90-20-20 BOEING: Amendment 39-6725. Docket No. 90-NM-79-AD.\n\n\tApplicability: Model 747 series airplanes, line numbers 1 through 783, certificated in any category. \n\n\tCompliance: Required as indicated, unless previously accomplished. \n\n\tTo prevent engine separation, accomplish the following: \n\n\tA.\tPrior to the accumulation of 10,000 total flight cycles or within the next 1,000 flight cycles after the effective date of this AD, whichever occurs later, perform either a detailed visual inspection or an ultrasonic inspection of the nacelle strut diagonal braces for cracks in accordance with Boeing Service Bulletin 747-54-2123, dated March 1, 1990. \n\n\t\tRepeat these inspections as follows:\n\n\t\t1.\tIf the immediately preceding inspection was accomplished visually, the next inspection must be conducted within 1,000 flight cycles. \n\n\t\t2.\tIf the immediately preceding inspection was accomplished ultrasonically, the next inspection must be conducted within 3,000 flight cycles.B.\tIf cracking is found, replace the nacelle strut diagonal brace with a serviceable brace prior to further flight. \n\n\tC.\tReplacement of the nacelle strut diagonal brace with a new production diagonal brace, which has revised internal and external surface finish, in accordance with the method described in Boeing Service Bulletin 747-54-2123, dated March 1, 1990, constitutes terminating action for the inspections required by paragraph A. of this AD. \n\n\tD.\tFor the purposes of compliance with this rule, two "touch-and-go" training flights may be considered equivalent to one flight cycle. \n\n\tE.\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 (ACO), FAA, Transport Airplane Directorate. \n\n\tNOTE: The request should be submitted directly to the Manager, Seattle ACO, and a copy sent to the cognizant FAA Principal Inspector (PI). The PI will then forward comments or concurrence to the Seattle ACO. \n\n\tF.\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 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 (39-6725, AD 90-20-20) becomes effective on October 23, 1990.