91-15-24 BOEING: Amendment 39-7089. Docket No. 91-NM-14-AD. \n\n\tApplicability: Model 747 series airplanes, line positions 2 through 720 for Auxiliary Power Unit (APU) pneumatic ducts in body sections 44, 46, and 48; and line positions 721 through 734 for APU pneumatic ducts in body sections 44 and 48; certificated in any category. \n\n\tCompliance: Required as indicated, unless previously accomplished. \n\n\tTo prevent damage to adjacent structure, pneumatic ducts, hydraulic lines, and/or electrical wiring as a result of failure of Auxiliary Power Unit (APU) pneumatic ducts, accomplish the following: \n\n\tA.\tPrior to the accumulation of 7,000 flight cycles, or within the next 3,000 flight cycles after the effective date of this AD, whichever occurs later, conduct a penetrant inspection, proof pressure test, and penetrant inspection again to detect cracks or ruptures in the APU pneumatic ducts in accordance with the Accomplishment Instructions, Items A. through G., K., O., and P.of Boeing Service Bulletin 747-36-2081, dated November 29, 1990. If cracks or ruptures are detected, prior to further flight, repair or replace in accordance with the service bulletin. The stress relieving procedure specified in Accomplishment Instructions, Items H., I., and J. of the service bulletin may be accomplished in conjunction with the penetrant inspection and proof pressure test required by this paragraph, and constitutes terminating action for the requirements of paragraph B. of this AD for all APU pneumatic ducts. \n\n\tB.\tPrior to the accumulation of 7,000 flight cycles after the accomplishment of the initial inspection required by paragraph A. of this AD, conduct an additional penetrant inspection, proof pressure test, and penetrant inspection of the APU pneumatic ducts, and stress relieve the APU pneumatic duct assemblies, in accordance with the Accomplishment Instructions, Items A. through K., O., and P. of Boeing Service Bulletin 747-36-2081, dated November 29, 1990.If cracks or ruptures are detected, prior to further flight, repair or replace in accordance with the service bulletin. \n\n\tC.\tReplacement of all APU pneumatic ducts in accordance with Accomplishment Instructions A., B., L. through O., and P. of Boeing Service Bulletin 747-36-2081, dated November 29, 1990; or in accordance with Boeing Service Bulletin 747-36-2092, dated June 28, 1990; constitutes terminating action for the requirements of this AD. \n\n\tD.\tAn alternative method of compliance or adjustment of the compliance time, which provides an acceptable level of safety, may be used when approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate. \n\n\tNOTE: The request should be forwarded through an FAA Principal Maintenance Inspector, who may concur or comment and then send it to the Manager, Seattle ACO. \n\n\tE.\tSpecial flight permits may be issued in accordance with FAR 21.197 and 21.199 to operate airplanes to a base in order tocomply with the requirements of this AD. \n\n\tF.\tThe penetrant inspections, proof pressure testing, and stress relieving shall be done in accordance with Boeing Service Bulletin 747-36-2081, dated November 29, 1990. APU pneumatic duct replacement shall be done in accordance with either Boeing Service Bulletin 747- 36-2081, dated November 29, 1990, or Boeing Service Bulletin 747-36-2092, dated June 28, 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. Copies may be inspected at the FAA, Northwest Mountain Region, Transport Airplane Directorate, 1601 Lind Avenue S.W., Renton, Washington, or at the Office of the Federal Register, 1100 L Street N.W., Room 8401, Washington, D.C. \n\n\tThis amendment (39-7089, AD 91-15-24) becomes effective on September 9, 1991.