AD 96-17-04

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 737-100 and -200 series airplanes, that requires inspections to detect cracking of the support fittings of the Krueger flap actuator and, if necessary, replacement of existing fittings with new steel fittings and modification of the aft attachment of the actuator. This amendment is prompted by reports of cracking due to fatigue and stress corrosion of the support fittings of the Krueger flap actuator. The actions specified by this AD are intended to prevent such cracking, which could result in fracturing of the actuator attach lugs, separation of the actuator from the support fitting, severing of the hydraulic lines, and resultant loss of hydraulic fluids. These conditions, if not corrected, could result in possible failure of one or more hydraulic systems, and subsequent reduced controllability of the airplane.

Final rule

96-17-04 BOEING: Amendment 39-9712. Docket 96-NM-04-AD. \n\n\tApplicability: Model 737-100 and -200 series airplanes, line positions 001 through 813 inclusive, 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 (d) 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 possible failure of one or more hydraulic systems and subsequent reduced controllability of the airplane, accomplish the following: \n\n\t(a)\tWithin one year after the effective date of this AD, perform an eddy current inspection to detect cracking of the support fitting of the Krueger flap actuator, in accordance with Boeing Service Bulletin 737-57-1129, Revision 1, dated October 30, 1981, as revised by Notices of Status Change 737-57-1129NSC1, dated July 23, 1982; 737-57-1129 NSC2, dated April 14, 1983; and 737-57-1129 NSC 3, dated May 18, 1995. \n\n\t\t(1)\tIf no cracking is found, repeat the inspection required by paragraph (a) of this AD thereafter at intervals not to exceed 3,000 hours time-in-service. \n\n\t\t(2)\tIf any cracking is found, prior to further flight, accomplish the replacement and modification specified in paragraph (b) of this AD. \n\n\t(b)\tReplacement of the support fitting with a steel fitting and modification of the actuator aft attachment in accordance with Boeing Service Bulletin 737-57-1129, Revision 1, dated October 30, 1981, as revised by Notices of Status Change 737-57-1129NSC1, dated July 23, 1982; 737-57-1129 NSC2, dated April 14, 1983; and 737-57-1129 NSC 3, dated May 18, 1995; constitutes terminating action for the repetitive inspections required by this AD. \n\n\t(c)\tAs of the effective date of this AD, no person shall install a support fitting having part number 69-37892-9, 69-37892-10, 69-37893-1, or 69-37893-2 on the Krueger flap actuator of any airplane. \n\n\t(d)\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 \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(e)\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(f)\tThe inspections, replacement, and modification shall be done in accordance with Boeing Service Bulletin 737-57-1129, Revision 1, dated October 30, 1981, as revised by Notice of Status Change 737-57-1129NSC1, dated July 23, 1982; Notice of Status Change 737-57-1129 NSC2, dated April 14, 1983; and Notice of Status Change 737-57-1129 NSC 3, dated May 18, 1995. 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(g)\tThis amendment becomes effective on September 17, 1996.

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 737-100 and -200 series airplanes was published in the Federal Register on March 13, 1996 (61 FR 10294). That action proposed to require inspections to detect cracking of the support fittings of the Krueger flap actuator and, if necessary, replacement of existing fittings with new steel fittings and modification of the aft attachment of the actuator. \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 Support for the Proposal \n\n\tOne commenter supports the proposal. \n\nRequest to Revise Proposed Inspection Requirements \n\n\tThe Air Transport Association (ATA), on behalf of its member operators, requests that the proposed requirement to perform repetitive eddy current inspections be replaced with a requirement to perform close visual inspections at 3,000-flight hour intervals, followed by an eddy current inspection or replacement of the fitting within a 4-year period. This commenter maintains that this alternative inspection program is: \n\n\t1. more consistent with the recommendations of the airframe manufacturer; \n\t2. equivalent in safety to that proposed in the notice; and \n\t3. more cost effective. \n\n\tFurther, this commenter states that, while the proposed eddy current inspection may be viewed as a more critical inspection process, it is not necessary to respond to the airworthiness concern. This commenter contends that, in order to determine whether a more stringent process is required (i.e., more stringent than the manufacturer's recommendations), the FAA should review service history data to determine whether cracking of the subject support fittings has actually become a fleet-wide problem. The commenter maintains that, while the one incident described in the preamble to the notice was certainly of concern, there is insufficient data to indicate that cracked support fittings is an industry problem. \n\n\tThe FAA does not concur. As explained in the preamble to the notice, the subject cracking in the fittings is attributed to stress corrosion combined with fatigue. The crack growth rate for such cracking is not known; however, it is known that material that the fitting is made from, 7075-T6 aluminum, is highly susceptible to stress corrosion cracking and has low toughness. It is also known that the critical crack size for this fitting is 0.165 inch. Cracks of this small size cannot be found with a high degree of confidence using a visual inspection technique. An eddy current inspection is a much more reliable method of finding such small cracks. \n\n\tAs for the service history of the subject problem, there have been several reports of cracking found in actuator attach support fitting assemblies on a number of in-service Model 737 series airplanes. There also have been two accidents involving hydraulic system failures that were associated with the failure of the actuator attach lugs on the support fittings. The FAA considers this a sufficient amount of service history to demonstrate that a potential unsafe condition associated with the subject cracking exists in airplanes equipped with the subject fittings. \n\n\tIn light of the small critical crack size, the high susceptibility to stress corrosion cracking of 7075-T6 material, and the ample service history relative to the addressed unsafe condition, the FAA does not find that the commenter's suggested alternative inspection program would provide an acceptable level of safety compared to that required by this final rule. \n\nRequest to Revise Proposed Inspection Intervals \n\n\tOne commenter requests that the proposed inspections be required in terms of flight cycles, rather than in terms of time-in-service. The commenter states that, because fatigue cracking of the actuator support fitting is caused by cycling of the Krueger flap, the maximum inspection intervals should be limited by flight cycles, not flight hours. \n\n\tThe FAA does not concur. The cracking mechanism associated with the addressed problem is stress corrosion cracking combined with fatigue. Although the commenter is correct that fatigue is cycle-driven, stress corrosion cracking is time- or flight hour-driven, since it is caused by a sustained tensile stress in a corrosive environment. Therefore, the FAA finds that a flight hour (time-in-service) inspection interval is appropriate for these inspections. \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\n Cost Impact \n\n\tThere are approximately 727 Model 737-100 and -200 series airplanes of the affected design in the worldwide fleet. The FAA estimatesthat 270 airplanes of U.S. registry will be affected by this AD, that it will take approximately 12 work hours per airplane (6 work hours per wing) to accomplish the required actions, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the AD on U.S. operators is estimated to be $194,400, or $720 per airplane, per inspection. \n\n\tThe 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 Regulatory 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 part39 of the Federal Aviation Regulations (14 CFR part 39) as follows: \n\nPART 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 amended by adding the following new airworthiness directive: