Airworthiness Directives; Boeing Model 747-100, -200, and -300 Series Airplanes Equipped With Pratt & Whitney JT9D Series Engines

DEPARTMENT OF TRANSPORTATION

14 CFR Part 39

[Docket No. 93-NM-151-AD; Amendment 39-8917; AD 94-10-10]

Airworthiness Directives; Boeing Model 747-100, -200, and -300

Series Airplanes Equipped With Pratt & Whitney JT9D Series Engines

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: This amendment adopts a new airworthiness directive (AD),

applicable to Boeing Model 747-100, -200, and -300 series airplanes,

that requires modification of the thrust reverser control system by

installing a solenoid-operated shut-off valve. This amendment is

prompted by incidents of deployment of the engine fan thrust reverser

during flight. The actions specified by this AD are intended to prevent

such deployment, which could result in reduced controllability of the

airplane.

DATES: Effective June 20, 1994.

The incorporation by reference of certain publications listed in

the regulations is approved by the Director of the Federal Register as

of June 20, 1994.

ADDRESSES: The service information referenced in this AD may be

obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle,

Washington 98124-2207. This information may be examined at the Federal

Aviation Administration (FAA), Transport Airplane Directorate, Rules

Docket, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of

the Federal Register, 800 North Capitol Street, NW., suite 700,

Washington, DC.

FOR FURTHER INFORMATION CONTACT: G. Michael Collins, Aerospace

Engineer, Propulsion Branch, ANM-140S, FAA, Transport Airplane

Directorate, Seattle Aircraft Certification Office, 1601 Lind Avenue,

SW., Renton, Washington 98055-4056; telephone (206) 227-2689; fax (206)

227-1181.

SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal

Aviation Regulations (14 CFR part 39) to include an airworthiness

directive (AD), applicable to Boeing Model 747-100, -200, and -300

series airplanes equipped with Pratt & Whitney JT9D series engines, was

published in the Federal Register on December 7, 1993 (58 FR 64386).

That action proposed to require modification of the thrust reverser

control system by installing a solenoid-operated shut-off valve.

Interested persons have been afforded an opportunity to participate

in the making of this amendment. Due consideration has been given to

the comments received.

One commenter supports the proposal.

Several commenters contend that the proposed modification is not

necessary and the proposed rule should be withdrawn. These commenters

state that the high reliability of both the Thrust Reverser Sequencing

Mechanism (TRSM) and the thrust reverser system preclude the need for

the proposed shut-off valve installation. These commenters note that

the manufacturer's service bulletin referenced in the proposal does not

clearly state that the shut-off valve will prevent an in-flight

deployment of the thrust reversers. The FAA does not concur. The

reliability of the TRSM or the thrust reverser system does not affect

the requirement for the solenoid-operated shut-off valve. An in-flight

deployment of a thrust reverser can occur even with a fully functioning

thrust reverser system with all components (including the TRSM)

operating within maintenance limits, if the pneumatic signal line vent

on the Directional Control Valve (DCV) becomes plugged. The

installation of the shut-off valve prevents this single malfunction

from causing such a deployment.

Further, the design of the thrust reverser control system is such

that there is a constant supply of pneumatic signal air to the DCV.

When the thrust levers are in the ``idle'' or ``forward thrust''

position, air flows through the DCV to the stow port of the gear

switching actuator. Signal air leakage from the stow chamber to the

deploy chamber occurs even with an actuator that is within overhaul

limits. The signal air that leaks into the deploy chamber is designed

to be vented through the actuator deploy port and the deploy signal

line to the DCV, where it exits through the DCV vent. If this vent path

becomes plugged, the pressure in the deploy chamber will rise.

Pressurization of the deploy chamber of the gear switching actuator can

cause the gear switching actuator to move to the ``deploy'' position.

Additionally, drive air is supplied to the thrust reverser

Pneumatic Drive Unit (PDU) through a Pressure Regulator Shut-off Valve

(PRSOV). The PRSOV opens when it receives pneumatic signal air from the

DCV. This signal is received whenever the thrust levers are moved to

the idle position, either in flight or on the ground. If the DCV vent,

deploy signal line, or gear switcher actuator deploy chamber port

becomes plugged and causes the gear switcher actuator to move to the

``deploy'' position, the thrust reverser will deploy when the thrust

lever is moved to the ``idle'' position. Plugging of this vent path has

caused approximately 30 (of 31) known events of in-flight deployment of

the thrust reversers on Model 747 airplanes. Such plugging has been

caused by improper deactivation procedures, pieces of air filter media,

and ice formed during flight.

The installation of the solenoid-operated shut-off valve required

by this AD will prevent the flow of pneumatic drive air to the PDU,

unless the airplane is on the ground and the reverse thrust levers are

in the ``reverse thrust'' position. The shut-off valve is installed

between the PRSOV and the thrust reverser PDU. The shut-off valve opens

only when the airplane air/ground logic is in the ground mode and

``reverse thrust'' is selected. Therefore, the shut-off valve is

designed to prevent in-flight deployment of the thrust reverser by

preventing the flow of drive air to the PDU (unless the airplane is on

the ground and ``reverse thrust'' is selected).

There has been only one reported in-flight deployment of a thrust

reverser on a Model 747 involving an engine that was equipped with the

solenoid-operated shut-off valve. However, this incident was determined

to have been caused by a maintenance crew that disconnected the PDU

drive shafts to the thrust reverser in order to deactivate the thrust

reverser, and inadvertently did not lock the reverser sleeve. By

disconnecting the two drive shafts, the two reverser locking devices

were disconnected. Thus, the improper deactivation of the reverser

contributed to the in-flight deployment incident. By disconnecting the

PDU drive shafts, the pneumatic drive was taken out of the system;

therefore, the shut-off valve had no effect on the deployment.

None of the other 30 incidents of in-flight deployments of the

thrust reversers on Model 747's have occurred on engines equipped with

the subject solenoid-operated shut-off valve. The FAA considers that

all of those events could have been prevented if the shut-off valve had

been installed.

For these reasons, the FAA has determined that installation of the

solenoid-operated shut-off valve is appropriate and warranted.

Several commenters do not support the need for the issuance of the

proposed rule. These commenters state that the Model 747 has been shown

to be controllable following all incidents of in-flight deployment of

the thrust reversers. They contend that these incidents of deployment

occurred at idle thrust, and that the proposed modification would only

help in preventing uncommanded deployments at idle thrust or where

improper lockout of the reverser sleeve had been performed. The FAA

does not concur with these commenters' inference that issuance of the

rule is not necessary. The FAA acknowledges that, in all of the known

events of in-flight thrust reverser deployments, the flight crews were

able to control the airplanes and land safely. At the time this rule

was proposed, there was insufficient data to demonstrate that the

airplane would be controllable in all phases of flight following

deployment of a thrust reverser. Recently, however, Boeing has

presented the results of a study it conducted on the controllability of

the airplane following an in-flight deployment event in several phases

of flight. The results of this study revealed that these airplanes

could experience certain control problems in the event of a thrust

reverser deployment occurring during high speed climb or possibly

during cruise. This additional information on the controllability of

the airplane following deployment of an outboard thrust reverser

further demonstrates the need to install the solenoid-operated shut-off

valve. For example, if the thrust lever is moved rapidly to the

``idle'' position from a high power setting, and the DCV vent is

plugged (e.g., by ice), the thrust reverser could deploy at a high

thrust level as the engine decelerates. The installation of the shut-

off valve is designed to prevent pneumatic drive air from powering the

PDU, unless the airplane is on the ground and ``reverser thrust'' is

selected, thereby preventing in-flight deployments.

Several commenters request that the proposed rule be revised to

allow the accomplishment of certain revised maintenance practices, in

lieu of installation of the shut-off valve. These commenters point out

that the majority of the incidents of in-flight deployments of thrust

reversers have involved improper or outdated maintenance practices

relative to deactivating the reverser. The Model 747 Maintenance Manual

has been revised to delete all deactivating procedures except one,

which involves disconnecting the PDU drive shafts and then ensuring

that the reverser sleeve is locked in the stowed position. The

commenters contend that, with such procedures now deleted from normal

maintenance practices, circumstances that previously would have led to

a deployment incident will now be eliminated. The FAA does not concur.

Although this new maintenance procedure may serve to preclude in-flight

deployments caused by improper deactivating procedures, the FAA finds

that no maintenance procedure can prevent in-flight deployments caused

by blocking of the DCV vent (by ice, foreign objects, etc.).

Additionally, the FAA points out that several revisions had been made

previously to the Model 747 Maintenance Manual in an attempt to prevent

in-flight deployments of the thrust reverser, yet deployments have

continued to occur. As stated elsewhere in this preamble, the FAA

considers that the installation of the subject shut-off valves would

have prevented 30 of the 31 known incidents of in-flight deployments on

Model 747's, since those incidents involved blocking of the DCV vent.

Numerous commenters request that the proposed compliance time of 24

months for modification be extended to between 36 months and 5 years.

Several of these commenters point out that there is a 300- to 400-day

lead time necessary for purchasing and obtaining necessary parts. This

lengthy lead time for parts delivery would seriously hamper affected

operators' ability to comply with the proposed rule within a 24-month

deadline, and would cause major scheduling problems, delays, and flight

cancellations. Other commenters request an extension to in order to

allow the modification to be accomplished during a regularly scheduled

``heavy'' maintenance visit (``D'' check) at a main base, where

required equipment and trained personnel would be available. The FAA

agrees that the compliance time can be extended somewhat. Based on

information provided by the commenters concerning parts availability,

the FAA has determined that a compliance time of 36 months is

appropriate in order to accommodate the time necessary for affected

operators to order, obtain, and install the modification; and will not

compromise safety. The final rule has been revised accordingly.

Certain commenters request that the proposed rule be revised to

permit the installation of unmodified engines after the effective date

of the AD and until the compliance time for modification. These

commenters point out that proposed paragraph (b) would require that, as

of the effective date of the rule, no operator install an unmodified

engine on any airplane. However, to comply with this paragraph,

operators would have to modify all spare engines prior to the effective

date of the AD; this would be impossible, due to the lead time

necessary to obtain parts, and would impose an economic burden on

operators. The FAA concurs. The final rule has been revised to permit

installation of an unmodified engine up to 18 months after the

effective date of the rule. This compliance time of 18 months is based

on a 1-year period that would be necessary to obtain parts, plus a 6-

month period to modify all spare engines.

One commenter requests that the proposed rule be revised to require

the installation of a new PDU to prevent the back driving of the thrust

reverser. This commenter has installed such a unit in accordance with

Boeing Service Bulletins 747-78-2084 and 747-78-2090, and Garrett

Service Bulletins 126712-78-1432 and 126236-78-1332. Since

installation, this commenter has not experienced any in-flight

deployments of the thrust reverser. The commenter recommends this

installation in lieu of the shut-off valve that would be required the

proposed AD. The FAA does not concur. The modification referred to by

the commenter entails the installation of a reversible PDU in place of

the gear-driven PDU that was installed on earlier Model 747 airplanes

equipped with Pratt & Whitney JT9D engines. The pneumatic control

system used with the reversible PDU is similar to the control system

used with the gear-driven PDU. One difference is that a ``directional

switcher actuator'' is used with the reversible PDU, in place of the

``gear switcher actuator'' used with the gear-driven PDU; however, both

actuators are similar in design, and their deploy chamber vent paths

are the same. Therefore, if the DCV vent, deploy signal line, or the

directional switcher actuator deploy chamber port becomes plugged, the

directional switcher actuator could move to the ``deploy'' position. If

this occurs, the thrust reverser will deploy when the thrust lever is

moved to the ``idle'' position, just as is the case for the gear-driven

PDU configuration. Installation of the subject shut-off valve will

prevent this from occurring. In light of this, the FAA has determined

that installation of the shut-off valve is necessary, regardless of the

type of PDU that is installed.

Other commenters request that the proposed rule be revised to allow

alternative approaches, in lieu of the modification, to address the

identified unsafe condition. The commenters state that the cost of the

specific proposed modification is an unnecessary burden on operators,

and consider that improved maintenance procedures are all that are

necessary to ensure safety of the system. For the reasons previously

described in this preamble, the FAA does concur with these commenters'

specific suggestions for ``alternative approaches.'' At this time,

based on all available data to date, the FAA has determined that the

modification required by the AD is the only action that will prevent

in-flight deployments of the thrust reverser caused by blockage of the

DCV vent. No maintenance action or acceptable alternative modification

has yet been presented to the FAA that will prevent a deployment caused

by this single modification. However, under the provisions of paragraph

(c) of the final rule, operators may request approval of the use of

alternative methods of compliance, provided that sufficient data is

presented to the FAA to justify the request.

Numerous commenters request that the economic impact information

presented in the preamble to the notice be revised to provide updated

costs. These commenters state that the costs indicated in the preamble

to the proposed rule were greatly underestimated. The FAA concurs. The

estimated modification costs that were presented in the preamble to the

proposal were based on information provided by the airplane

manufacturer at the time the proposed rule was being developed. The

manufacturer has now provided updated costs for the parts and labor

necessary to modify affected airplanes, and the economic impact

information, below, has been revised accordingly.

The economic impact information has also been revised to update the

current number of airplanes that will be affected by this AD.

After careful review of the available data, including the comments

noted above, the FAA has determined that air safety and the public

interest require the adoption of the rule with the changes previously

described. The FAA has determined that these changes will neither

increase the economic burden on any operator nor increase the scope of

the AD.

There are approximately 221 Model 747-100, -200, and -300 series

airplanes of the affected design in the worldwide fleet. The FAA

estimates that 124 airplanes of U.S. registry will be affected by this

AD, that it will take approximately 128 work hours per airplane to

accomplish the required actions, and that the average labor rate is $55

per work hour. Required parts will cost approximately $75,356 per

airplane. Based on these figures, the total cost impact of the AD on

U.S. operators is estimated to be $10,217,104, or $82,396 per airplane.

The number of required work hours, as indicated above, is presented

as if the accomplishment of the modification required by this AD were

to be conducted as a ``stand alone'' action. However, the 36-month

compliance time specified in paragraph (a) of this AD allows ample time

for the modification to be accomplished coincidentally with scheduled

major airplane inspection and maintenance activities, thereby

minimizing the costs associated with special airplane scheduling.

The total cost impact figure discussed above is based on

assumptions that no operator has yet accomplished any of the

requirements of this AD action, and that no operator would accomplish

those actions in the future if this AD were not adopted.

The FAA recognizes that the obligation to maintain aircraft in an

airworthy condition is vital, but sometimes expensive. Because AD's

require specific actions to address specific unsafe conditions, they

appear to impose costs that would not otherwise be borne by operators.

However, because of the general obligation of operators to maintain

aircraft in an airworthy condition, this appearance is deceptive.

Attributing those costs solely to the issuance of this AD is

unrealistic because, in the interest of maintaining safe aircraft, most

prudent operators would accomplish the required actions even if they

were not required to do so by the AD.

A full cost-benefit analysis has not been accomplished for this AD.

As a matter of law, in order to be airworthy, an aircraft must conform

to its type design and be in a condition for safe operation. The type

design is approved only after the FAA makes a determination that it

complies with all applicable airworthiness requirements. In adopting

and maintaining those requirements, the FAA has already made the

determination that they establish a level of safety that is cost-

beneficial. When the FAA, as in this AD, makes a finding of an unsafe

condition, this means that this cost-beneficial level of safety is no

longer being achieved and that the required actions are necessary to

restore that level of safety. Because this level of safety has already

been determined to be cost-beneficial, a full cost-benefit analysis for

this AD would be redundant and unnecessary.

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 final

rule does not have sufficient federalism implications to warrant the

preparation of a Federalism Assessment.

For the reasons discussed above, I certify that this action (1) is

not a ``significant regulatory action'' under Executive Order 12866;

(2) is not a ``significant rule'' under DOT Regulatory Policies and

Procedures (44 FR 11034, February 26, 1979); and (3) will not have a

significant economic impact, positive or negative, on a substantial

number of small entities under the criteria of the Regulatory

Flexibility Act. A final evaluation has been prepared for this action

and it is contained in the Rules Docket. A copy of it may be obtained

from the Rules Docket at the location provided under the caption

ADDRESSES.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by

reference, Safety.

Adoption of the Amendment

Accordingly, pursuant to the authority delegated to me by the

Administrator, the Federal Aviation Administration amends 14 CFR part

39 of the Federal Aviation Regulations as follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C.

106(g); and 14 CFR 11.89.

Sec. 39.13 [Amended]

2. Section 39.13 is amended by adding the following new

airworthiness directive:

94-10-10 Boeing: Amendment 39-8917. Docket 93-NM-151-AD.

Applicability: Model 747-100, -200, and -300 series airplanes;

equipped with Pratt & Whitney JT9D series engines; certificated in

any category.

Compliance: Required as indicated, unless accomplished

previously.

To prevent inadvertent engine fan thrust reverser deployment

during flight, which could result in reduced controllability of the

airplane, accomplish the following:

(a) Within 36 months after the effective date of this AD, modify

the thrust reverser control system to include a solenoid-operated

shut-off valve in accordance with Boeing Service Bulletin 747-78-

2052, Revision 4, dated March 23, 1989.

Note: Airplanes on which the modification has been accomplished

previously in accordance with Boeing Service Bulletin 747-78-2052,

Revision 3, dated August 27, 1987, are considered to be in

compliance with this paragraph.

(b) As of 18 months after the effective date of this AD, no

person shall install a Pratt & Whitney JT9D series engine on any

airplane unless the thrust reverser control system installed on that

engine has been modified to include a solenoid-operated shut-off

valve in accordance with Boeing Service Bulletin 747-78-2052,

Revision 4, dated March 23, 1989.

(c) An alternative method of compliance or adjustment of the

compliance time that provides an acceptable level of safety may be

used if approved by the Manager, Seattle Aircraft Certification

Office (ACO), FAA, Transport Airplane Directorate. Operators shall

submit their requests through an appropriate FAA Principal

Maintenance Inspector, who may add comments and then send it to the

Manager, Seattle ACO.

Note: Information concerning the existence of approved

alternative methods of compliance with this AD, if any, may be

obtained from the Seattle ACO.

(d) Special flight permits may be issued in accordance with

Secs. 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR

21.197 and 21.199) to operate the airplane to a location where the

requirements of this AD can be accomplished.

(e) The modification shall be done in accordance with Boeing

Service Bulletin 747-78-2052, Revision 4, dated March 23, 1989,

which contains the following list of effective pages:

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Revision level

Page No. shown on page Date shown on page

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1-5, 12-13,-19-21, 35, 38, 46 4 March 23, 1989.

6-7, 18, 39, 41.............. 3 August 27, 1987.

15, 23-24, 36, 42-45, 47-49 2 January 9, 1976.

30-31, 40.

10, 16, 22, 37............... 1 March 28, 1975.

8-9, 11, 14, 17, 25-29, 32-34 Original July 19, 1974.

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This incorporation by reference was approved by the Director of

the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR

part 51. Copies may be obtained from Boeing Commercial Airplane

Group, P.O. Box 3707, Seattle, Washington 98124-2207. Copies may be

inspected at the FAA, Transport Airplane Directorate, 1601 Lind

Avenue, SW., Renton, Washington; or at the Office of the Federal

Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

(f) This amendment becomes effective on June 20, 1994.

Issued in Renton, Washington, on May 10, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-11780 Filed 5-18-94; 8:45 am]

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