Fatigue Evaluation of Structure

SUMMARY: This action amends the fatigue requirements for damage-

tolerant structure on transport category airplanes to require a

demonstration using sufficient full-scale fatigue test evidence that

widespread multiple-site damage will not occur within the design

service goal of the airplane; and inspection thresholds for certain

types of structure based on crack growth from likely initial defects.

This change is needed to ensure the continued airworthiness of

structures designed to the current damage tolerance requirements, and

to ensure that should serious fatigue damage occur within the design

service goal of the airplane, the remaining structure can withstand

loads that are likely to occur, without failure, until the damage is

detected and repaired.

EFFECTIVE DATE: April 30, 1998.

FOR FURTHER INFORMATION CONTACT:

Richard Yarges, FAA, Airframe and Airworthiness Branch (ANM-115),

Transport Airplane Directorate, Aircraft Certification Service, 1601

Lind Avenue SW., Renton Washington 98055-4056; telephone (425) 227-

2143, facsimile (425) 227-1320.

SUPPLEMENTARY INFORMATION:

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5948).

Internet users may access the FAA web page at http://www.faa.gov or

the Federal Register's web page at http://www.access.gop.gov/su___docs

to download recently published rulemaking documents.

Any person may obtain a copy of this final rule 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. Communications must reference the amendment or docket

number of this final rule.

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Distribution System, which describes the application procedure.

Small Entity Inquiries

The Small Business Regulatory Enforcement Fairness Act of 1996

(SBREFA) requires the FAA to report inquiries from small entities

concerning information on, and advice about, compliance with statutes

and regulations within the FAA's jurisdiction, including interpretation

and application of the law to specific sets of facts supplied by a

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If you are a small entity and have a question, contact your local

FAA official. If you do not know how to contact your local FAA

official, you may contact Charlene Brown, Program Analyst Staff, Office

of rulemaking, ARM-27, Federal Aviation Administration, 800

Independence Avenue, SW, Washington, DC 20591, 1-888-551-1594. Internet

users can find additional information on SBREFA in the ``Quick Jump''

section of the FAA's web page at http://www.faa.gov and may send

electronic inquiries to the following Internet address: 9-AWA-

[email protected]

Background

This amendment is based on Notice of Proposed Rulemaking (NPRM) 93-

9, which was published in the Federal Register on July 19, 1993 (58 FR

38642). The notice was issued because of the need: (1) To ensure that

widespread, multiple site fatigue cracking will not occur during the

period of service for which the airplane is designed to operate; and

(2) to prescribe criteria for establishing the thresholds for damage-

tolerance based inspections.

In addition three minor changes requested by both U.S. and European

manufacturers of transport category airplanes, aimed at harmonizing the

U.S. and European certification requirements, were also proposed in

this notice.

Section 25.571 of 14 CFR part 25 requires that applicants for an

airplane type certificate address the technical issue of structural

fatigue (other than sonic fatigue) in one of two ways: (1) A damage-

tolerance evaluation of the structure; or (2) a safe-life fatigue

evaluation of the structure.

Of the two methods of evaluation, the first is preferred and the

second may only be used if the applicant establishes that it is

impractical to use a damage-tolerance approach. Even so, several in-

service incidents and accidents resulting from structural fatigue

failures have demonstrated the need to improve the damage-tolerance

evaluation requirements of part 25.

A damage-tolerance evaluation consists of engineering calculations

and tests aimed at establishing what kind of inspections are needed,

and how often they need to be repeated on an airplane's structure while

in service. The inspection frequency is set to assure that, should

serious fatigue damage begin to develop before the design service goal

of the airplane is reached, it will be found and repaired before it

grows to proportions that represent a hazard to the airplane.

This methodology has proven to be successful in many applications

and, in fact, is part of the reason for the excellent overall safety

record that has been achieved in the U.S. Nevertheless, there are two

issues that have been debated within the technical community that are

not clearly dealt with by the damage-tolerance methodology:

1. When in an airplane's life should the first inspection in the

inspection cycle be conducted (the threshold inspection)?

2. When in an airplane's life can safety no longer be effectively

maintained by the damage tolerance inspection program prescribed at the

time of certification of the airplane type (the onset of widespread

cracking)?

These are complex issues that are discussed at some length in

Notice 93-9. This rulemaking attempts to incorporate into part 25 some

technical judgments on these issues that offer a high degree of safety

to the flying public, without overburdening the air transportation

system with unnecessary inspections or tests. To this end, the FAA

proposed that Sec. 25.571 of the FAR, ``Damage-tolerance and fatigue

evaluation of structure,'' be revised:

1. To require sufficient full-scale fatigue testing to ensure that

widespread, multiple-site fatigue damage does not occur within the

design service goal of the airplane; and

2. To require that thresholds for inspections be based on anaylses

and tests considering the damage-tolerance concept, manufacturing

quality, and susceptibility to in-service damage. (The idea of basing

the time of the threshold inspection on the time it takes a crack to

grow from a manufacturing defect that is likely to escape manufacturing

[[Page 15709]]

quality control inspection to the time the crack represents a hazard to

the airplane is known as the ``rogue flaw'' concept for establishing

inspection thresholds.)

A revision to companion draft Advisory Circular (AC) 25.571-1A was

prepared for the proposed rulemaking, to provide guidance on means that

the FAA would accept as showing compliance with the regulation. As with

all advisory circulars, this draft was intended only to provide

guidance on acceptable means of compliance, without eliminating the

flexibility for future applicants to identify other means of compliance

with the proposed rule. That draft revision (AC 25.571-1X) was not

available at the time that Notice 93-9 was issued and was subsequently

made available to the public for comment on October 19, 1993 (58 FR

53987). As a result, the FAA has received two sets of comments from the

public, one in response to the draft AC and one in response to the

proposed rule. Some of the earlier comments were made without the

benefit of the commenter knowing the contents of the draft AC. Because

of this, the FAA has considered both sets of comments in preparing the

final rule contained herein, and in revising the AC. The announcement

of the FAA's issuance of the revised AC will be published in the

Federal Register once it is available to the public.

Interested persons have been given an opportunity to participate in

this rulemaking, and due consideration has been given to all matters

presented. Comments received in response to Notice 93-9 are discussed

below.

Discussion of Comments

The FAA received many comments in response to Notice 93-9, most of

which state support for the added requirement for full-scale fatigue

testing of new airplane types. Commenters included airplane

manufacturers, the National Transportation Safety Board, the Airline

Pilots Association, the Aerospace Industries Association, the General

Aviation Manufacturers Association, airplane operators, and others.

Only a few commenters state that full-scale fatigue testing should not

be required.

One commenter states that full-scale fatigue tests should not be

mandated because these tests do not adequately account for actual

conditions experienced in service and therefore cannot accurately

predict in-service problems. The commenter further states that such

tests have never predicted widespread fatigue damage that later became

a problem in the fleet. The FAA does not concur with this comment. It

is widely recognized in the aviation engineering community that

``scatter factors'' need to be applied to fatigue test results, because

such tests cannot account for the individual construction variations

and the individual service experience of each airplane. Nevertheless,

important results have been, and will continue to be, obtained from

such tests, including the prediction of widespread fatigue damage. The

FAA, airplane manufacturers, and others have come to recognize that

full-scale fatigue testing provides an indispensable, although

admittedly incomplete, source of information about what to expect in

service from airframe structures. As was pointed out by another

commenter who favors the new requirement, full-scale fatigue test

evidence must be coupled with prudent exploratory fleet inspections to

ensure continued airworthiness.

The FAA received several comments about the full-scale fatigue

testing of derivative or modified type designs. These commenters point

out that full-scale fatigue test data generated during the original

certification of an airplane type, and other data, can sometimes be

used to determine when widespread multiple-site fatigue damage will, or

will not, occur on the modified designs. These commenters state that

additional full-scale fatigue testing would not be necessary in all

cases. The FAA concurs with these comments. The working of

Sec. 25.571(b) in the final rule has been changed along the lines of

one comment that had been jointly developed by the Aerospace Industries

Association, the Association Europeenne des Constructeurs de Materiel

Aerospatial, and the FAA's Technical Oversight Group for Aging

Airplanes. This change uses the words ``sufficient full-scale fatigue

test evidence'' in place of ``sufficient full-scale testing.''

The same commenters also state that guidance should be provided in

the form of an advisory circular (AC) on the subject of when and how

much fatigue testing would be necessary for modification and derivative

certification programs. The FAA concurs. In fact, draft AC 25.571-1X

does contain some guidance. Based on comments provided to the docket

for this rulemaking and in response to the draft AC, the FAA has

revised and expanded the guidance regarding the relevant factors in

determining whether, and to what extent, fatigue testing may be

necessary for derivatives and modifications of type designs. Generally,

these factors relate to the applicability and reliability of previously

developed test evidence for determining that the airplane will remain

free of widespread fatigue damage until its design service goal is

reached.

Another commenter points out that two lifetimes of fatigue testing

cannot ``ensure'' that widespread multiple-site damage will not occur

within the design lifetime of an airplane (since no fatigue test can

duplicate the exact configuration and operating history of each

airplane). The commenter states that the requirement of the rule should

be to ensure hat widespread multiple-site damage will not ``normally''

occur. The FAA agrees that two lifetimes of fatigue testing cannot

ensure that widespread fatigue damage will not occur within the design

lifetime of an airplane; however, guidance on this statistical fact is

best addressed in the AC. Therefore, as a result of this comment, the

FAA has revised the AC in this regard.

The FAA also received comments that full-scale fatigue testing

represents a prohibitive expense for small entities that perform

modifications of type designs produced by others and would put them out

of business. These commenters note that the FAA has certificated

airplane modifications for damage tolerance in the past, relying on

analytical methods that are based upon test data and using conservative

assumptions, but without full-scale fatigue testing. They state that

they are small entities that the FAA did not consider.

As discussed previously, the objective of this rulemaking is to

ensure that transport category airplanes will remain free of widespread

fatigue damage within their design service goals. For reasons discussed

below, the FAA considers that most modifications can be found to meet

this objective without additional full-scale testing. However, it is

true that in some cases involving extensive structural modification

(such as a cargo conversion project) it may be necessary for the FAA to

require a modifier to conduct full-scale fatigue testing to demonstrate

freedom from widespread fatigue damage within the design service goal

of an airplane type. The FAA acknowledges that such testing may be

expensive. In these cases, the FAA has determined that the safety

interests of the flying public must take precedence over the economic

interests of airplane modifiers. This final rule does not preclude

modifiers from conducting such projects, but, if they cannot otherwise

meet the objectives of this rule, they will need to consider the costs

of full-scale fatigue testing along with the other compliance costs

when they evaluate the economic viability of a particular modification

project.

[[Page 15710]]

The FAA does not, however, concur that the overall economic impact

of this final rule on these small entities is significant. First, as

discussed in the preceding paragraphs, the full-scale fatigue testing

requirement of the proposed rule has been revised such that it is not

always necessary to conduct one for a modification project, and most

modifications would not necessitate one. The companion AC to the rule

has been expanded to provide guidance on acceptable means of showing

compliance for modifications. This guidance discusses how small, simple

design changes, using a design comparable to the original structure,

could be analytically determined to be equivalent to the original

structure in their propensity for widespread fatigue damage (e.g.,

modification of the fuselage structure for mounting an antenna using a

design that is similar to the original airplane in that area). In

addition, the amendment will not impose any additional costs on these

small entities on projects for which they have already applied for

supplemental type certificates; nor will it impose any additional costs

on projects for which they would apply for supplemental type

certificates in the near future, since the designs that would be

affected by this amendment would probably not enter service until at

least 5 to 10 years after its adoption. This is because, in general, in

accordance with Sec. 21.101 of 14 CFR part 21, modifiers of type

designs need only comply with the regulations that were used to

certificate the original model.

One other commenter states that the rule could be interpreted to

require full-scale fatigue testing of modifications specified in

service bulletins, which would actually impede safety by delaying the

issuance of needed service bulletins. The FAA does not concur with this

comment. Service bulletin modifications are in the same general

category as other modifications, and most would not necessitate full-

scale fatigue testing. Further, if circumstances necessitate

airworthiness directive (AD) action to mandate a modification specified

in a manufacturer's service bulletin before fatigue testing of the

modification is complete, there is nothing in the rule that prevents

the FAA from doing so.

One commenter also suggested replacing the sentence in current

Sec. 25.571(b) that states, ``Damage at multiple sites due to prior

fatigue exposure must also be included where the design is such that

this type of damage is expected to occur,'' with the following

sentence: ``Special consideration for WFD must be included where the

design is such that this type of damage could occur.'' Although the

commenter provided no explanation of this suggestion, the FAA considers

that it has merit. The FAA concurs that requiring ``special

consideration for WFD'' emphasizes that, in addition to demonstrating

that WFD will not occur within the design service goal, the applicant

for type certificate must also consider ways to prevent or control the

effects of WFD that may occur beyond the design service goal. This is

necessary to fulfill the objective of Sec. 25.571(a) to avoid

catastrophic failure due to fatigue throughout the operational life of

the airplane.

Many commenters object to basing all inspection thresholds on the

so-called ``rogue flaw'' concept, as would be required by the proposed

amendment to Sec. 25.571(a)(3). These commenters state that

indiscriminately applying this approach to all airplane structures

would result in an exorbitant increase in airplane inspection costs,

because it would necessitate detailed inspections earlier in an

airplane's life and would not significantly enhance safety. Although

most of these commenters aknowledge the necessity of using the ``rogue

flaw'' concept to establish inspection thresholds for certain types of

airframe design details, it was argued that the current industry

practice for establishing the inspection thresholds (consisting of

predicting the onset of cracking from fatigue testing and service

experience) is adequate for most commonly used airframe designs. Some

commenters endorsed a proposal that had previously been jointly

submitted by the Aerospace Industries Association, the Association

Europeene des Constructeurs De Material Aerospatial (AECMA), and the

Technical Oversight Group for Aging Airplanes (hereinafter referred to

as the AIA/AECMA/TOGAA comment). This group proposed that rogue flaw

based inspection thresholds be limited to single load path structure,

or other structure where it cannot be demonstrated that load path

failure, partial failure, or crack arrest will be detected and repaired

prior to failure of the remaining structure. The FAA concurs with these

comments. These criteria have been incorporated into the final rule,

and will ensure that the rogue flaw method of establishing inspection

thresholds is not applied indiscriminately, but will be applied where

necessary.

Following close of the comment period, and after the FAA had

reviewed these comments and decided to incorporate the language

proposed by AIA/AECMA/TOGAA into the final rule, Boeing, which had

participated in the development of the AIA/AECMA/TOGAA comment, became

aware of the FAA's decision. (This resulted from a series of

communications between Boeing and the FAA regarding an ongoing program

to determine the appropriate criteria for establishing fatigue

inspection thresholds for the Model 757 and 767 airplanes; the

communications were otherwise unrelated to this rulemaking.) At

Boeing's request, AIA filed an additional comment, objecting to

inclusion of this language in the final rule, and recommending instead

that it be incorporated into AC 25.571-1X. AIA stated that the FAA's

decision was in conflict with the AIA/AECMA/TOGAA comments, which had

been based on the commenters' conclusion that the general requirement

of Sec. 25.571(a)(3) that inspections be established ``as necessary to

prevent catastrophic failure'' was sufficient to ensure that rogue

flaws would be considered appropriately, as described in their proposed

revision to the AC.

Although the FAA concurs with the commenter's position that rogue

flaws in certain types of structure must be considered, the FAA does

not concur that revising the AC alone, and relying on the general

language of Sec. 25.571(a)(3), is sufficient to ensure adequate

consideration. Advisory circulars are not mandatory and explicitly

describe ``one means, but not the only means,'' of complying with the

relevant regulations. Therefore, because the FAA considers it essential

that rogue flaws be considered, the final rule has been amended, as

described previously.

One commenter states that the sentence added to Sec. 25.571(a)(3)

should be revised to state that thresholds for inspection should also

be based on service experience and fatigue testing, followed by a

``tear-down'' examination of the test article. Although the FAA agrees

that there may be important factors, it is more appropriate to discuss

them as acceptable means of compliance in the companion advisory

circular, and not in the rule itself. This will provide maximum

flexibility for future applicants to identify means of fulfilling the

rule's objectives.

One manufacturer asks for confirmation that its particular method

of establishing thresholds for inspection be allowed under the current

rulemaking. The FAA considers it inappropriate, in the context of this

rulemaking, to evaluate any one manufacturer's particular methodology.

Such an evaluation would normally be

[[Page 15711]]

accomplished during the certification process for an airplane type.

One commenter states that the proposed rule implies that simulated

manufacturing defects must be inflicted on the full-scale fatigue test

from the start. The FAA disagrees. As proposed, the purpose of the

full-scale fatigue test requirement is to establish that the structure

will be substantially free from widespread fatigue damage at least

until its design service goal is reached. In contrast, the purpose for

the consideration of manufacturing defects is to establish thresholds

for inspection (or other procedures) for certain types of structure.

Although the latter could involve full-scale fatigue testing in which

the test article is inflicted with simulated manufacturing defects,

and, in fact, the FAA's certification evaluation of a model type design

may reveal that this is the necessary way of establishing a threshold

in exceptional cases, it is not the FAA's intent to require this in

general.

One commenter states that it is not normally possible to complete a

full-scale fatigue test prior to issuance of a type certificate. The

commenter recommends that AC 25.571-1A be revised to allow completion

of the full-scale fatigue test after type certification. The FAA agrees

with this comment. As noted by the commenter, taken literally, the

proposed rule would have required that the testing be completed prior

to issuance of a type certificate. However, as reflected in the

preamble of the NPRM, the FAA recognized that this may not be realistic

and would have allowed completion of testing after issuance of the type

certificate. In light of the comment, the FAA has reconsidered this

issue and determined that the rule must be revised to address this

potential conflict. As revised, the rule allows testing to be completed

after issuance of the type certificate, provided:

1. Before issuance of the type certificate the Administrator has

approved a plan for completing the required tests, and

2. The Type Certificate contains an airworthiness limitation in the

airworthiness limitations section of the instructions for continued

airworthiness required by Sec. 25.1529 that no airplane may be operated

beyond a number of cycles equal to one-half the number of cycles

accumulated on the fatigue test article, until such testing is

completed.

The FAA considers that the first condition is necessary to ensure

that, at the time of type certification, the TC holder has at least

identified an acceptable method of complying with this rule's

requirements. The FAA considers that the second condition is necessary

to ensure that, following type certification, the testing proceeds so

that the affected airplanes receive the safety benefits that this rule

is intended to provide. Although these conditions were not specified in

the NPRM, the final rule actually provides relief from the literal

requirement of the NPRM to complete testing prior to issuance of a TC.

It is also a logical outgrowth of the proposal in that it resolves the

conflict between the proposed rule language and the preamble discussion

in a way that ensures that the rule's objectives are fulfilled.

Several commenters recommend that the words ``within the design

lifetime of the airplane,'' used in the sentence added to

Sec. 25.571(b), be changed to ``within the design service goal of the

airplane.'' It was pointed out that it is difficult for manufacturers

to know at the time an airplane is first certificated exactly how long

it will be used. The expected service period is set as a goal for

fatigue design at that time; therefore, the words ``design service

goal'' are more appropriate. Furthermore, it was pointed out that the

term ``lifetime'' implies a fixed service period for an airplane, after

which it would be retired. These commenters state that this does not

represent the intent of the proposed rule. The FAA concurs with this

comment, and the words ``design service goal'' have been substituted

for ``design lifetime.''

Several commenters state that the proposed requirements for

operating past the original design service goal are not clear. They

note that an industry team, the Structural Audit Evaluation Task Group

(SAETG) of the Airworthiness Assurance Working Group (AAWG), conducted

an extensive activity to determine possible actions for airplane models

that reach that point. (The SAETG and AAWG are sub-groups of the

Aviation Rulemaking Advisory Committee (ARAC), which submits rulemaking

recommendations to the FAA). The commenters state that the

recommendations of the SAETG should be addressed concurrently with the

present change to Sec. 25.571. The FAA does not agree with this

comment. Although the FAA is addressing the recommendations of the

SAETG at this time, that action covers only 11 specific models of

airplanes whose fleet leaders have already exceeded their design

service goal. These recommendations consist of suggested actions on how

to implement the guidance material they generated. Although the FAA

agrees that additional guidance may be appropriate for airplanes

affected by the present rulemaking on the subject considered by the

SAETG, the urgency of that action is not great because the design

service goal of these airplanes would not be reached for at least

another 20 years. Furthermore, one of the SAETG recommendations is that

their guidance should not be extended beyond the 11 specific models it

covers until it has actually been tried. To attempt to establish

guidance for airplanes affected by the present rulemaking based on the

SAETG recommendations at this time would only serve to delay the

issuance of the present rulemaking. Therefore, the most expeditious

manner of obtaining the benefits of the proposed refinement for the

damage-tolerance evaluations is to adopt the present rule change and to

continue discussions with the ARAC and others on how best to address

the SAETG recommendations.

One commenter states support for the new requirement for full-scale

testing, provided the companion Advisory Circular (AC 25.571-1X)

follows the Certification Maintenance Requirements (CMR) guidelines (AC

25-19 dated 11/28/94). The CMR guidelines referred to by this commenter

are guidelines on how inspection programs for airplane systems should

be established at the time of certification. The FAA does not agree

with this comment. There are presently fundamental differences in

methodology between the way inspection programs are established for

airplane systems and airplane structures. Attempts at resolving those

differences have not been fruitful in the past, and there is no

guarantee that they will be any more fruitful in the future. Therefore,

evaluation of the appropriateness of using the CMR guidelines to

establish structural inspection programs as part of the present

rulemaking would result in a delay that the FAA considers unacceptable.

Several commenters state that the rule should specify the size of

the initial manufacturing flaw or fatigue scatter factor criteria,

either in the rule itself or in the accompanying AC. Although the FAA

does not concur that an absolute size should be specified for the

initial manufacturing flaw, guidance on acceptable means of compliance

has been provided in the revised version of the AC on both subjects.

Regulatory Evaluation

Proposed 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

[[Page 15712]]

intended regulation justify its costs. Second, the Regulatory

Flexibility Act of 1980 requires agencies to analyze the economic

effect of regulatory changes on small entities. Third, the Office of

Management and Budget directs agencies to assess the effects of

regulatory changes on international trade. In conducting these

analyses, the FAA has determined that this rule: (1) will generate

benefits that justify its costs as defined in the Executive Order; (2)

is significant as defined in DOT's Regulatory Policies and Procedures;

(3) will not have a significant impact on a substantial number of small

entities; and (4) could affect international trade. These analyses,

available in the docket, are summarized below.

Estimated Costs and Benefits

Based on the opinions of industry and agency experts, the FAA

estimates that development and certification costs associated with the

requirement for an inspection threshold based on initial manufacturing

defects will be negligible. However, this provision could affect

operating costs, depending on the degree to which it impinges on the

timing of initial inspections. This evaluation conservatively estimates

that an additional 500,000 work hours will be required to inspect a

fleet of 1,000 airplanes as result of the requirement to base

inspection thresholds on assumed manufacturing defects. Assuming a

fully burdened compensation rate of $65 per hour, this provision will

increase operating costs by approximately $32.5 million over the life

of a 1,000 airplane fleet.

The cost of a full-scale fatigue test for a representative

transport category airplane design is statistically estimated using a

sample of four different airplane models, ranging from a 45-seat

airplane to a large widebody transport. In its comments on NPRM 93-9,

the Aerospace Industries Association (AIA) notes that certification

requirements could double the number of work hours for such testing. To

account for this, full-scale fatigue test costs for each airplane model

were inflated by multiplying the labor cost component by a factor of

two. The relationship between these adjusted fatigue test costs and

airplane size--measured by the number of seats--was then estimated

using ordinary least squares. This yields a cost estimate of $540,000

for each seat in a proposed model. The cost of a full-scale fatigue

test for a 162-seat airplane design, for example, would be

approximately 162 times $540,000 or $87.5 million. for a 1,000 airplane

fleet, this would equal $87,500 per airplane.

Total costs are estimated for a representative type certification

using the following assumptions: (1) The hypothetical airplane model is

assumed to have 162 seats; (2) 50 percent of testing costs are incurred

in the year 2000, one-third of the remaining costs are incurred in each

of the years 2001, 2002, and 2003; (3) production commences in the year

2002; (4) 100 airplanes are produced per year for 10 years; (5) the

first airplanes enter service in 2002; (6) for each airplane,

inspection costs related to the ``rogue flaw'' requirement are

uniformly distributed in the interval bounded by one-fourth and one-

half the design service goal (i.e., between the 5th and 9th years of

operation); (7) total burdened cost per work hour is $65; (8) the

discount rate is 7 percent; and (9) each airplane is retired at the end

of its 20-year design service goal.

Under these assumptions, undiscounted fleet certification and

operating costs--including the costs of a full-scale fatigue test and

the inspection threshold provision--equal $120.0 million or $120,000

per airplane. On a discounted (1997 dollar) basis, fleet costs equal

$78.6 million or $78,600 per airplane.

The benefits of the rule depend on the inherent variability of

structural fatigue analysis and on the efficacy of actions taken in

response to the results of such analysis. For example, the ``rogue-

flaw'' inspection threshold requirement will prevent an accident only

if: (1) The threshold occurs before an accident would otherwise occur;

and (2) the resulting inspection identifies the damaged structure.

Nevertheless, based on the accident history and the likelihood of

ancillary benefits, the FAA finds that the benefits of the rule justify

its costs.

An examination of the service history identified 39 domestic

accidents or incidents involving structural fatigue during the period

1974-1990. The National Transportation Safety Board (NTSB) identified

improper maintenance and/or corrosion as important contributing factors

in 17 of the events. Of the remaining 22 events, 12 involved the

landing gear and 10 involved the wing, fuselage, or other structure.

Although only two of these 10 events resulted in fatalities,

several other events had catastrophic potential (in one case a wing

spar failed, and in five other cases passenger cabin decompression

occurred). In at least one case, the NTSB concluded that the accident

was the probable result of a manufacturing defect.

During the same period, air carriers accumulated approximately 148

million flight hours. Thus, between 1974 and 1990, the overall event

rate was (10/148)=0.0676 per million flight hours. The historical fatal

accident rate was (2/148)=0.014.

Assuming that the average air carrier airplane has 162 seats, 69

percent of which are occupied; the airplane replacement cost is $30

million; and the value of an averted fatality is $2.7 million; then the

economic value of one accident in which an airplane is destroyed and

there are no survivors is approximately $345.9 million. If the rule

prevents one such accident, its undiscounted benefits will exceed its

undiscounted costs by a ratio of $345.9 million/$120.0 million or 2.88.

Assuming that the probability of an avoided accident is proportional to

the size of the complying fleet in any given year, then the expected

discounted benefits of such an avoided accident will exceed discounted

costs by a ratio of approximately 1.34.

Prevented accidents, however, do not exhaust the benefits of this

rule. Full-scale fatigue testing is already industry practice. This

reflects, in part, benefits such as timely correction of deficiencies

to prevent early cracking, and verification of inspection and

maintenance procedures. In addition to obvious safety implications,

early identification of premature cracking will allow repairs to be

accomplished during scheduled maintenance visits, thus lessening the

economic impact of withdrawing an airplane from revenue service. While

it is difficult to account for these ancillary benefits, the accident

history gives some indication of their potential.

A review of records on accidents that occurred between 1974 and

1989 shows that at least five fleetwide inspections involving

approximately 900 airplanes were ordered as a result of accidents

involving failure of airplane structure. During these inspections, at

least 72 airplanes were found to have fatigue cracks. Cost information

specifically related to these inspections is unavailable. However, a

review of some recent Airworthiness Directives (AD) and Service

Bulletin data indicates that a minimum of 20 work hours (10 hours

elapsed time) are necessary to carry out the required inspections.

Minimum out-of-service time is 15-20 hours--approximately one day. If

the cracking is predicted by full-scale fatigue testing and planned for

in normal maintenance, unscheduled downtime may be averted. The number

of required work hours (and downtime) would be much greater of the

examination reveals extensive

[[Page 15713]]

cracking since this finding would necessitate additional inspections

and structural repair. If cracking is predicted from a full-scale

fatigue test, it can be detected at an earlier stage of development in

the operating airplanes, resulting in less costly repairs, requiring

less downtime to accomplish.

The cost of the unscheduled downtime for a fleetwide inspection, in

which each airplane is withdrawn from revenue service for one day, can

be estimated using the same production, operating history, and discount

rate assumptions listed above. Assuming that the probability of an

unscheduled inspection is uniformly distributed over each airplane's

service life and that the revenue lost per airplane per day out of

service is $40,000, the FAA estimates that the expected discounted

savings from averting an average of one unscheduled inspection/repair

day per airplane (over the service life of the fleet) is approximately

$12.1 million. Thus, regardless of the number of accidents avoided, if

the rule averts an average of 6.5 days of downtime per airplane over

the life of the fleet, the expected discounted benefits of the rule

will equal the discounted costs.

Regulatory Flexibility Determination

The Regulatory Flexibility Act of 1980 (RFA) was enacted by

Congress to ensure that small entities are not unnecessarily and

disproportionately burdened by government regulations. The RFA requires

agencies to review rules which may have a ``significant economic impact

on a substantial number of small entities.'' Entities potentially

affected by the rule include manufacturers of transport category

airplanes and aircraft modification firms.

While manufacturers of transport category airplanes generally

support full scale fatigue testing, some aircraft modifiers--including

some small firms--object to this requirement on the grounds that it

constitutes and excessive burden. As noted previously, however, the

final rule may require full-scale fatigue testing--covering, when

applicable, modifications to future transport airplane designs--for

four reasons.

First, the rule will not affect existing airplane types. The

amendment will not impose additional costs on existing applications for

supplemental type certification, nor will it affect applications made

in the near future. The airplanes that would be affected by this

amendment would not enter service for at least 5 to 10 years after its

adoption.

Second, in the case of future type designs, it is difficult to

predict whether anyone would seek approval for subsequent

modifications, and, if so, how extensive the modifications would be and

whether full-scale testing would be necessary for them (based on

experience, the FAA concludes that most modifications of future designs

will not require full-scale fatigue testing). Thus, it is impossible to

conclude that there will be a significant effect on a substantial

number of small entities.

Third, even assuming that small entities would propose such

modifications, the FAA has determined that the safety interests of the

flying public take precedence over the economic interests of airplane

modifiers. The FAA finds, that, under the circumstances where existing

test evidence is insufficient to meet the objectives of this rule,

there are no alternatives to full-scale testing that would enable small

entities to meet these objectives.

Fourth, the FAA remains open to considering technical innovations

that provide alternatives to full scale testing. Such innovations could

form the basis for finding that sufficient full-scale test evidence

exists based on testing performed during initial type certification.

International Trade Impact Assessment

The Office of Management and Budget directs Federal Agencies to

assess whether or not a rule or regulation will affect any trade-

sensitive activity. The FAA has assessed the potential for this rule to

affect domestic transport category airplane manufacturers, aircraft

modification firms, and air carriers.

The FAA determines that the rule will have little or no effect on

trade for either U.S. firms marketing transport category airplanes in

foreign markets or foreign firms marketing transport category airplanes

in the U.S. This follows since full scale fatigue testing for such

airplanes is already industry practice, both domestically and abroad.

Also, domestic and foreign manufactured airplanes would both be subject

to the inspection threshold provision of the rule if they are

certificated in the U.S.

Similarly, the FAA determines that the rule will have little or no

effect on foreign firms competing for U.S. aircraft modification work,

or U.S. firms competing for foreign aircraft modification work.

The FAA recognizes that the rule could affect the competition for

international air travel by imposing more conservative inspection

requirements on U.S. carriers. However, it is unlikely that, in

validating the FAA's certification of a future airplane design, another

civil aviation authority would escalate the inspection threshold

required by this rule. Nevertheless, if a foreign civil aviation

authority determines that the inspection threshold is too conservative

and, thus, chooses not to impose this requirement, U.S. carriers

operating future airplane models subject to this rule could incur

larger inspection costs relative to foreign carriers operating foreign

registered airplanes of the same models. The FAA estimates that the

discounted 20-year cost of the inspection threshold provision is

approximately $12,000 per airplane. Under the average passenger

capacity and load factor assumptions described above, and assuming an

average of 1,600 departures per airplane per year, this equals

approximately $0.003 per enplaned passenger.

Federalism Implications

The regulations adopted herein will not have substantial direct

effects on the States, on the relationship between the national

government and the States, or on the distribution of power and

responsibilities among the various levels of government. Therefore, in

accordance with Executive Order 12612, it is determined that this rule

will not have sufficient federalism implications to warrant the

preparation of a Federalism Assessment.

International Civil Aviation Organization (ICAO) and Joint Aviation

Regulations

In keeping with U.S. obligations under the Convention on

International Civil Aviation, it is FAA policy to comply with ICAO

Standards and Recommended Practices to the maximum extent practicable.

The FAA has determined that this rule does not conflict with any

international agreement of the United States.

Paperwork Reduction Act

In accordance with the Paperwork Reduction Act of 1995 (44 U.S.C.

3507(d)), there are no reporting or recordkeeping requirements

associated with this rule.

Regulations Affecting Intrastate Aviation in Alaska

Section 1205 of the FAA Reauthorization Act of 1996 (110 Stat.

3213) requires the Administrator, when modifying regulations in Title

14 of the CFR in a manner affecting intrastate aviation in Alaska, to

consider the extent to which Alaska is not served by transportation

modes other than aviation, and to establish such

[[Page 15714]]

regulatory distinctions as he or she considers appropriate. Because

this final rule applies to the certification of future designs of

transport category airplanes and their subsequent operation, it could

affect intrastate aviation in Alaska. The Administrator has considered

the extent to which Alaska is not served by transportation modes other

than aviation, and how the final rule could have been applied

differently to intrastate operations in Alaska. However, the

Administrator has determined that airplanes operated solely in Alaska

would present the same safety concerns as all other affected airplanes;

therefore, it would be inappropriate to establish a regulatory

distinction for the intrastate operation of affected airplanes in

Alaska.

Unfunded Mandates Reform Act

Title II of the Unfunded Mandates Reform Act of 1995 (the Act),

enacted as Public Law 104-4 on March 22, 1995, requires each Federal

agency, to the extent permitted by law, to prepare a written assessment

of the effects of any Federal mandate in a proposed or final rule that

may result in the expenditure by State, local, and tribal governments,

in the aggregate, or by the private sector, of $100 million or more

(adjusted annually for inflation) in any one year. Section 204(a) of

the Act, 2 U.S.C. 1534(a), requires the Federal agency to develop an

effective process to permit timely input by elected officers (or their

designees) of State, local, and tribal governments on a proposed

``significant intergovernmental mandate.'' A ``significant

intergovernmental mandate'' under the Act is any provision in a Federal

agency regulation that would impose an enforceable duty upon State,

local, and tribal governments, in the aggregate, of $100 million

(adjusted annually for inflation) in any one year. Section 203 of the

Act, 2 U.S.C. 1533, which supplements section 204(a), provides that

before establishing any regulatory requirements that might

significantly or uniquely affect small governments, the agency shall

have developed a plan that, among other things, provides for notice to

potentially affected small governments, if any, and for a meaningful

and timely opportunity to provide input in the development of

regulatory proposals.

This rule does not contain a Federal intergovernmental or private

sector mandate meeting that criterion, therefore the requirements of

the Act do not apply.

List of Subjects in 14 CFR Part 25

Aircraft, Aviation safety, Reporting and recordkeeping

requirements.

Adoption of the Amendment

In consideration of the foregoing, the Federal Aviation

Administration (FAA) amends 14 CFR part 25 of the Federal Aviation

Regulations (FAR) as follows:

PART 25--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES

1. The authority citation for part 25 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701-44702, 44704.

2. Section 25.571 is amended by revising the introductory text of

paragraph (a), and paragraph (a)(3), the introductory text of paragraph

(b), and paragraphs (b)(1), (b)(5)(ii), and (e)(1) to read as follows:

Sec. 25.571 Damage-tolerance and fatigue evaluation of structure.

(a) General. An evaluation of the strength, detail design, and

fabrication must show that catastrophic failure due to fatigue,

corrosion, manufacturing defects, or accidental damage, will be avoided

throughout the operational life of the airplane. This evaluation must

be conducted in accordance with the provisions of paragraphs (b) and

(e) of this section, except as specified in paragraph (c) of this

section, for each part of the structure that could contribute to a

catastrophic failure (such as wing, empennage, control surfaces and

their systems, the fuselage, engine mounting, landing gear, and their

related primary attachments). For turbojet powered airplanes, those

parts that could contribute to a catastrophic failure must also be

evaluated under paragraph (d) of this section. In addition, the

following apply:

* * * * *

(3) Based on the evaluations required by this section, inspections

or other procedures must be established, as necessary, to prevent

catastrophic failure, and must be included in the Airworthiness

Limitations Section of the Instructions for Continued Airworthiness

required by Sec. 25.1529. Inspection thresholds for the following types

of structure must be established based on crack growth analyses and/or

tests, assuming the structure contains an initial flaw of the maximum

probable size that could exist as a result of manufacturing or service-

induced damage:

(i) Single load path structure, and

(ii) Multiple load path ``fail-safe'' structure and crack arrest

``fail-safe'' structure, where it cannot be demonstrated that load path

failure, partial failure, or crack arrest will be detected and repaired

during normal maintenance, inspection, or operation of an airplane

prior to failure of the remaining structure.

(b) Damage-tolerance evaluation. The evaluation must include a

determination of the probable locations and modes of damage due to

fatigue, corrosion, or accidental damage. Repeated load and static

analyses supported by test evidence and (if available) service

experience must also be incorporated in the evaluation. Special

consideration for widespread fatigue damage must be included where the

design is such that this type of damage could occur. It must be

demonstrated with sufficient full-scale fatigue test evidence that

widespread fatigue damage will not occur within the design service goal

of the airplane. The type certificate may be issued prior to completion

of full-scale fatigue testing, provided the Administrator has approved

a plan for competing the required tests, and the airworthiness

limitations section of the instructions for continued airworthiness

required by Sec. 25.1529 of this part specifies that no airplane may be

operated beyond a number of cycles equal to \1/2\ the number of cycles

accumulated on the fatigue test article, until such testing is

completed. The extent of damage for residual strength evaluation at any

time within the operational life of the airplane must be consistent

with the initial detectability and subsequent growth under repeated

loads. The residual strength evaluation must show that the remaining

structure is able to withstand loads (considered as static ultimate

loads) corresponding to the following conditions:

(1) The limit symmetrical maneuvering conditions specified in

Sec. 25.337 at all speeds up to Vc and in Sec. 25.345.

* * * * *

(5) * * *

(ii) The maximum value of normal operating differential pressure

(including the expected external aerodynamic pressures during 1 g level

flight) multiplied by a factor of 1.15, omitting other loads.

* * * * *

(e) * * *

(1) Impact with a 4-pound bird when the velocity of the airplane

relative to the bird along the airplane's flight path is equal to

Vc at sea level or 0.85Vc at 8,000 feet,

whichever is more critical;

* * * * *

[[Page 15715]]

Issued in Washington, D.C. on March 26, 1998.

Jane F. Garvey,

Administrator.

[FR Doc. 98-8379 Filed 3-30-98; 8:45 am]

BILLING CODE 4910-13-M