AD 98-02-01 SUPERSEDED

This amendment adopts a new airworthiness directive (AD), applicable to all Boeing Model 737-100, -200, -300, -400, and -500 series airplanes, that requires removing the yaw damper coupler; replacing its internal rate gyroscope with a new or overhauled unit; and performing a test to verify the integrity of the yaw damper coupler, and repair, if necessary. This amendment is prompted by an FAA determination that requiring replacement of the internal rate gyroscope will significantly increase the reliability of the yaw damper coupler system. The actions specified by this AD are intended to prevent sudden uncommanded yawing of the airplane due to potential failures within the yaw damper system, and consequent injury to passengers and crewmembers.

Final rule

98-02-01 BOEING: Amendment 39-10283. Docket 97-NM-45-AD. \n\n\tApplicability: All Model 737-100, -200, -300, -400, and -500 series airplanes, certificated in any category. \n\n\tNOTE 1: This AD applies to each airplane identified in the preceding applicability provision, regardless of whether it has been modified, altered, or repaired in the area subject to the requirements of this AD. For airplanes that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (c) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and, if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it. \n\n\tCompliance: Required as indicated, unless accomplished previously. \n\n\tTo prevent sudden uncommanded yawing of the airplane due to potential failures within the yaw damper system, and consequent injury to passengers and crewmembers, accomplish the following: \n\n\t(a)\tRemove the yaw damper coupler, replace the internal rate gyroscope with a new or overhauled unit, and perform a test to verify the integrity of the yaw damper coupler, all in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate, at the applicable time specified in paragraph (a)(1) or (a)(2) of this AD. \n\n\t\t(1)\tFor airplanes on which the yaw damper coupler has accumulated less than 12,000 hours time-in-service since its last maintenance activity as of the effective date of this AD: Perform the actions within 6,000 hours time-in-service after the effective date of this AD; and thereafter at intervals not to exceed 9,000 hours time-in-service. \n\n\t\t(2)\tFor airplanes on which the yaw damper coupler has accumulated 12,000 or more hours time-in-service since its last maintenance activity as of the effective date of this AD: Perform the actions within 3,000 hours time-in-service after the effective date of this AD; and thereafter at intervals not to exceed 9,000 hours time-in-service. \n\n\t(b)\tIf the yaw damper coupler fails the test required by paragraph (a) of this AD, prior to further flight, repair the coupler in accordance with a method approved by the Manager, Seattle ACO. \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 ACO. 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\tNOTE 2: 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 maybe 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)\tThis amendment becomes effective on February 17, 1998.

A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an airworthiness directive (AD) that is applicable to all Boeing Model 737-100, -200, -300, -400, and -500 series airplanes was published in the Federal Register on June 25, 1997 (62 FR 34185). That action proposed to require removing the yaw damper coupler; replacing its internal rate gyroscope with a new or overhauled unit; and performing a test to verify the integrity of the yaw damper coupler, and repair, if necessary. \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\nSupport for the Proposal\n\n\tThree commenters support the proposal.\n\n Findings of Critical Design Review Team \n\n\tOne commenter requests the second paragraph of the Discussion section that appeared in the preamble to the proposed rule be revised to accurately reflect the findings of the Critical Design Review (CDR) team. The commenter asks that the FAA delete the one sentence in that paragraph, which read: "The recommendations of the team include various changes to the design of the flight control systems of these airplanes, as well as correction of certain design deficiencies." The commenter suggests that the following sentences should be added: "The team did not find any design issues that could lead to a definite cause of the accidents that gave rise to this effort. The recommendations of the team include various changes to the design of the flight control systems of these airplanes, as well as incorporation of certain design improvements in order to enhance its already acceptable level of safety." \n\n\tThe FAA does not find that a revision to this final rule in the manner suggested by the commenter is necessary, since the Discussion section of a proposed rule does not reappear in a final rule. The FAA acknowledges that the CDR team did not find any design issue that could lead to a definite cause of the accidents that gave rise to this effort. However, as a result of having conducted the CDR of the flight control systems on Boeing Model 737 series airplanes, the team indicated that there are a number of recommendations that should be addressed by the FAA for each of the various models of the Model 737. In reviewing these recommendations, the FAA has concluded that they address unsafe conditions that must be corrected through the issuance of AD's. Therefore, the FAA does not concur that these design changes merely "enhance (the Model 737's) already acceptable level of safety." \n\nConnection Between the Proposed Rule and AD 97-14-03 \n\n\tSeveral commenters request that the FAA clarify how the requirements of AD 97-14-03, amendment 39-10060 (62 FR 34623, June 27, 1997), which requires replacement of the yaw damper coupler with a new unit (that has yet to be certified), and the proposal affect each other. The commenters state that the planned design required by AD 97-14-03 will eliminate the subject of the proposed rule (use of an electro-mechanical internal rate gyro). One commenter suggests that accomplishment of the requirements of AD 97-14-03 be considered as an alternative method of compliance for the actions specified in the proposal. Another commenter requests that accomplishment of the requirements of AD 97-14-03 be considered terminating action for the requirements of the proposal. Further, one commenter requests that a note be added to the proposed AD indicating whether the actions required by AD 97-14-03 terminate the test and replacement required by this proposed rule, or whether those test and replacement requirements must be continued. \n\n\tThe FAA clarifies that the requirements of this AD and AD 97-14-03 are related. This final rule requires, in part, removal of the yaw damper coupler, and replacement of its internal rate gyroscope with a new or overhauled unit. AD 97-14-03 requires replacement of the yaw damper coupler with a new unit. However, since that new unit has not yet been certified, the FAA cannot consider the requirements of AD 97-14-03 to be terminating action for the requirements of this AD, and the actions required by paragraph (a) of this AD must be accomplished on a repetitive basis. Once a new yaw damper coupler is designed, developed, and certified, the FAA may consider installation of that new unit to be terminating action for the requirements of this AD.\n\nTesting of the Yaw Damper Coupler \n\n\tOne commenter requests clarification concerning the requirement for testing of the yaw damper coupler specified in the proposal. Specifically, the commenter asks whether the yaw damper coupler must be tested in a shop or on the airplane. The commenter also requests clarification concerning which documents should be referenced for test procedures (i.e., the Airplane Maintenance Manual or the Component Maintenance Manual). The commenter also suggests thatthe test procedures be provided in a logical sequence based on whether the test is accomplished on the airplane or in a shop. (The commenter submitted sample procedures for tests accomplished on the airplane or in a shop.) \n\n\tThe FAA concurs that clarification is necessary. Since the manufacturer currently has no service information that describes maintenance procedures for the yaw damper coupler, this AD requires that maintenance actions be accomplished in accordance with a method approved by the FAA. Therefore, the individual operator is responsible to establish logical, sequential maintenance procedures (for accomplishment of actions either in a shop or on the airplane), and to submit those procedures to the FAA for approval. \n\nLast Maintenance Activity \n\n\tOne commenter requests clarification of the phrase "since last maintenance activity." The commenter states that because this phrase is unclear, the FAA should publish another proposal. \n\n\tThe FAA clarifies that the phrase "since last maintenance activity" applies to maintenance activity in which it was positively established that the yaw damper coupler was functioning properly and did not require repair. However, the FAA considers that the phrase is understandable and is commonly used throughout the aviation industry. Therefore, the FAA does not concur that this phrase is unclear, or that publication of another proposal is warranted.\n\nSignificant Increase in Reliability of Yaw Damper Coupler System \n\n\tOne commenter, the manufacturer, requests that the word "significantly" be omitted from the following phrase, which appeared in the Discussion section of the proposal: "The FAA made this determination ... replacement of the internal rate gyroscope ... will significantly increase the reliability of the yaw damper coupler system." The commenter states that the data it provided the FAA indicate that there would be a maximum increase in reliability of 30 to 40 percent, which the commenter considers to be a moderate (rather than significant) increase in reliability. \n\n\tThe FAA does not concur. There are no specific quantitative or standard definitions of the terms "significant" and "moderate." In this case, the FAA considers it appropriate to define an increase in reliability of 30 to 40 percent as "significant." Additionally, since the Discussion section of a proposal does not reappear in a final rule, the FAA finds that no change to this final rule is necessary. \n\nRudder Limiting Device \n\n\tOne commenter, the manufacturer, requests that reference to the "rudder limiting device" be removed from the Discussion section of the proposal. The commenter states that the discussion of the rudder limiting device is confusing because it is not related to the yaw damper failure modes. In addition, the commenter points out that certain information discussing the rudder limiting devices is outdated. \n\n\tThe FAA acknowledges that there may have been some confusion about including adiscussion of the rudder limiting device; however, the FAA considers that the confusion would not be so great as to warrant not including that information. Furthermore, the Discussion section of the proposal does not reappear in the final rule. Therefore, the FAA finds that no change to this final rule is necessary. \n\nConclusion \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 as proposed.\n\nCost Impact \n\n\tThere are approximately 2,675 Model 737 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 1,091 airplanes of U.S. registry will be affected by this proposed AD, that it would take between 8 and 13 work hours per airplane to accomplish the required actions, and that the average labor rate is $60 per work hour. Required parts will cost approximately $2,500 per airplane. Based on these figures, the cost impactof the AD on U.S. operators is estimated to be between $3,251,180 and $3,578,480, or between $2,980 and $3,280 per airplane. \n\n\tThe cost impact figures discussed above are 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\nRegulatory Impact \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\n\tAir transportation, Aircraft, Aviation safety, Safety. \n\nAdoption of the Amendment \n\n\tAccordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:\n\n PART 39 - AIRWORTHINESS DIRECTIVES \n\n\t1. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701. § 39.13 (Amended) \n\t2. Section 39.13 is amendedby adding the following new airworthiness directive:

Information pertaining to this rulemaking action may be examined at the Federal Aviation Administration (FAA), Transport Airplane Directorate, Rules Docket, 1601 Lind Avenue, SW., Renton, Washington.

T. Tin Truong, Aerospace Engineer, Systems and Equipment Branch, ANM-130S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 227-2552; fax (425) 227-1181.