AD 97-04-01

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 737 series airplanes, that requires modification of the aileron centering spring and trim mechanism. This amendment is prompted by a review of the design of the flight control systems on Model 737 series airplanes. The actions specified by this AD are intended to prevent jamming of the aileron control system during flight due to fracturing of the springs in the aileron centering units; this condition, if not corrected, could result in reduced lateral control of the airplane.

Final rule

97-04-01 BOEING: Amendment 39-9925. Docket 96-NM-153-AD. \n\n\tApplicability: Model 737 series airplanes; as listed in Boeing Service Bulletin 737-27-1155, dated October 26, 1989; as revised by Notices of Status Change No. \n737-27-1155NSC1, dated January 25, 1990, No. 737-27-1155NSC2, dated February 15, 1990, and No. 737-27-1155NSC3, dated May 17, 1990; 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 otherwise 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 jamming of the aileron control system during flight, which could result in reduced lateral control of the airplane, accomplish the following: \n\n\t(a)\tWithin 18 months after the effective date of this AD, accomplish the requirements of paragraphs (a)(1), (a)(2), and (a)(3) of this AD, as applicable, in accordance with Boeing Service Bulletin 737-27-1155, dated October 26, 1989; as revised by Notice of Status Change No. 737-27-1155NSC1, dated January 25, 1990, and Notice of Status Change No. 737-27-1155NSC2, dated February 15, 1990, and Notice of Status Change No. 737-27-1155NSC3, dated May 17, 1990. \n\n\t\t(1)\tFor Groups 1 and 2 airplanes: Replace the aileron centering springs, part number (P/N) 69-39429-2, with improved springs, P/N 69-39429-3, in accordancewith the service bulletin and Notices of Status Change. \n\n\t\t(2)\tFor Groups 1 and 2 airplanes: Install a two-piece plug, P/N 69-78072-1, in the weight reduction hole in the feel cam in accordance with the service bulletin and Notices of Status Change. \n\n\t\t(3)\tFor Group 1 airplanes: Replace the two eyebolts, P/N 69-39423-1, of the aileron centering spring attachment with new eyebolts, P/N 69-74646-1, in accordance with the service bulletin and Notices of Status Change. \n\n\t(b)\tAs of the effective date of this AD, no person shall install the items specified in paragraphs (b)(1) and (b)(2) of this AD on any airplane, as specified: \n\n\t\t(1)\tFor Groups 1 and 2 airplanes: Aileron centering springs having P/N 69-39429-2 shall not be installed. \n\n\t\t(2)\tFor Group 1 airplanes: Eyebolts, P/N 69-39423-1, of the aileron centering spring attachment shall not be installed. \n\n\t(c)\tAn alternative method of compliance or adjustment of the compliance time that provides an acceptable level ofsafety 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\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 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 replacement and installation shall be done in accordance with Boeing Service Bulletin 737-27-1155, dated October 26, 1989; as revised by Notice of Status Change No. 737-27-1155NSC1, dated January 25, 1990, and Notice of Status Change No. 737-27-1155NSC2, dated February 15, 1990, and Notice of Status Change No. 737-27-1155NSC3, dated May 17, 1990. 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. \n\n\t(f)\tThis amendment becomes effective on March 21, 1997.

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 certain Boeing Model 737 series airplanes was published in the Federal Register on August 28, 1996 (61 FR 44247). That action proposed to require modification of the aileron centering spring and trim mechanism. \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\nRequest to Revise Statement of 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\nRequest to Withdraw the Proposal\n\n\tOne commenter contends that the proposal is not justified since it cannot be supported by data. The commenter does not consider that the proposal contributes to improving the safety aspects of Model 737 airplanes. The commenter states that the Critical Design Review (CDR) team's report does not indicate that there is any evidence to tie the referenced service documents to any in- service problems or accidents. The commenter adds that the FAA has not indicated that it has reviewed any routine component tear-down reports that would support the proposed actions. The commenter concludes that the FAA does not understand the enormity of the proposed action. The FAA infers from these remarks that the commenter requests the proposed rule be withdrawn.\n\n\tThe FAA does not concur. The FAA has received at least 26 reports from two operators of Model 737 series airplanes indicating that fractured springs were found in the aileron centering units. The cause of the fracturing was attributed to fatigue cracking. Two of the reports indicated that the fractured springs had become lodged in a centering cam hole, which caused binding of the aileron control system. The FAA's position is that this condition is a potential unsafe condition that must be corrected in order to ensure the safety of the affected fleet. \n\nRequest to Delay Issuance of the Final Rule \n\n\tThe Air Transport Association (ATA) of America, on behalf of two of its members, requests that the FAA postpone issuing the final rule untilBoeing revises the service bulletin cited in the proposal to incorporate the three Notice of Status Change documents referenced in the proposed AD. The ATA indicates that this will ensure no confusion exists concerning service bulletin recommendations.\n\n\tThe FAA does not concur with the commenter's request to delay issuance of the final rule. The FAA has been advised that Boeing has no plans to revise the referenced service bulletin to incorporate the Notice of Status Change documents. To delay this action would be inappropriate, since the FAA has determined that an unsafe condition exists and that the required modification must be accomplished to ensure continued safety. \n\nRequests to Revise Compliance Time \n\n\tOne commenter requests that the proposed compliance time be shortened from 18 months to 12 months to provide an acceptable level of safety. The commenter provides no data in support of its request.\n\n\tA second commenter requests that the proposed compliance time be extended beyond 18 months to ensure that adequate parts and trained personnel are available to accomplish the modification. The commenter did not submit data to substantiate its request. \n\n\tThe FAA does not concur with the commenters' requests to revise the compliance time. As explained in the preamble to the proposal, the FAA's intent is that the modification be performed during a regularly scheduled maintenance visit for the majority of the affected fleet when the airplanes would be located at a base where special equipment and trained personnel would be readily available, if necessary. The FAA finds that 18 months corresponds closely to the interval representative of most of the affected operators' normal maintenance schedules. The FAA considers that this interval will provide an acceptable level of safety. \n\nRequest to Allow Measurement of Thickness of Aileron Centering Spring \n\n\tThe ATA, on behalf of one of its members, requests that operators be allowed to measure the thickness of the aileron centering spring in lieu of determining the part number. The ATA indicates that the part number of the aileron centering spring cannot be determined by inspecting the part because it is impossible to read the part number on the spring. However, the difference in diameter between the spring having part number 69-39429-2 and the spring having part number 69-39429-3 can be distinguished by measurement. The ATA states that if a part has a thickness greater than 0.13 inch, this should constitute compliance with the AD. One ATA member indicates that drawings show that the spring having part number 69-39429-2 is made from 0.125-inch diameter wire, and that the spring having part number 69-39429-3 is made from 0.135-inch diameter wire. \n\n\tThe FAA does not concur. The FAA acknowledges that, except for a removable tag, the spring is not marked with a part number. However, it is not necessary to read the part number of the spring. If an operator previously has performed the actions described in the referenced service bulletin, the correct spring should be installed on the airplane; if not, an incorrect spring would have been installed. In the unlikely event that the spring has been changed due to a maintenance action apart from incorporation of the referenced service bulletin, it is difficult to determine which spring is installed. The only way to ensure that the proper spring is installed is to perform the actions of the referenced service bulletin. Further, the FAA does not agree that the springs having part numbers 69-39429-2 and 69-39429-3 are made from different diameter wire; the FAA has determined that both springs are made from 0.135-inch diameter wire.\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 1,631 Model 737 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 830 airplanes of U.S. registry will be affected by this AD. \n\n\tThe FAA estimates that 485 Group 1 airplanes will be affected by this AD. For Group 1 airplanes, the FAA estimates that it will take approximately 2 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 $707 per airplane. Based on these figures, the cost impact of the AD on U.S. operators of Group 1 airplanes is estimated to be $401,095, or $827 per airplane. \n\n\tThe FAA estimates that 345 Group 2 airplanes will be affected by this AD. For Group 2 airplanes, the FAA estimates that it will take approximately 2 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 $224 per airplane. Based on these figures, the cost impact of the AD on U.S. operators of Group 2 airplanes is estimated to be $118,680, or $344 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, Incorporation by reference, 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\n\t2. Section 39.13 is amended by adding the following new airworthiness directive: