91-11-01 BOEING: Amendment 39-6997. Docket No. 90-NM-210-AD. Supersedes AD 86- 23-06 R1. \n\n\tApplicability: Model 747 series airplanes, listed in Boeing Alert Service Bulletin 747- 53A2265, Revision 7, dated January 25, 1990, certificated in any category. \n\n\tCompliance: Required as indicated, unless previously accomplished. \n\n\tTo prevent sudden decompression of the fuselage, accomplish the following: \n\n\tA.\tWithin the next 500 flight cycles after the effective date of this AD, or prior to accumulating the flight limit specified in Boeing Drawing 624U0001, Sheet 3, Revision A, dated December 14, 1989, whichever occurs later, accomplish the inspections contained in Boeing Alert Service Bulletin 747-53A2265, Revision 7, dated January 25, 1990. Repeat these inspections at intervals not to exceed those specified in the drawing. \n\n\tB.\tIf any cracking is found, repair in accordance with FAA-approved procedures prior to further flight. Concurrent with performing any repair, visually inspect adjacent structures in accordance with Section III of Boeing Alert Service Bulletin 747-53A2265, Revision 7, dated January 25, 1990, and repair any cracks in accordance with FAA-approved procedures prior to further flight. \n\n\tC.\tFor the purpose of complying with this AD, the number of landings may be determined to equal the number of pressurization cycles where the cabin differential pressure was greater than 2.0 PSI. \n\n\tD.\tFor Model 747SR airplanes only, based on continued mixed operation of lower cabin differentials, the initial inspection thresholds and the repetitive inspection intervals specified in this AD may be multiplied by a 1.2 adjustment factor. \n\n\tE.\tFor structure that has been installed during previous airplane modification/repair, the inspection thresholds referenced in paragraph A. of this AD are measured from the time of replacement of that structure. \n\n\tF.\tAn alternative method of compliance or adjustment of the compliance time, which providesan 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\tG.\tSpecial flight permits may be issued in accordance with FAR 21.197 and 21.199 to operate airplanes to a base in order to comply with the requirements of this AD. \n\n\tH.\tInstallation of new and improved body frame structure in accordance with FAA- approved procedures or Boeing Service Bulletin 747-53-2272, dated January 12, 1987, is considered terminating action for the repetitive inspections required by this AD for the structure replaced and other adjacent structure (considered to be stingers, clips, and skin associated with the frame). \n\n\tAll persons affected by this directive who have not already received the appropriate service documents from the manufacturer may obtain copies upon request to Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124. These documents may be examined at the FAA, Northwest Mountain Region, Transport Airplane Directorate, 1601 Lind Avenue SW., Renton, Washington.\n \n\tThis amendment supersedes Amendment 39-5583, AD 86-23-06 R1. \n\tThis amendment (39-6997, AD 91-11-01) becomes effective on June 24, 1991.