AD 94-10-10 ACTIVE

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.

Final rule

94-10-10 BOEING: Amendment 39-8917. Docket 93-NM-151-AD. \n\n\tApplicability: Model 747-100, -200, and -300 series airplanes; equipped with Pratt & Whitney JT9D series engines; certificated in any category. \n\n\tCompliance: Required as indicated, unless accomplished previously. \n\n\tTo prevent inadvertent engine fan thrust reverser deployment during flight, which could result in reduced controllability of the airplane, accomplish the following: \n\n\t(a)\tWithin 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. \n\nNOTE: 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. \n\n\t(b)\tAs of 18 months after the effective date of this AD, no person shallinstall 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. \n\n\t(c)\tAn 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. \n\nNOTE: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO. \n\n\t(d)\tSpecial flight permits may be issued in accordance with sections 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. \n\n\t(e)\tThe 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: \n\n\nPage Number\nRevision Level\nShown on Page \n Date \nShown on Page \n1-5, 12-13,\n19-21, 35, \n38, 46 \n4\nMarch 23, 1989 \n6-7, 18, 39,\n41 \n3\nAugust 27, 1987 \n15, 23-24, 36,\n42-45, 47-49\n30-31, 40\n2\nJanuary 9, 1976 \n10, 16, 22,\n37 \n1\nMarch 28, 1975 \n8-9, 11, 14,\t\t\n17, 25-29, \n32-34 \n Original\nJuly 19, 1974 \n\n\tThis 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. \n\n\t(f)\tThis amendment becomes effective on June 20, 1994.

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. \n\n\tInterested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received. \n\n\tOne commenter supports the proposal. \n\n\tSeveral 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 valveinstallation. 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. \n\n\tFurther, 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 thestow 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. \n\n\tAdditionally, 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. \n\n\tThe 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). \n\n\tThere 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. \n\n\tNone of the other 30 incidents of in-flight deployments of the thrust reversers on Model 747's have occurred on engines equipped withthe 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. \n\n\tFor these reasons, the FAA has determined that installation of the solenoid-operated shut- off valve is appropriate and warranted. \n\n\tSeveral 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 wereable 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 deployat 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. \n\n\tSeveral 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. \n\n\tNumerous commenters request that the proposed compliance time of 24 months for modification be extended to between 36 months and5 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 finalrule has been revised accordingly. \n\n\tCertain 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. \n\n\tOne 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 "gearswitcher 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. \n\n\tOther 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 onoperators, 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. \n\n\tNumerous commenters request that the economic impact information presented in the preamble to the noticebe 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. \n\n\tThe economic impact information has also been revised to update the current number of airplanes that will be affected by this AD. \n\n\tAfter 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. \n\n\tThere 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. \n\n\tThe 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. \n\n\tThe 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. \n\n\tThe 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. \n\n\tA fullcost-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. \n\n\tThe 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. \n\n\tFor 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." \n\nList of Subjects in 14 CFR Part 39 \n\nAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the Amendment \n\nAccordingly, 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: \n\nPART 39 - AIRWORTHINESS DIRECTIVES \n\n1.\tThe 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. § 39.13 - (Amended) \n\n2.\tSection 39.13 is amended by adding the following new airworthiness directive:

The service information referenced in this AD may be obtained from Boeing\nCommercial 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.

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.