AD 94-17-17 ACTIVE

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 747 series airplanes, that requires inspections of the inboard and outboard strut chords, stiffeners, and web to detect cracks and loose fasteners; repair of the chords, stiffeners, or web, if necessary; and replacement of any loose fastener. This amendment is prompted by reports of fatigue cracks and loose fasteners found in the forward lower spar web of the inboard strut on Model 747 series airplanes equipped with Pratt & Whitney JT9D-3 and JT9D-7 series engines. The actions specified by this AD are intended to prevent separation of the strut from the wing of the airplane due to fatigue cracking.

Final rule

94-17-17 BOEING: Amendment 39-9012. Docket 94-NM-05-AD. \n\n\tApplicability: Model 747 series airplanes equipped with Pratt & Whitney JT9D-3 or JT9D-7 series engines, excluding JT9D-70 series engines; line numbers 001 through 510 inclusive; certificated in any category. \n\n\tCompliance: Required as indicated, unless accomplished previously. \n\n\tTo prevent separation of the strut from the wing of the airplane due to fatigue cracking, accomplish the following: \n\n\t(a)\tPerform a detailed visual inspection of the inboard and outboard strut forward lower spar chords, stiffeners, and web to detect cracks and loose fasteners, in accordance with the procedures described in Boeing Service Bulletin 747-54-2160, dated September 9, 1993, at the time specified in paragraph (a)(1), (a)(2), or (a)(3) of this AD, as applicable. Repeat this inspection thereafter at intervals not to exceed 2,000 landings or 8,000 hours time-in-service, whichever occurs first. \n\n\t\t(1)\tFor airplanes that have accumulated less than 6,000 total landings as of the effective date of this AD: Perform the initial inspection at the later of the times specified in paragraphs (a)(1)(i) and (a)(1)(ii) of this AD. \n\n\t\t\t(i)\tPrior to the accumulation of 6,000 total landings on the strut. \t\t\t\t\n\t\t\t\tOr \n\n\t\t\t(ii)\tWithin 12 months after the effective date of this AD. \n\n\t\t(2)\tFor airplanes that have accumulated 6,000 or more total landings, but less than 15,000 total landings, as of the effective date of this AD: Perform the initial inspection within 9 months after the effective date of this AD. \n\n\t\t(3)\tFor airplanes that have accumulated 15,000 or more total landings as of the effective date of this AD: Perform the initial inspection within 6 months after the effective date of this AD. \n\n\t(b)\tIf any crack is found during any inspection required by paragraph (a) of this AD, prior to further flight, repair in accordance with procedures specified in Chapter 54-10-03 of the 747 Structural Repair Manual. \n\n\t(c)\tIf any loose fastener is found during any inspection required by paragraph (a) of this AD, prior to further flight, replace the fastener in accordance with procedures specified in Chapter 51-30-02 of the 747 Structural Repair Manual. \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: 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 toa location where the requirements of this AD can be accomplished. \n\n\t(f)\tThe inspection shall be done in accordance with Boeing Service Bulletin 747-54-2160, dated September 9, 1993. 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 30, 1994.

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 747 series airplanes was published in the Federal Register on March 17, 1994 (59 FR 12560). That action proposed to require inspections of the inboard and outboard strut chords, stiffeners, and web to detect cracks and loose fasteners; repair of the chords, stiffeners, or web, if necessary; and replacement of any loose fastener. \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 proposed rule. \n\n\tOne commenter, Boeing, requests that the proposed rule be withdrawn since only six cracks have been reported in the 20 years these airplanes have been in service. In addition, these cracks were found only on the inboard struts during normal maintenance. Boeing asserts that the probability of another strut with a pre-existing firewall crack experiencing loads that approach the ultimate design condition is extremely remote. Boeing adds, however, that the airplane maintenance manual is being revised to include an inspection of the lower spar web in the case of an unusual overload event. Boeing also indicates that the results of one operator's inspections revealed only one loose fastener on one airplane out of 24 inspected. Boeing states that the firewall webs will be reinforced during the strut modification program referenced in the proposal. \n\n\tThe FAA does not concur with the commenter's request to withdraw this AD. The FAA has determined that inspections mandated by the issuance of this AD are necessary to correct the unsafe condition presented by separation of the strut from the wing of the airplane due to fatigue cracking in the web. These inspections are necessary until the 0.025-inch inboard webs and the 0.032-inch outboard webs are replaced as part of the strut modification program discussed in the proposal. Sonic fatigue analysis performed by Boeing in support of the decision to replace the webs has revealed that the existing inboard and outboard webs on Model 747 series airplanes addressed in this AD are inadequate. Recent findings from an investigation of an incident involving a Model 747 series airplane indicate that fatigue cracking found on the web of the airplane resulted from flexing or vibration of the web material. This fatigue cracking resulted in the in-flight loss of an engine during airplane operation in severe turbulence. Since the FAA considers it probable for other airplanes to have cracked webs, and since no requirements exist to restrict airplane operation in severe turbulence, the FAA finds that this AD action is necessary in order to prevent in-flight engine loss. \n\n\tSeveral commenters request that inspections of the outboard webs be removed from the proposed rule so that the AD is consistent with the referenced service bulletin. The commenters indicate that no cracking or loose fasteners have been found on the outboard webs. The commenters also state that the design thickness of the webs (0.032 inch for the outboard web; 0.025 inch for the inboard web) represents a 28 percent difference, which should be considered significant. \n\n\tThe FAA does not concur. The thickness of the inboard web of the incident airplane mentioned previously measured 0.027 inch. The FAA finds that there is little difference between the inboard and outboard webs with standard manufacturing tolerances. Each web is similar in configuration, loading, and sonic environments. In light of these considerations, the FAA concludes that the outboard strut web is subject to the same unsafe condition as the inboard strut web and must be included in this AD. \n\n\tSeveral commenters request that the compliance times for inspections of the outboard strut be extended. One commenter requests that the compliance times be revised to align with those recommended in the referenced service bulletin, less the amount of time provided for public comment on the proposal. Another commenter requests that the compliance times be extended to be consistent with a load analysis based upon the actual web thickness of the outboard strut. This commenter assumes that the proposed repetitive inspection interval is based on a load analysis of the inboard strut structure. Another commenter requests that the compliance times for the initial inspection of the inboard strut be extended to 15 months. The commenter does not provide any justification for this request, however. \n\n\tThe FAA concurs that the compliance times can be extended somewhat. The compliance times specified in paragraphs (a)(1) and (a)(3) of this AD have been revised to reflect the times recommended in the manufacturer's service bulletin. The FAA finds that the proposed compliance time of 6 months specified for airplanes that have accumulated 6,000 or more total landings, but less than 15,000 total landings (those airplanes applicable to paragraph (a)(2) of the final rule) may be extended to 9 months. Although the service bulletin specifies a recommended compliance time of 12 months for those airplanes, the FAA finds that, based on the service history of affected Model 747 series airplanes that fall within this group, 9 months represents the maximum interval of time allowable for these airplanes to continue to operate without compromising safety. \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\tThis is considered to be interim action. The manufacturer has advised that it is currently developing a modification program for the engine strut that will positively address the unsafe condition addressed by this AD. Once this modification program is developed, approved, and available, the FAA may consider additional rulemaking. \n\n\tThere are approximately 380 Model 747 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 140 airplanes of U.S. registry will be affected by this AD, that it will take approximately 22 work hours per airplane to accomplish the required actions, and that the average labor rate is $55 per work hour. Based on these figures, the total cost impact of the AD on U.S. operators is estimated to be $169,400, or $1,210 per airplane. \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 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\n List 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\nPART 39 - AIRWORTHINESS DIRECTIVES \n\n\t1.\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\n\t2.\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.

Tim Backman, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Transport Airplane Directorate, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (206) 227-2776; fax (206) 227-1181.