposal is costly. To ensure that this rule is as cost effective as possible, we specifically request comments on whether there are other categories of airplanes or ways to distinguish among airplanes that would focus this rule on those where the benefits would be greatest. Any comments provided should include data to support the suggested exclusions or distinctions.

3. Fuel Tanks

The requirements proposed today would apply the proposed new FRM or IMM requirements to existing fuel tanks with a fleet average flammability exposure level that exceeds 7 percent. Main fuel tanks on existing airplanes, i.e., those that are designed both to feed fuel directly to one or more engines and to hold the required fuel reserves continually throughout each flight, are unlikely to be affected as they should have a fleet average flammability exposure level well below 7 percent.

For any fuel tank that is normally emptied and has a fleet average flammability exposure level that exceeds 7 percent average flammability exposure, if any portion of the tank is located in the fuselage contour, the proposal would require TC STC and field approval holders to develop IMM or FRM that reduces the flammability exposure to 3 percent average flammability exposure and that meets the 3 percent warm day requirements.

All other tanks with a fleet average flammability exposure level exceeding 7 percent would need to incorporate IMM, or FRM. If FRM is installed it would need to provide a fleet average flammability exposure at one of two levels: Tanks on airplanes manufactured pursuant to a type certificate applied for prior to June 6, 2001 would have to have an exposure level no greater than 7 percent; tanks on airplanes manufactured pursuant to a type certificate applied for after June 6, 2001 would have to have an exposure level either no greater than 3 percent or equivalent to that of a comparable conventional unheated aluminum tank (which could be either more or less than 3 percent).

The ARAC found fuel tanks that are normally emptied have higher flammability exposure times than main tanks. Because these tanks contain a high percentage of ullage during a significant portion of most flights, a larger number of potential ignition sources are exposed to fuel vapor space for an extended time. Additionally, when they are within the fuselage contour, they are not naturally cooled by external air, which causes the fuel vapor to be flammable for a significant portion of the airplane operating time.

Auxiliary fuel tanks are developed by TC holders, STC holders and, occasionally, by operators via field approvals, to increase the fuel capacity available on a type-certificated airplane. There are 74 different STCs for auxiliary fuel tanks in the airplanes potentially affected by the proposed rule. There are also field approvals for auxiliary tanks installed by airplane operators. Data submitted to the FAA as a result of SFAR 88 shows that fifteen of these auxiliary tanks have high flammability exposure fuel tanks. Some of these tanks have been installed in airplanes such as the DC-9 and DC-10 that do not have any other fuel tanks with high flammability exposure. Production of these airplane models ended long ago, so many of these airplanes will be at or near the end of their intended operational life at the end of the proposed compliance time given to the operators to incorporate FRM or IMM. Requiring the affected certificate holders to develop service instructions and the operators to incorporate FRM for these older fuel tanks increases the cost of the proposed rulemaking with fewer benefits than incorporation of FRM on newer airplane models. Therefore, the FAA specifically requests comments on including these auxiliary fuel tanks in the proposal. Information on the number of fuel tanks installed in the fleet and the remaining useful life of the affected airplanes should be provided.

Portions of fuel tanks that are located within the fuselage contour include those in either the pressurized or unpressurized section of the fuselage or those whose surfaces make up part of the pressurized compartment. Fuel tanks located within the cargo compartment and center wing tanks are considered to be located in the fuselage contour. Many center tanks have portions that extend from the center wing box to the wing. The compartments of the tank located within the wing would also be considered part of the tank located within the fuselage contour and the same flammability requirements would apply. Fuel tanks located in the horizontal stabilizer, which also include segments located inside the fuselage and portions that extend outside the fuselage contour, would be assessed in the same way. Fuel tanks have also been located within the vertical stabilizer. If no portion of these tanks is in the fuselage, these tanks would not be considered as located within the fuselage boundary.

4. Airplane Operators

The rule proposed today would also apply to operators of the affected aircraft other than those who operate pursuant to 14 CFR part 135, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons On Board Such Aircraft. We are excluding part 135 operators, because we have determined that only a few airplanes operated under part 135 would be subject to the rule. This is because part 135 is currently limited to a carrying of capacity of 10 or fewer passengers and a payload of no more than 7,500 lb. We are in the process of revising part 135 and may consider increasing the payload capacity as part of that revision. If an increase in payload capacity is contemplated, we may also consider requiring FRM or IMM under part 135.

As discussed previously, in an effort to enhance the cost effectiveness of this rule, we specifically request comments on whether other categories of operations should be excluded. Any comments provided should include data to support the suggested exclusions or distinctions.

D. Proposed Requirements for Manufacturers and Holders of Type Certificates, Supplemental Type Certificates and Field Approvals

1. New Airplane Designs

Currently, § 25.981(c) establishes a requirement that fuel tank installation on all airplanes for which the type certificate was applied for after 2001 must have either a “means to minimize the development of flammable vapors in the fuel tanks” that would “reduce the likelihood of flammable vapors, or a “means to mitigate the effects of an ignition of fuel vapors * * *.” We propose amending this section to address new airplane designs.

We propose to require those airplanes incorporating FRM to limit the fleet average flammability exposure to 3 percent, and to limit warm day exposure to 3 percent, for all normally emptied fuel tanks located, in whole or in part, in the fuselage. All other fuel tanks could either meet the 3 percent average flammability exposure limitation or have a level that is no higher than the exposure level in a conventional unheated aluminum wing tank that is cooled by exposure to ambient temperatures during flight. The advantage of the first option is that manufacturers using unconventional designs would not be required to conduct the modeling on an equivalent unheated aluminum wing tank that is a purely theoretical design. The advantage of the second option is that a manufacturer could increase the level of acceptable exposure based on the exposure characteristics of this theoretical wing design.

TC Applicants have proposed newer technology airplanes using composite wing skins or fuel tank designs with little exposed surface area. These designs may result in average fuel tank flammability exposure above the levels recommended by the ARAC. We expect future applicants will propose similar designs. For these airplane types, the applicant would have the option of demonstrating compliance by analyzing the fleet average flammability exposure of an equivalently designed wing made of aluminum for the model under evaluation. The thermal characteristics of the wing treated as a single fuel tank, as well as airplane specific parameters such as climb, cruise and descent profiles and flight length distribution, would be used as inputs to the flammability exposure analysis defined in Appendix L. This analysis would establish the maximum allowable flammability for the airplane model under evaluation.

The safety objective of an “unheated aluminum wing tank” that is proposed as the standard in this notice is consistent with the ARAC recommendation and 14 CFR 25.981(c). It does not provide a numerical standard to apply in future type certification programs and the demonstration of compliance requires the applicant to conduct an analysis of their design to establish the flammability of a conventional unheated aluminum wing tank. In certain cases the compliance demonstration would be simplified if a numerical standard were provided in the regulation. Therefore we are proposing to establish a numerical flammability exposure standard of 3 percent that can be used. This approach may have implementation advantages and should achieve the safety level intended by the ARAC recommendation and the current approach of § 25.981(c). We specifically request comments on which approach would be more workable and effective. If, based on comments received, we determine that a numerical standard alone is preferable, we may revise the final rule to adopt this approach.

In addition to designing normally emptied fuel tanks that meet the proposed requirements, the TC holder would be required to provide airworthiness limitations designed to prevent exceeding the exposure limits of this rule or degrading the performance and reliability of FRM or IMM provided by the TC holder. For example, the manufacturer may state that any changes to the fuel system may invalidate its exposure analysis. In such an instance, the party making subsequent changes would need to conduct its own exposure analysis to ensure that the affected fuel tanks remain within the applicable limits. Likewise, a manufacturer may limit the type of jet fuel acceptable for its systems, as a jet fuel with a lower flash point may invalidate the initial exposure analysis.

As discussed earlier, today's proposal would not apply to airplanes designed solely for all-cargo operations. This exclusion applies to airplanes that, either as a result of initial type certification or through later design changes, have no passenger carrying capability, except for carriage of supernumeraries.

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Airplanes designed for all-cargo operations would continue to be subject to the existing requirements of § 25.981(c), which requires either means to minimize the development of flammable vapors in the fuel tanks or IMM. On the other hand, if an airplane that is designed for all-cargo operations is converted to an airplane equipped to carry passengers, including a “combi” airplane (part cargo, part passenger), this design change would make the airplane subject to these proposed requirements.

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These are cargo handlers and other persons who are typically carried on cargo-only airplanes to assist in the cargo operations.

2. Existing Airplane Designs

Holders of existing TCs would be required to first conduct a fleet average flammability exposure to determine whether the rule proposed today would apply to their fuel tanks. If the exposure level for normally emptied fuel tanks within the fuselage exceeds 7 percent, design changes and instructions for IMM or FRM that limit both overall and warm day fleet flammability exposure levels (discussed later) to no more than 3 percent would need to be developed. All other normally emptied fuel tanks exceeding a 7 percent exposure limit would require design changes limiting exposure to 7 percent unless manufactured pursuant to a type certificate applied for after June 6, 2001, in which case the potentially more stringent requirements of existing § 25.981(c) would continue to apply.

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Once design changes are developed, a second exposure analysis would need to be conducted to validate the design changes.

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If this proposed amendment is not issued until after affected pending certification projects are completed, the final rule may revise the retrofit requirements proposed in § 25.1815 to reference Amendment 25-102 as the appropriate standard for fuel tanks on these airplanes other than those located in the fuselage.

Even if no changes to existing fuel tanks are required based on the fleet average exposure analysis, the manufacturer would be required to develop the same type of airworthiness limitations as those required for new airplane designs.

The affected TC holders would also be required to submit compliance plans for the flammability analysis and the development of service instructions for an FRM or IMM. The contemplated compliance schedules and submissions are discussed later in this document.

Finally, today's proposal would require production cut-in for all airplanes manufactured after the required design changes are available. This section would apply only if the FAA has jurisdiction over the organization responsible for final assembly of the airplane. Section 25.1821(a) uses the same terminology as Annex 8 to the Convention on International Civil Aviation, which defines the limits of the FAA's authority under international law. In most cases, this refers to final assembly within the United States; there are limited circumstances where final assembly may occur in United States, but the responsible organization is under the jurisdiction of a foreign authority. It is also possible that final assembly could be done in another country by an organization over which the FAA has jurisdiction, such as a production certificate holder.

3. Auxiliary Fuel Tanks

Manufacturers and installers of auxiliary fuel tanks, whether manufactured under an amended TC, an STC or a field approval, would be required to conduct both an initial fleet

average exposure analysis and an impact assessment. The first analysis would determine the exposure of the tanks for which they are responsible, while the second would determine whether those tanks negatively impact the flammability exposure of the tanks originally installed on the airplane.

Changes to TCs, including installation of auxiliary fuel tanks or changes in the capacity of fuel tanks, may result in increased fuel tank flammability exposure or adversely affect FRM or IMM.

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Accordingly, the proposed rule would require a flammability exposure analysis of the auxiliary fuel tank design, an impact assessment to determine any adverse impact its design may have on the original or modified type design, and development of a flammability impact mitigation means (FIMM) to address adverse changes in flammability exposure.

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With the adoption of rules requiring the retrofit of fuel tanks in certain airplanes, we have to consider different issues in deciding what standards applicants for design change approvals must meet. Otherwise, the safety improvements that result from TC holder compliance with these requirements could be undone by later modifications. Therefore, even if we determine under § 21.101 that it is not necessary to require these applicants to comply with the latest airworthiness standards, it is still necessary for them to show that the change would not degrade the level of safety provided by the TC holder's compliance with the rule proposed today.

STC holders or applicants for an amended TC affected by the proposed rule would need to conduct a flammability analysis using the “Monte Carlo” method defined in proposed Appendix L and discussed later in this document. A number of inputs are required to conduct this analysis. Airplane specific data, such as fuel management, fuel tank thermal characteristics, or airplane climb rate may not be readily available from the original TC holder. We intend the STC holders to obtain the information by working with the TC holder and operators of airplanes that have their tanks installed. Applicants would need to work with prospective customers. Operators have business agreements with the original TC holders and in many cases access to TC holder information they obtained when they purchased the airplane. Conservative assumptions or business agreements with the original TC holders are other possible methods of gathering airplane type specific data needed for the analysis.

If an increase in exposure above the allowable limits is identified, the holder of the STC or field approval would have to develop a FIMM and demonstrate how it will mitigate the impact of the increased exposure. One of the easiest methods may be simply deactivating the auxiliary tank or sealing off the venting to the affected tank. As another example, if an auxiliary fuel tank vents into a TC holder's tank for which FRM is provided, the venting may have to be modified to prevent adversely affecting the FRM's performance.

Finally, a validation analysis would be required for the auxiliary tanks that demonstrates that the auxiliary tank flammability exposure levels, as modified with the addition of FRM or IMM, do not exceed the acceptable limits. Likewise, a validation analysis would be required to demonstrate that the FIMM is effective in maintaining the level of exposure in other tanks determined by the manufacturer of the other tank. As is the case for TC holders of existing airplanes, holders of STCs and field approvals would need to develop future airworthiness limitations and meet all mandated compliance schedules should they decide not to deactivate the fuel tank.

For applicants for STCs and TC amendments, this proposal includes other design changes that could affect flammability exposure. Because this rule would require retrofit of airplanes to reduce flammability exposure, it would be counterproductive to allow future design changes that might negate the safety benefits of those retrofits.

Any design change to a TC subject to the requirements proposed in today's document that adds an auxiliary fuel tank, increases fuel tank capacity, or increases the flammability exposure of the existing fuel tank would have to meet the requirements of § 25.981 proposed today. This requirement is intended to apply primarily to future design changes, but it may also apply to design change projects that are pending when this rule is issued. For example, in addition to applying for a new TC for the Airbus Model A380, Airbus has also applied for an amendment to that TC for the Model A380-800F (freighter derivative). Among other design changes, this TC amendment would incorporate a new fuel tank in the fuselage contour that is normally emptied. Under this proposal, this fuel tank would have to be shown to meet the requirements of proposed § 25.981. Because of the increased technical complexity of auxiliary fuel tank installations resulting from this proposal once this final rule is adopted, field approvals will no longer be granted for these tanks on airplanes affected by this rule.

4. Methods of Mitigating the Likelihood of a Fuel Tank Explosion

As noted above, TC and STC holders may need to make design changes to their fuel tanks located, in whole or in part, within the fuselage to decrease their level of flammability exposure. The rule proposed today offers two options, IMM or FRM.

a. Flammability Analysis Using the Monte Carlo Method

For all fuel tanks, an analysis must be performed to determine whether the fuel tank, as originally designed, meets the fleet average flammability exposure limits discussed above. By “average,” we mean that the analysis of each fuel tank must be averaged over the entire flammability exposure evaluation time (FEET) (see footnote 8) of each airplane in the entire fleet. To determine the flammability exposure of fuel tanks, the ARAC used a specific methodology referred to as the Monte Carlo method.

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We are proposing that any analysis of a fuel tank must be performed in accordance with this methodology, as detailed in proposed Appendix L and in the FAA document, Fuel Tank Flammability Assessment Method Users Manual.

14

We considered approving alternative methodologies in lieu of Appendix L, but we found that no other alternative considered all factors that influence fuel tank flammability exposure, which is the safety objective of this proposal.

13

This methodology determines the fuel tank flammability exposure for numerous simulated airplane flights during which various parameters such as ambient temperature, flight length, fuel flash point are randomly selected. The results of these simulations are averaged together to determine the fleet average fuel tank flammability exposure.

14

As indicated in Appendix L, we intend to incorporate the users manual by reference into the final rule.

The Monte Carlo method,

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as commonly understood by scientists, is

useful for obtaining numerical solutions to problems which are too complicated to solve analytically. The method provides approximate solutions to a variety of mathematical problems by performing statistical sampling experiments on a computer. The method applies to problems with no probabilistic content as well as to those with inherent probabilistic structure.

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History of Monte Carlo method

The method is called after the city in the Monaco principality, because of a roulette, a simple random number generator. The name and the systematic development of Monte Carlo methods dates from about 1944.

The real use of Monte Carlo methods as a research tool stems from work on the atomic bomb during the second world war. This work involved a direct simulation of the probabilistic problems concerned with random neutron diffusion in fissile material; but even at an early stage of these investigations, von Neumann and Ulam refined this particular “ Russian roulette” and “splitting” methods. However, the systematic development of these ideas had to await the work of Harris and Herman Kahn in 1948. About 1948 Fermi, Metropolis, and Ulam obtained Monte Carlo estimates for the eigenvalues of Schrodinger equation.

In about 1970, the newly developing theory of computational complexity began to provide a more

precise and persuasive rationale for employing the Monte Carlo method. The theory identified a class of problems for which the time to evaluate the exact solution to a problem within the class grows at least exponentially with M. The question to be resolved was whether or not the Monte Carlo method could estimate the solution to a problem in this intractable class to within a specified statistical accuracy in time bounded above by a polynomial in M. Numerous examples now support this contention. Karp (1985) shows this property for estimating reliability in a planar multiterminal network with randomly failing edges. Dyer (1989) establish it for estimating the volume of a convex body in M-dimensional Euclidean space. Broder (1986) and Jerrum and Sinclair (1988) establish the property for estimating the permanent of a matrix or, equivalently, the number of perfect matchings in a bipartite graph. Discussion derived from

History of the Monte Carlo Method

, Sabri Pllana,

http://geocities.com/College Park/Quad/2435/index.html.

Our use of this method to analyze fuel tank flammability exposure and define acceptable limits is based on the recommendation of the ARAC, which compared the flammability exposure of conventional unheated aluminum wing fuel tanks to that of tanks that are located within the fuselage contour and heated by adjacent equipment. Use of the Monte Carlo method allows us to consider variables from within defined distributions that represent possible operating conditions for the flight. The results of a large number of flights can then be used to approximate average flammability exposure over a large fleet of airplanes.

Variables include those affecting all airplanes in the transport category airplane fleet, such as: (1) Ground, overnight, and cruise air temperatures likely to be experienced worldwide; (2) fuel properties; and (3) conditions when the tank in question will be considered flammable. In addition, the analysis factors in specific airplane models characteristics, such as climb and descent profiles, fuel management, heat transfer characteristics of fuel tanks, maximum airplane operating temperature limitations, maximum airplane range for the airplane model, and the effectiveness of FRM (if installed).

The flammability analysis must include any model variations and derivatives for which the TC holder has obtained approval that affect fuel tank flammability exposure. Model variations that may affect fuel tank flammability could include changes in the fuel tank volume or usable fuel capacity, changes in the fuel management procedures, and engine changes that might affect parameters such as airplane climb rate or bleed air available if needed by an FRM. Other examples of configuration differences that may affect fuel tank flammability exposure are provided in the discussion of § 25.1817. The flammability analysis would also include all modifications and changes mandated by ADs that affect fuel tank flammability exposure as of the effective date of the rule. These ADs would only be those issued against any configurations developed by TC holders. The analysis would not address any ADs issued against modifications defined by a third party STC installed on affected airplanes. The result would be a configuration that is clearly understood by both industry and the FAA.

Mass loading and changes in fuel vapor concentration caused by fuel condensation and vaporization have been excluded from the flammability exposure analysis. The method used by the ARAC to establish the flammability exposure value as the benchmark for fuel tank safety for wing fuel tanks did not include the effects of cooling of the wing tank surfaces and the associated condensation of vapors from the tank ullage. If this effect had been included in the wing tank flammability exposure calculation, it would have resulted in a significantly lower wing tank flammability exposure benchmark value. The ARAC analysis also did not consider the effects of the low fuel condition (or “mass loading”) which would lower the calculated flammability exposure value for fuel tanks that are routinely emptied, such as center wing tanks. When the amount of fuel is reduced to very low quantities within a fuel tank, there may be insufficient fuel in the tank to allow vaporization of fuel to the concentration that would be predicted for any particular temperature and pressure.

The effect of condensation and vaporization in reducing the flammability exposure of wing tanks is comparable to the effect of the low fuel condition in reducing the flammability exposure of center tanks. Therefore, we consider these effects to be offsetting, so that by eliminating their consideration, the analysis will produce results for both types of tanks that are comparable. Accordingly, both factors have been excluded when establishing the flammability exposure limits in this proposal. During development of the harmonized special conditions for the Boeing 747, the FAA and the European Joint Aviation Authorities (JAA)/EASA agreed that using the ARAC methodology provides a suitable basis for determining the flammability of a fuel tank and consideration of these effects should not be permitted.

Using these variables, the Monte Carlo method would then be applied to a statistically significant number of flights (1,000,000), where each of the factors described above is randomly selected. The flights selected are representative of the fleet using the defined distributions of the variables. For example, flight one may be a short flight on a cold day with an average flash point fuel. Flight two may be a long flight on an average day with a low flash point fuel. This process is repeated until 1,000,000 flights have been defined in this manner.

For every one of the 1,000,000 flights, the Monte Carlo program calculates the amount of time the bulk average fuel temperature and ambient pressure in the fuel tank or compartment of interest would result in the fuel vapor being within the flammable range. This calculation is then used, in combination with the oxygen concentration in the fuel tank (if an FRM is installed), to establish whether the fuel tank is flammable. Averaging the results for all 1,000,000 flights provides an average flammability exposure for the fleet of airplanes of a particular model type.

The determination of whether the fuel tank ullage is flammable is based on the temperature of the fuel in the tank or the compartment of interest, determined by the tank thermal model, the atmospheric pressure in the fuel tank, and properties of the fuel loaded for a given flight, which is randomly selected from data provided in tables in this appendix.

The Monte Carlo methodology has previously been recommended by ARAC and has been used in previous analyses by the affected certificate holders in evaluating the flammability exposure of fuel tanks conducted as part of evaluating the findings of SFAR 88. Therefore we expect the affected type certificate holders already have a good understanding and can comply with this requirement within the proposed timeframe of 150 days.

b. Ignition Mitigation Means

The proposed rule maintains the option introduced by Amendment 25-102 for affected manufacturers to use ignition mitigation as a means of protecting the airplane from the hazards associated with fuel tank flammability. IMM is a passive system that requires little attention once installed. IMM does not prevent an ignition in the fuel tank; rather, material absorbs the heat created by the fire. While a small fire could occur, an IMM system eliminates the

possibility of a catastrophic fuel tank explosion.

We acknowledge that IMM presents maintenance challenges. The mitigation means (such as polyurethane foam, metal foil products and explosion suppression systems discussed within AC 25.981-2) must be reinstalled exactly as removed when the fuel tanks are opened up for maintenance actions. Replacement is particularly difficult because all voids must be removed. It also appears that the materials used for mitigation (particularly the polyurethane foams) may be prone to compression, thus reducing the usable life of the material.

Nevertheless, given the potential effectiveness of IMM, the FAA believes we should continue to allow installation of IMM as a means of compliance with the requirements proposed today. A detailed discussion of acceptable means of compliance for manufacturers choosing to comply with the IMM option is provided in AC-25.981-2.

c. Flammability Reduction Means

Alternatively, a TC or STC holder could decide to use an FRM that limits the exposure level of the tanks. For fuel tanks that are normally emptied and located within the fuselage contour, the exposure would have to be limited to 3 percent under two sets of conditions, overall fleet exposure and warm day fleet exposure. Both of these conditions would be evaluated using the Monte Carlo method described below. For all other fuel tanks, the 3 percent limit would apply only to the overall fleet exposure.

The proposed flammability exposure requirements are intended to provide an additional layer of protection to the existing certification standards that require designs to preclude fuel tank ignition sources. This balanced risk management approach of precluding ignition sources and reducing flammability exposure in certain fuel tanks provides two independent layers for preventing fuel tank explosions in those tanks. The proposed requirements could be met by a highly reliable “single-string” (non-redundant) inerting-based FRM, allowing for limited operation of airplanes with an inoperative FRM until repairs could be made. These requirements could also be met by a cooling-based FRM. Compliance with these requirements has been shown to be practical using existing technology.

i. Accounting for System Reliability and Performance Issues

As discussed in the background section of this document, previous studies of inerting-based FRM showed that, if inerting systems were required to be operational for all flights, the system would be required to have at least some redundant design features and would not be practical. That is, it would require most components to be duplicated to provide a back-up function in the event the primary component failed. A requirement for a redundant FRM that would continue to operate after component failure would increase the weight and complexity of an inerting system. This may result in a system that would not be practical for commercial airplanes at this time. The overall fleet flammability exposure analysis would assume some periods of inoperability. However, we would require that the contribution to average flammability exposure due to either reliability (during periods when the system is inoperative) or system performance (during periods when the system does not have the capacity to maintain a non flammable tank), be limited to 1.8 percent. This gives the designer freedom to engineer the system, and allows for some operation of airplanes with an inoperative FRM until repairs can be made at an appropriate maintenance facility.

ii. Warm Day Fleet Flammability Exposure

The warm day exposure analysis is intended to ensure minimum FRM system performance levels when there is the greatest risk to safe flight. Therefore, the 3 percent flammability exposure limit excludes system reliability related contributions that are included in the overall fleet flammability exposure assessment. Compliance with this proposal would require conducting an analysis in accordance with Appendix L for each of the specific phases of flight during warmer day conditions defined in the proposal. The flammability exposure of the tank in question would be determined for the ground, takeoff and climb phases as separate values, without including the times when the FRM is not available because of failures of the system or dispatch with the FRM inoperative. The fleet flammability exposure level of each fuel tank for ground, takeoff, and climb phases of flight during warm days must not exceed 3 percent of the flammability exposure evaluation time in each of the three phases.

iii. Reliability Reporting

Today's proposal, if adopted, would require that the applicant demonstrate effective means to ensure collection of FRM reliability data so that the effects of component failures can be assessed on an on-going basis. The proposed reporting requirement applies to applicants and holders of the affected TCs, STCs, and field approvals.

The rule would require the TC or STC holder to provide the FAA with summaries of the FRM reliability data and compliance with Appendix K on a quarterly basis for the first five years after the FRM is installed and operational. After that time, continued quarterly reporting requirements may be replaced with other reliability tracking methods approved by the FAA oversight office. The requirement for quarterly reports may be eliminated if the FAA determines that the reliability of the FRM meets, and will continue to meet, the requirements of the rule.

Operators would not be required to report FRM reliability information. We intend TC holders to gather the needed data from operators using existing reporting systems that are currently used for airplane maintenance, reliability and warranty claims. We anticipate the operators would provide this information through existing business arrangements between the TC holders and the airlines.

iv. Reliability Indication and Maintenance Access

The proposed rule would require that indicators be provided to identify failures of the FRM, so that appropriate actions can be taken to maintain the reliability of the FRM. The need to provide indication of the FRM status will depend on the particular FRM design. Various design methods may be used to make sure an FRM meets the reliability and performance requirements. These may include a combination of system integrity monitoring and indication, redundancy of components, and maintenance actions. A combination of maintenance indication or maintenance check procedures could be used to limit exposure to latent failures within the system, or high inherent reliability may be used to make sure the system will meet the fuel tank flammability exposure requirements.

The need for FRM indications and the frequency of checking system performance (maintenance intervals) must be determined as part of the FRM fuel tank flammability exposure analysis. The determination of a proper maintenance interval and procedure will follow completion of the certification testing and demonstration of the system's reliability and performance prior to certification or as part of the FAA review process for airplanes manufactured under existing

TCs or auxiliary fuel tanks under existing STCs.

The rule would also require that sufficient accessibility to FRM status indications be provided for maintenance personnel. We intend that maintenance personnel or the flightcrew have access to any indications that must be accessed at intervals established by the FRM design approval holder when demonstrating compliance with the reliability requirements for the FRM. Access doors and panels to the fuel tanks with FRMs and to any other enclosed areas that could contain hazardous atmosphere under either normal conditions or failure conditions would need to be permanently stenciled, marked, or placarded to warn maintenance personnel of the possible presence of a potentially hazardous atmosphere. The proposal for markings does not alter the existing requirements that must be addressed when entering airplane fuel tanks.

d. Service Instructions and Service Bulletins

If the flammability exposure analysis shows that the average exposure level for any fuel tank exceeds 7 percent, the TC holder would be required to develop design changes and service instructions for either FRM or IMM.

Modifications incorporated into existing airplanes, including safety related changes (design and/or maintenance) that are mandated by AD, are typically made by operators using service instructions developed by the TC holders, commonly referred to as service bulletins. In this proposal, service instructions must contain sufficient information for the operator to incorporate the design change and any associated procedures and airworthiness limitations. They may include specific step-by-step procedures and information needed by the operator, such as parts lists and drawings. Therefore, the proposed rule would require TC holders to develop and submit for approval by the FAA, not just data defining a proposed design change, but all of the information necessary to enable an operator to comply with the proposed operational rules, discussed later.

e. Critical Design Configuration Control Limitations (CDCCL)

If adopted, the rule would require defining airworthiness limitations, including Critical Design Configuration Control Limitations (CDCCL), inspections, and other procedures for fuel tanks to prevent exceeding the applicable flammability exposure limits. For this proposal, CDCCL include those features of the design that must be present or maintained for compliance with the requirements of § 25.981(b) and (c) for the operational life of the airplane. For example, certain fuel tanks may rely on natural cooling to meet the flammability exposure levels within this proposal. Changes to the airplane, such as installing a fuel re-circulation system, hydraulic heat exchanger in the fuel tank, or a heat source adjacent to the fuel tank, may affect fuel tank flammability. The CDCCL would be necessary in this example to prohibit the addition of heat to the fuel tank. Another example of CDCCL might include limits on operation with certain fuel types such as JP-4. We expect all fuel tanks, even those in airplanes that do not have high flammability fuel tanks, would need to have CDCCL defined so that future modifications do not increase the flammability above the mandatory limit. The proposal applies the same requirements already applied to fuel tank ignition source prevention in § 25.981(b) to the FRM or IMM.

The proposal also includes the requirement that visible means identifying CDCCL are present. Our intent here is to prevent alterations to critical features of the system. As the visible identifications are critical to the FRM or IMM system, they are also considered to be CDCCL. Any tampering or removal would be in violation of the CDCCL. These CDCCL, inspections, or other procedures would be documented as airworthiness limitations in the ICA.

Under the proposal, all fuel tanks, regardless of flammability exposure, must be subject to airworthiness limitations consisting of CDCCL, inspections, or other procedures. The purpose of these limitations is to prevent increasing the flammability exposure of the tanks above that permitted under this section and to prevent degradation of the performance of any means installed in accordance with this section. For example, certain fuel tanks may rely on natural cooling or use of certain fuel types to meet the flammability levels within this proposal. Therefore, CDCCL may be required that define the critical features, such as—

• Flammability exposure of the unheated aluminum wing tank,

• Cooling rate,

• Limits on heat input,

• Limits on use of high volatility fuels such as JP-4,

• Quantity of engine bleed air flow that is used for inerting,

• Limits on penetrations of the fuel tank,

• Limits on any changes to fuel management that may affect FRM,

• Limits on changes to any placards or means used to visibly identify critical design features of the fuel tank system that must not be compromised for the operational life of the airplane.

As discussed above, airworthiness limitations, such as those proposed today, are part of the ICA. TC holders would need to make available to affected parties pertinent changes to the ICAs. (The term “make available” is used in the same sense that it is used in § 21.50.) We do not intend by this proposal to alter or interfere with the existing commercial relationships between TC holders and these other persons. We anticipate that TC holders would be able to be reasonably compensated for developing these documents, as they are under current practice.

The proposed rule would require creation of an Airworthiness Limitations Section (ALS), unless previously established. The ALS is required by current part 25 and includes those items that have mandatory inspection or replacement times related to fuel systems and structure. The ALS is included in the ICA, approved as part of certification, and distributed with an airplane on delivery. In this way the ALS is visible to all who need it and who would be required to comply with it under §§ 91.1509, 121.917, 125.509 and 129.117 of this proposal. The current part 25 ALS and ICA requirements apply only to airplane types for which the TC application was made after Amendment 25-54 (adopted in 1981) and were developed for structural considerations. As a result, they are not applicable to many current airplanes and do not currently contain information for other systems.

For those TC holders of airplanes that currently do not have an ALS, the intent of this proposal is to require an ALS only for fuel tank safety related limits. This proposal would not require that the ALS for these airplanes include the other requirements for an ALS established under Amendment 25-54 to part 25, or a later amendment. For those TC holders or applicants with airplanes certified to Amendment 25-54 or later, the existing ALS would be revised to include the fuel tank system airworthiness limitation items (ALI).

f. Compliance Planning

Historically, the FAA has worked together with the TC holders when safety issues arise, in order to identify solutions and actions that need to be taken. Some of the safety issues that have been addressed by this process include those involving aging aircraft structure, thrust reversers, cargo doors,

and wing icing protection. While some manufacturers have promptly addressed these safety issues and developed service instructions, others have not applied the resources necessary to develop service instructions in a timely manner. This has caused delay in the adoption of corrective action(s). A more uniform and expeditious response is necessary to address fuel tank safety issues. Because this proposal sets a precedent in introducing part 25 requirements for holders of existing TCs, changes to existing TCs, and manufacturers, it is the FAA's expectation that they will work closely with the FAA oversight office in putting together a compliance plan for developing the required FRM or IMM.

In order to provide TC holders and the FAA with assurance that the TC holders understand what means of compliance is acceptable and have taken necessary actions (including assigning sufficient resources) to achieve compliance with the proposed rule, we are proposing a compliance planning requirement. This requirement is based substantially on “The FAA and Industry Guide to Product Certification,” which describes a process for developing project-specific certification plans for type certification programs. This Guide may be found in the docket. This planning requirement would not apply to future applicants for TCs. Since this type of planning routinely occurs at the beginning of the certification process, no additional compliance planning is required for future applicants.

The Guide recognizes the importance of ongoing communication and cooperation between applicants and the FAA. The proposed planning schedule, while regulatory in nature, is intended to encourage establishment of the same type of relationship in the process of complying with this rule, if adopted.

One of the items required in the plan is, “If the proposed means of compliance differs from that described in FAA advisory material, a detailed explanation of how the proposed means will comply with this section.” FAA advisory material is never mandatory, because it describes one means, but not the only means of compliance. In the area of type certification, applicants frequently propose acceptable alternatives to the means described in advisory circulars. But when an applicant chooses to comply by an alternative means, it is important to identify this as early as possible in the certification process to provide an opportunity to resolve any issues that may arise that could lead to delays in the certification schedule.

The same is true for the fuel tank flammability reduction requirement. As discussed earlier, timely compliance with this section is necessary to enable operators to comply with the operational requirements of this proposal. Therefore, this item in the plan would enable the FAA oversight office to identify and resolve any issues that may arise with the compliance plan without jeopardizing the TC holders ability to comply with this section by the compliance time.

i. Compliance Plan for Flammability Exposure Analysis

The proposed rule would require submission of a compliance plan within 60 days of the effective date of the final rule for the flammability exposure analysis required by the proposed rule. The intent of the proposal is to promote early planning and communication between the certificate holders and the FAA. The exposure analysis would need to be completed within 150 days of the rule's effective date. Thus, the 60 day planning submission should provide sufficient time for the FAA to discuss any concerns that it may have over how the affected party intends to analyze fleet average flammability exposure.

ii. Compliance Plan for Design Changes and Service Instructions

Under today's proposal, each holder of an existing TC would need to submit to the FAA oversight office a compliance plan for developing design changes and service instructions within 210 days of the rule's effective date.

TC holders and applicants would have to correct a deficient plan, or deficiencies in implementing those plans, in a manner identified by the FAA oversight office. Deficiencies in the compliance plan would need to be corrected within 30 days of notification by the FAA. This approach differs from the original type approval process. Applicants for type certificates face commercial pressures, not regulatory deadlines, so the FAA can permit them to resolve identified deficiencies on their own schedule. Such leeway is not appropriate here because operators who are subject to regulatory deadlines are dependent on TC holders' timely compliance with these requirements. However, before the FAA formally notifies a TC holder or applicant of deficiencies, we will contact it to try to understand the deficiencies and develop a means of correcting them. Therefore, the notification referred to in this paragraph should document the agreed corrections.

The ability of an operator to comply with the proposed operating rules will be dependent on TC holders complying with the requirement to develop design changes and service instructions. The FAA intends to carefully monitor compliance and take appropriate action if necessary. Failure to comply by the dates specified in the final rule would constitute a violation of the requirements and could subject the violator to certificate action to amend, suspend, or revoke the affected certificate (49 U.S.C. 44709). It could also subject the violator to a civil penalty of not more than $25,000 per day per certificate until § 25.1815 is complied with (49 U.S.C. 46301).

iii. Compliance Plan for Auxiliary Fuel Tanks

The proposed rule would also establish a timeframe in which affected STC holders, applicants for an amended TC, and operators using fuel tanks pursuant to a field approval must submit for approval (to the FAA oversight office) a flammability exposure analysis for their design changes. The proposal includes a 12-month timeframe to complete the analysis. Any applicant whose STC or TC amendment is not approved within the 12-month compliance period would have to complete the analysis before approval.

The proposed rule would also require submission for approval of an impact assessment of the fuel tank system, as modified by the STC holder's design change. The purpose of this proposal is to identify any features of the modification to the original type design that may violate the critical design configuration control limitations developed by the original TC holder. For example, if an FRM that utilized inerting is incorporated into an airplane, a CDCCL would likely be developed that would limit venting of air into the fuel tank, because it could introduce oxygen into the tank, resulting in a flammable vapor space. In this case the STC holder would need to assess its design and identify any violation of the CDCCL identified for the FRM. Results from the analysis would be provided to the FAA in the form of a report or summary letter.

Supplemental type certificate holders would have to submit the impact assessment within six months after we approve the TC holder's CDCCL. Applicants whose design changes are not approved within that six-month period would have to submit the assessment before approval of the change. Once the CDCCL is approved, the TC holder would be required to make them available to other affected persons, including those subject to this

section. We consider the six-month period more than enough to perform the required assessment. The resulting service instructions would be required to show compliance with the applicable flammability requirements and to address any adverse effects of the design change on any IMM or FRM developed by the TC holder.

g. Compliance Schedule

Table 2 contains compliance dates for the required submissions. This table provides specific dates for each Boeing and Airbus model airplane that has fuel tanks whose average flammability exposure exceeds 7 percent. A compliance time of 24 months from the effective date of the final rule is proposed for all other models subject to this proposal (if the flammability exposure analysis shows an average exposure level exceeding 7 percent). We established the compliance dates proposed in this table after consideration of the time needed by the TC holders to develop the means to address fuel tank flammability exposure. We anticipate development of an FRM or IMM would take the affected TC holder about 2 years. The dates in the proposal were based on the assumption that it would be adopted well before the end of 2005. However, the rulemaking process took longer than originally anticipated. Consequently, given the specific compliance dates I the proposed rulemaking and the likelihood that finalization of the rules will be later than expected, there may not be as much time allowed for compliance as originally planned. We recognize that compliance intervals may need to be adjusted and will consider your comments on this condition.

On February 17, 2004, the FAA Administrator announced that the agency is developing a proposal for new rules that would require reducing the flammability exposure of new production transport category airplanes and existing transport category airplanes with high-flammability fuel tanks. Since then, Boeing has announced plans to incorporate FRM in newly produced airplanes and to make service instructions available for the airplane models listed in this notice. Boeing has also submitted applications for type certification of flammability reduction systems. On February 15, 2005, we published a Special Conditions No. 25-285-SC for flammability reduction means on the Boeing Model 747 (70 FR 780068563). Airbus flew an A320

16

in August 2003 with the prototype FAA inerting system, but has not committed to production incorporation or development of service instructions for any flammability reduction means on its airplane models.

16

Flight-Testing of the FAA Onboard Inert Gas Generation System on an Airbus A320

, DOT/FAA/AR-03/58, dated June 2004.

While Airbus and Boeing may have less than 2 years from the effective date of the final rule to develop an FRM or IMM for some of their models, we know that both companies have been considering these improvements well in advance of this rulemaking. The proposed compliance dates are thus staggered to allow the engineering resources of the TC holders to develop design means for all of their models. The proposed dates are established based on both our assessment of when it is feasible for TC holders to comply and the risks associated with particular airplane models, due to the flammability of the fuel tanks and numbers of airplanes in the fleet. For example, the Boeing Model 747 is first, followed by the Boeing Model 737. The first Airbus model affected is the A320. The proposed dates will support the retrofit of airplanes at the earliest reasonable time to achieve the safety benefits intended by this rulemaking.

The compliance times proposed for airplane and fuel tank manufacturers are also used as the basis for the proposed compliance dates for introduction of these systems into the operators' fleets under parts 91, 121, 125, and 129. Extension of the compliance dates for development of the service instructions by the certificate holders would either reduce the amount of time available to operators or delay full deployment of these safety improvements. As discussed later in this proposal for the operational requirements, incorporation of FRM or IMM will likely require access inside the fuel tanks.

Table 2

Model

Service instruction submittal date

Boeing

747 Series

December 31, 2005.

737 Series

March 31, 2006.

777 Series

March 31, 2006.

767 Series

September 30, 2006.

757 Series

March 31, 2007.

707/720 Series

December 31, 2007.

Airbus

A319, A320, A321 Series

December 31, 2006.

A300, A321 Series

June 30, 2007.

A330, A340 Series

December 31, 2007.

All other affected models

Within 24 months of effective date of this amendment.

E. Proposed Requirements for Airplane Operators

The proposed operating rules would prohibit the operation of certain transport category airplanes operated under parts 91, 121, 125, and 129 beyond specified compliance dates, unless the operator of those airplanes has incorporated approved IMM, FRM or FIMM modifications and associated airworthiness limitations for the affected fuel tanks. The proposed rules would not apply to airplanes used only in all-cargo operations.

This rulemaking also includes a proposal to create new subparts that pertain to the support of continued airworthiness and safety improvements in the following parts of Title 14 Code of Federal Regulations:

• Part 91, General Operating and Flight Rules;

• Part 121, Operating Requirements: Domestic Flag and Supplemental Operation;

• Part 125, Certification and Operation: Airplanes Having a Seating Capacity of 20 or More Passengers or a Maximum Payload Capacity of 6,000 Pounds or More; and Rules Governing Persons On Board Such Aircraft; and

• Part 129, Operations: Foreign Air Carriers and Foreign Operators of U.S.-registered Aircraft Engaged in Common Carriage.

As discussed earlier, this proposal does not include part 135, since the number of airplanes in part 135 operation that would be affected by these proposals is relatively small. In the event changes to part 135 result in a greater number of affected airplanes operating under that part, the FAA will reassess the need to apply these proposed requirements to that part.

The FAA believes that inclusion of certain rules under the new subparts will enhance the reader's ability to readily identify rules pertinent to continued airworthiness. Unless stated otherwise, our purpose in moving requirements to the new subparts is to ensure easy visibility of those requirements applicable to the continued airworthiness of the airplane. We do not intend to change their legal effect in any other way. The new subparts are substantially the same and accordingly are not discussed separately here. Table 3 illustrates what proposed and existing requirements will be included in the new subparts. Each new subpart is titled “Continued Airworthiness and Safety Improvements.” The proposed new subparts consist of relocated, revised, and new regulations pertaining to continued airworthiness of the airplane.

Table 3.—New Subparts for Parts 91, 121, 125, and 129

Part 91 new/relocated rules within proposed subpart K

Part 121 new/relocated rules

within proposed subpart Y

Part 125 new/relocated rules

within proposed subpart M

Part 129 new/relocated rules

within proposed subpart B

§ 91.1501, Applicability (new)

§ 121.901, Applicability

§ 125.501, Applicability

§ 129.101, Applicability.

§ 91.1503, Reserved

§ 121.903, Reserved

§ 125.503, Reserved

§ 129.103, Reserved.

§ 91.1505, fuel tank system maintenance program

§ 121.905, Electrical wiring interconnection systems (EWIS) maintenance program

§ 125.505, Fuel tank system inspection program

§ 129.105, Electrical wiring interconnection systems (EWIS) maintenance program.

§ 91.1507, Repairs assessment for pressurized fuselages (formerly § 91.401(a))

§ 121.907, Fuel tank system maintenance program

§ 125.507, Repairs assessment for pressurized fuselages (formerly § 125.248(a))

§ 129.107, Fuel tank system maintenance program.

§ 91.1509, Reserved

§ 121.909, Reserved

§ 125.509, Reserved

§ 129.109, Reserved.

§ 91.1511, Reserved

§ 121.911, Reserved

§ 125.511, Reserved

§ 129.111, Reserved.

§ 121.913, Aging airplane inspections and records reviews (formerly § 121.368)

§ 129.113, Supplemental inspections for U.S.-registered aircraft (formerly § 129.16).

§ 121.915, Repairs assessment for pressurized fuselages (formerly § 121.370(a))

§ 129.115, Repairs assessment for pressurized fuselages (formerly § 129.32(a)).

§ 91.1513, Reserved

§ 121.917, Supplemental inspections (formerly § 121.370(a)

§ 129.117, Aging airplane inspections and records reviews for U.S.-registered aircraft (formerly § 129.33).

1. Requirement to Install and Operate FRM, IMM or FIMM

The proposed rules would prohibit certificate holders from operating any affected airplane after dates specified, unless IMM, FRM or FIMM, as applicable, are installed and operational for any fuel tank for which they are required. The safety objective of these proposed rules is to have the required modifications installed and operational at the earliest opportunity.

The proposed rule would require that operators of the affected airplanes incorporate applicable maintenance program changes before returning an airplane to service after accomplishing any required modifications.

For some of the affected airplanes, manufacturer compliance with the proposed requirements may not result in any design changes, but would result in development of airworthiness limitations in the form of maintenance actions, operational procedures, or CDCCL, as previously discussed. In these cases the affected operators would be required to incorporate these limitations within one year after their approval by the FAA oversight office. The FAA will inform the affected operators and principal inspectors of the availability of the approved information.

Once an operator revises its maintenance or inspection program, it is important to make sure that later alterations to the airplane do not degrade the level of safety provided by these revisions. The proposed rules would require future applicants for approval of design changes to develop new airworthiness limitations for new auxiliary fuel tanks and other design changes affecting fuel tank flammability. To ensure that these airworthiness limitations are implemented, operators who incorporate these design changes into their airplanes would be required to revise their maintenance and inspection programs to incorporate the corresponding airworthiness limitations.

Today's proposal would require operators to submit the proposed maintenance and inspection program changes to their FAA Principal Inspector for review and approval.

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This review would include the integration of the applicable airworthiness limitations for the TC and any STC and field approved auxiliary fuel tank to ensure their consistency and compatibility in the maintenance or inspection program. Guidance will be provided to operators and principal inspectors regarding how to address any deviations that may be proposed by the affected operators from the information approved by the FAA oversight office. As airworthiness limitations, these cannot be changed without FAA approval, nor are they subject to maintenance review board or other maintenance program development processes.

17

A part 91 operator would send the relevant information to either their principal inspector or Flight Standards District Office, as applicable.

2. Authority To Operate With an Inoperative FRM, IMM or FIMM

Generally, the FAA does not require operators to use or maintain equipment installed on airplanes prior to a uniform compliance date. In this proposal, we take a different approach. The safety advantages associated with a fuel tank system equipped with an FRM or IMM design, as modified by any FIMM, are so compelling that we propose requiring that operators use these systems as soon as they are available. We have accommodated the difficulties faced by operators in making the required design changes by providing a phased-in compliance schedule that extends up to seven years after the manufacturer's compliance date for each model. Accordingly, an operator may not operate any airplane with fuel tanks equipped with FRM, IMM or FIMM, unless those systems are fully operational. The sole exception is when the systems are inoperative and the conditions and limitations specified in the operator's Minimum Equipment List (MEL) are met.

The method used to allow operation of an airplane when an FRM is inoperative would be to include the FRM dispatch relief in the FAA-approved MEL. The MEL contains a list of equipment that may be inoperative for a defined period of time. Under § 91.213 and similar regulations, the airplane may be dispatched with inoperative equipment in accordance with the Master Minimum Equipment List (MMEL).

The FAA Flight Operations Evaluation Board (FOEB) would establish the MMEL dispatch relief interval for an FRM based on data submitted by the applicant to the FAA. The expected MMEL dispatch relief interval is one of the contributing factors affecting the overall system reliability analyses that must be established early in the design of the FRM. The proposed requirements of Appendix K allow the designer to choose to design a highly reliable FRM and then request longer MMEL dispatch relief intervals when submitting their data to the FOEB.

This proposal does not recommend the adoption of a specific MMEL dispatch inoperative interval at this time. However, the comments received from the NTSB on to the proposed special conditions for the Boeing 747 indicate that the FRM should be treated like other non-redundant safety equipment, such as the flight data recorder. The recorders are allowed a 3-day MMEL inoperative interval. We specifically request public comment on the proposal to allow the current FOEB process to establish the MMEL interval rather than requiring a specific maximum interval.

3. Compliance Schedule

To achieve the safety benefits of this initiative, we believe it is necessary to have a mandatory schedule for phasing in the design changes rather than to rely solely on market forces to drive the production and availability of parts and normal maintenance scheduling for the installation of the FRM, IMM, or FIMM. Accordingly, this rule, if adopted, would require at least 50 percent of the affected airplanes be outfitted within four years after the relevant TC holder is required to comply with the proposed requirements. The remainder of the operator's fleet would have to comply with the final rule within seven years after the specified date. The affected fleet would include those airplanes that have field or STC approved auxiliary fuel tanks. Certificate holders that operate only one airplane of an affected model would have to modify that airplane within the seven-year compliance period.

The proposed compliance schedule of 7 years after TC holders to develop service instructions, while long, should allow for the approval of the service instructions for IMM, FRM, or FIMM, manufacture of modification parts for a large fleet of airplanes, and accomplishment of the modifications with minimum disruption of normal maintenance schedules. Typically, fuel tanks are only accessed during heavy maintenance checks that are done on a schedule that is established during development of the maintenance program. The compliance dates proposed for the operational rules were developed to allow for the majority of the modifications to be done during these heavy maintenance checks. Introduction of FRM, IMM or FIMM outside of normally scheduled maintenance would increase the cost to the operators, because extra tank entry and airplane down time would be needed.

Some airplane types or specific airplanes within an operator's fleet may not be scheduled for normally scheduled maintenance, where the fuel tanks would be opened, during the 7-year compliance time after service instructions become available. These airplanes would require incorporation of modifications outside of the normally scheduled maintenance. We have determined the number of airplanes that would be affected is small and that further lengthening the compliance period would not achieve the safety benefits of this proposal in a timely way. Also, we anticipate that some of the upcoming ADs to address ignition source issues will occur in this time period and in some cases will require fuel tank entry. Compliance with the AD may provide additional opportunities for incorporating approved FRM, IMM or FIMM if not occurring during normal scheduled maintenance. These issues are further discussed in the regulatory evaluation.

F. Additional Provisions

1. Relationship of This Proposal to Aging Airplane Regulatory Initiatives

As part of our broader review of several important initiatives comprising the Aging Airplane Program, we have revised certain compliance dates in existing rules and pending proposals so that operators can make required modifications during scheduled maintenance. Changing compliance dates affects our ability to expedite some aspects of this program but reduces the costs of the rules and proposals in place to deal with aging airplanes. Notice of these changes and a description of our Aging Airplane Program review appeared in the

Federal Register

on July 30, 2004 (69 FR 45936). In addition to this Fuel Tank Flammability Reduction proposal, the actions affected by these revisions include:

• Aging Aircraft Program (Widespread Fatigue Damage (proposal),

• Aging Airplane Safety (interim final rule), and

• Enhanced Airworthiness Program for Airplane Systems/Fuel Tank Safety (proposal).

Today's proposal, if adopted, would also affect compliance with SFAR 88 and potentially make it less costly. The safety reviews following the TWA 800 accident led us to require that the fuel quantity indication system wiring entering high flammability tanks incorporate either adequate separation or energy limiting devices, known as transient suppression devices, on the Boeing 737 and 747 to protect the tank from ignition sources. As part of the safety reviews of SFAR 88, we have identified other models that likewise would need a transient suppression device. We have determined that if FRM are incorporated in high flammability fuel tanks, ADs requiring installation of devices to protect the fuel quantity system wiring will no longer be needed. We have not yet estimated the potential savings and have not included these savings in the current regulatory evaluation. We specifically request comments regarding the savings that would be achieved if electrical energy limiting devices were not required on wiring entering high flammability fuel tanks affected by this proposal.

2. FAA Advisory Material

We are developing extensive guidance material to supplement the proposed rule, including a revised AC 25.981-2 to include guidelines on conducting a fuel tank flammability exposure assessment using the Monte Carlo methodology and developing IMM and FRM. It will also include guidance on development of the airworthiness limitations section, confined space hazards and markings, documentation required by the FAA, and reporting methods. We have incorporated some comments on these topics from a group of specialists at the Aerospace Industries Association, which included airplane manufacturers, airline operators and manufacturers of inert gas generating equipment.

18

The group provided advice on fuel tank inerting and use of the Monte Carlo methodology. We will invite public comments on the proposed ACs (which references the Monte Carlo User's Manual) by separate notice published in the issue of the

Federal Register

.

18

A copy of the AIA report is included in the docket for this rulemaking.

3. FAA Oversight Office

We are also requiring affected persons to submit various compliance materials to the FAA Oversight Office, defined in proposed § 25.1803(c). The FAA Oversight Office is the aircraft

certification office or office within the Transport Airplane Directorate having oversight responsibility for the relevant TC or STC, as delegated by the Administrator. For example, with respect to fuel-tank flammability issues, TC and STC holders must obtain approvals from the responsible office in the FAA's Aircraft Certification Service. In other contexts, we have described the FAA office performing these functions as the “cognizant FAA office.”

Table 4 lists the FAA offices that currently oversee issuance of TCs and amended TCs for manufacturers of large transport category airplanes.

Table 4.—FAA Offices That Oversee Type Certificates

Airplane manufacturer

FAA Oversight Office

Aerospatiale

Transport Airplane Directorate, International Branch.

Airbus

Transport Airplane Directorate, International Branch.

BAE

Transport Airplane Directorate, International Branch.

Boeing

Seattle Aircraft Certification Office.

Bombardier

New York Aircraft Certification Office.

Embraer

Transport Airplane Directorate, International Branch.

Fokker

Transport Airplane Directorate, International Branch.

Gulfstream

Atlanta Aircraft Certification Office.

Lockheed

Atlanta Aircraft Certification Office.

Boeing/McDonnell-Douglas Corp

Los Angeles Aircraft Certification Office.

4. Workplace Safety Issues

Because we would require that maintenance personnel be given access to FRM installations, the proposal could increase occupational safety risks for these personnel. A large percentage of the work involved in properly inspecting and modifying airplane fuel tanks and their associated systems must be done in the interior of the tanks. Performing the necessary tasks requires inspection and maintenance personnel to physically enter the tank, where environmental hazards exist. These hazards exist in any fuel tank (regardless of whether a nitrogen inerting system is installed) and include fire and explosion, toxic and irritating chemicals, oxygen deficiency, and the confinement to the fuel tank itself. To prevent related injuries, operator and repair station maintenance organizations have developed specific procedures for identifying, controlling, or eliminating the hazards of fuel-tank entry. In addition, government agencies have adopted safety requirements for use when entering fuel tanks and other confined spaces. These same procedures would be applied to the reduced oxygen environment likely to be present in an inerted fuel tank.

Introduction of nitrogen enriched air within the fuel tanks and the possibility of nitrogen enriched air in compartments adjacent to the fuel tanks if leakage occurs creates additional risk. Lack of oxygen in these areas could be hazardous to maintenance personnel, the passengers, or flight crew. Existing certification requirements address these hazards. This proposal requires markings to emphasize the potential hazards associated with confined spaces and areas where a hazardous atmosphere could be present as a result specifically of the addition of FRM. We would require that the access doors and panels to the fuel tanks with FRMs and to any other enclosed areas that could contain hazardous atmosphere under either normal conditions or failure conditions be permanently stenciled, marked, or placarded to warn of hazards.

Fuel tanks are confined spaces

19

and contain high concentrations of fuel vapors that must be exhausted from the fuel tank before entry. Other precautions such as measurement of oxygen concentrations before entering a fuel tank are already required. Addition of the FRM that utilizes inerting may result in reduced oxygen concentrations due to leakage of the system in locations in the airplane where service personnel would not expect it. These gases may be under pressure because of the FRM design, and any hazards associated with working in adjacent spaces near the opening should be identified in the marking of the opening to the confined space.

19

Our worker safety requirements apply to confined spaces, which are partly or fully enclosed areas big enough for a worker to enter and perform assigned work and with limited or restricted means of entry or exit. Such areas are not designed for someone to work in regularly but for tasks such as inspection, cleaning, maintenance, and repair. (Reference U.S. Department of Labor Occupational Safety & Health Administration (OSHA), 29 CFR § 1910.146(b).) This proposal would not significantly change the procedures used by maintenance personnel to enter fuel tanks and is not intended to conflict with existing government agency requirements (e.g., OSHA).

Designs currently under consideration locate the FRM in the fairing below the center wing fuel tank. Access to these areas is obtained by opening doors or removing panels, which could allow some ventilation of the spaces adjacent to the FRM. But this may not be enough to avoid creating a hazard. Therefore, unless the design eliminates this hazard, we intend that marking be provided to warn service personnel of possible hazards associated with the reduced oxygen concentrations in the areas adjacent to the FRM. Appropriate markings would be required for all inerted fuel tanks, tanks adjacent to inerted fuel tanks and all fuel tanks communicating with the inerted tanks via plumbing. The plumbing includes, but is not limited to, plumbing for the vent system, fuel feed system, refuel system, transfer system and cross-feed system. The markings should also be stenciled on the external upper and lower surfaces of the inerted tank adjacent to any openings, to ensure maintenance personnel understand the possible contents of the fuel tank.

Advisory Circular 25.981-2 will provide additional guidance regarding markings and placards.

IV. Rulemaking Analyses and Notices

Authority for This Rulemaking

The FAA's authority to issue rules regarding aviation safety is found in Title 49 of the United States Code. Subtitle I, Section 106, describes the authority of the FAA Administrator. Subtitle VII, Aviation Programs, describes in more detail the scope of the agency's authority. This rulemaking is promulgated under the authority described in Subtitle VII, Part A, Subpart III, Section 44701, “General requirements.” Under that section, the FAA is charged with promoting safe flight of civil aircraft in air commerce by prescribing.

• Minimum standards required in the interest of safety for the design and performance of aircraft;

• Regulations and minimum standards in the interest of safety for inspecting, servicing, and overhauling aircraft; and

• Regulations for other practices, methods, and procedures the Administrator finds necessary for safety in air commerce.

This regulation is within the scope of that authority because it prescribes—

• New safety standards for the design of transport category airplanes, and

• New requirements necessary for safety for the design, production, operation, and maintenance of those airplanes, and for other practices, methods and procedures relating to those airplanes.

Paperwork Reduction Act

This proposal contains the following new information collection requirements. As required by the Paperwork Reduction Act of 1955 (44 U.S.C. 3507(d)), the Department of Transportation has sent the information requirements associated with this proposal to the Office of Management and Budget for its review.

Title:

Transport Category Airplane Fuel Tank Flammability Reduction Safety Improvements.

Summary:

This proposal would require certain certificate holders to develop means to reduce the flammability of high flammability exposure fuel tanks on certain large turbine-powered transport category airplanes. In addition, this proposal requires operators of the affected airplanes with high flammability exposure fuel tanks to incorporate FRM. The current requirements for fuel tank flammability exposure for new designs would be revised to add requirements for inerting systems if inerting is used to minimize flammability exposure. This proposal also proposes to expand the coverage of part 25 to include requirements that must be complied with by current holders of these certificates. Certificate holders would be required to provide a quarterly report to the FAA that contains reliability data for the FRM. There is no specific reporting requirement for operators. Data collected by the certificate holders from operators would be obtained through normal business agreements.

Proposed subpart I would also require that TC holders submit to the FAA a plan detailing how they intend to comply with its requirements. This information would be used by the FAA to assist the TC holder in complying with requirements. The compliance plan would be necessary to ensure that TC holders fully understand the requirements, correct any deficiencies in planning in a timely manner, and are able to provide the information needed by the operators for the operators' timely compliance with the rule.

Reporting:

When scheduled or unscheduled maintenance and inspections are performed, including tasks that are not identified as ALI or Certification Maintenance Requirements, the operators are only required to report specific discrepancies and corrective actions in accordance with § 121.703. This proposal would not mandate any additional reporting above the current requirements for ALI by the operators. We do not intend that operators report to the FAA the results of routine inerting system operational checks, or discrepancies found .

The proposed reporting requirement applies to applicants and holders of the affected certificates. There is no proposed additional requirement within this rulemaking for operators to report FRM reliability information. We intend for certificate holders to gather the needed data from operators using existing reporting systems that are currently used for airplane maintenance, reliability and warranty claims. The operators would provide this information through existing or new business arrangements between the certificate holders and the airlines.

Use of:

This proposal would support the information needs of the FAA in approving design approval holder and operator compliance with the proposed rule.

Respondents (including number of): The likely respondents to this proposed information requirement are the affected type certificate holders such as Boeing, Airbus and several auxiliary fuel tank manufacturers.

Frequency:

The proposal would require the certificate holders to submit a report to the FAA once each quarter for a period up to 5 years.

Average Annual Burden Estimate:

The burden would consist of the work necessary to:

• Develop the design and the data for STCs to install fuel tank inerting systems,

• Develop and incorporate a maintenance plan into the existing maintenance programs,

• Record the results of the installation and maintenance activities.

The largest paperwork burden would be a one-time effort (spread over 3 years) associated with the STC applications. This one-time total burden would be as follows:

Documents required to show compliance with the proposed rule

One-time pages

Present value discounted cost (in millions of $2005)

Specifications for Fuel Tank STC

8,000

2.7

Manuals (Flight Manuals, Operations, and Maintenance) for Fuel Tank STC

12,000

2.7

Production for Fuel Tank STC

500

0.4

Documentation for FAA/EASA Certification

1,000

13.4

Total

21,500

19.2

The yearly burden for each of the 3 years would have a present value of about $6.4 million and involve 7,167 pages.

This proposed rulemaking would result in a minimal annual recordkeeping and reporting burden. All records that would be generated to verify the installation, to record any fuel tank system inerting failures, and to record any maintenance would use forms currently required by the FAA.

The FAA computed the annual recordkeeping (Total Pages) burden by analyzing the necessary paperwork requirements needed to satisfy each process of the proposed rulemaking.

The agency is seeking comments to—

• Evaluate whether the proposed information requirement is necessary for the proper performance of the roles of the agency, including whether the information will have practical utility;

• Evaluate the accuracy of the agency's estimate of the burden;

• Improve the quality, utility, and clarity of the information to be collected; and

• Minimize the burden of the collection of information on those who are to respond using appropriate automated, electronic, mechanical, or

other technological collection techniques or other forms of information technology.

Individuals and organizations may submit comments to the FAA on the information collection requirement by February 21, 2006. You should send your comments to the address listed in the

ADDRESSES

section of this document.

Under the Paperwork Reduction Act of 1995, (5 CFR 1320.8(b)(2)(vi)), an agency may not conduct or sponsor, and a person is not required to respond to, a collection of information unless it displays a currently valid OMB control number. The OMB control number for this information collection will be published in the

Federal Register

, after the Office of Management and Budget approves it.

International Compatibility

In keeping with U.S. obligations under the Convention on International Civil Aviation, it is FAA policy to comply with International Civil Aviation Organization (ICAO) Standards and Recommended Practices to the maximum extent practicable. The FAA has determined that there are no ICAO Standards and Recommended Practices that correspond to these proposed regulations.

Regulatory Evaluation, Regulatory Flexibility Determination, International Trade Assessment, and Unfunded Mandates Assessment

Regulatory Evaluation

This portion of the preamble summarizes our analysis of the economic impacts of this NPRM. It also includes summaries of the initial regulatory flexibility determination. We suggest readers seeking greater detail read the full regulatory evaluation, a copy of which we have placed in the docket for this rulemaking.

Changes to Federal regulations must undergo several economic analyses. First, Executive Order 12866 directs that each Federal agency shall propose or adopt a regulation only upon a reasoned determination that the benefits of the intended regulation justify its costs. Second, the Regulatory Flexibility Act of 1980 requires agencies to analyze the economic impact of regulatory changes on small entities. Third, the Trade Agreements Act (19 U.S.C. 2531-2533) prohibits agencies from setting standards that create unnecessary obstacles to the foreign commerce of the United States. In developing U.S. standards, this Trade Act requires agencies to consider international standards and, where appropriate, to be the basis of U.S. standards. Fourth, the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-4) requires agencies to prepare a written assessment of the costs, benefits, and other effects of proposed or final rules that include a Federal mandate likely to result in the expenditure by State, local, or tribal governments, in the aggregate, or by the private sector, of $100 million or more annually (adjusted for inflation).

In conducting these analyses, we determined this rule: (1) Is a “significant regulatory action” as defined in section 3(f) of Executive Order 12866, and is “significant” as defined in DOT's Regulatory Policies and Procedures; (2) would have a significant economic impact on a substantial number of small entities; (3) has a neutral international trade impact; and (4) does not impose an unfunded mandate on state, local, or tribal governments, or on the private sector. These analyses, available in the docket, are summarized as follows.

Total Benefits and Costs of This Rulemaking

We estimated that the proposed rule would prevent an expected 4 catastrophic passenger accidents over the analysis period. If all accidents ha

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