90-04-01 BOEING: Amendment 39-6502. Docket No. 89-NM-173-AD. \n\n\tApplicability: Model 747-400 series airplanes, as listed in Boeing Service Bulletin 747- 26-2135, Revision 1, dated December 21, 1989, certificated in any category. \n\n\tCompliance: Required as indicated, unless previously accomplished. \n\n\tTo preclude operation of the APU without fire detection, accomplish the following: \n\n\tA.\tWithin the next ten days after the effective date of this AD, unless previously accomplished, add the following to the Limitations Section of the FAA-approved Airplane Flight Manual (AFM). This may be accomplished be inserting a copy of this AD in the AFM: \n\n\tAUXILIARY POWER UNIT: "The DC power main battery switch must remain in the ON position during all APU operation including the 60 second cool down cycle following APU shutdown." \n\n\tB.\tWithin the next 90 days after the effective date of this AD, modify the APU electrical power circuit in accordance with Boeing Service Bulletin 747-26-2135, Revision 1, dated December 21, 1989. \n\n\tC.\tThe Airplane Flight Manual limitation required by paragraph A., above, may be removed following incorporation of the modification required by paragraph B., above. \n\n\tD.\tAn alternate means 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, FAA, Northwest Mountain Region. \n\n\tNOTE: The request should be forwarded through an FAA Principal Maintenance Inspector (PMI), who will either concur or comment, and then send it to the Manager, Seattle Aircraft Certification Office. \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 to comply with the requirements of this AD. \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 Airplanes, P.O. Box 3707, Seattle, Washington 98124. These documents may be examined at the FAA, Northwest Mountain Region, Transport Airplane Directorate, 17900 Pacific Highway South, Seattle, Washington, or at the Seattle Aircraft Certification Office, 9010 East Marginal Way South, Seattle, Washington. \n\n\tThis amendment (39-6502, AD 90-04-01) becomes effective on March 19, 1990.