90-21-13 BOEING: Amendment 39-6751. Docket No. 90-NM-67-AD. \n\n\tApplicability: Model 747 series airplanes, line position 001 through 331, identified in Boeing Service Bulletin 747-29-2051, Revision 1, dated August 8, 1980, certificated in any category.\n \n\tCompliance: Required within the next 4,000 flight hours or 12 months after the effective date of this AD, whichever occurs first, unless previously accomplished. \n\n\tTo prevent damage to hydraulic and pneumatic systems and flight deck/cabin air contamination, accomplish the following: \n\n\tA.\tModify the hydraulic system in the number 2 and 3 struts in accordance with the Accomplishment Instructions of Boeing Service Bulletin 747-29-2051, Revision 1, dated August 8, 1980.\n \n\tB.\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 Inspector (PI), who will either concur or comment, and then send it to the Manager, Seattle Aircraft Certification Office. \n\n\tC.\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 The 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 S.W., Renton, Washington. \n\n\tThis amendment (39-6751, AD 90-21-13) becomes effective on November 12, 1990.