91-03-17 BOEING: Amendment 39-6884. Docket No. 90-NM-163-AD. \n\n\tApplicability: Model 747 series airplanes, listed in Boeing Service Bulletin 747-57-2256, dated March 8, 1990, certificated in any category. \n\n\tCompliance: Required as indicated, unless previously accomplished. \n\tTo prevent flap track failures, accomplish the following: \n\n\tA.\tPrior to the accumulation of 30,000 flight hours, or 8 years time-in-service, on current production flap tracks, whichever occurs first, or within 2,000 flight cycles after the effective date of this AD, whichever occurs later, perform a borescope inspection of the forward four bolt holes on each side of the affected trailing edge flap tracks for corrosion and adequate cadmium plating, in accordance with the procedures specified in Boeing Service Bulletin 747-57-2256, dated March 8, 1990, or Revision 1, dated November 15, 1990 (hereafter referred to as "the Bulletin"). If the cadmium plating is adequate, as specified in the Bulletin, and no corrosion or cracks are found, no further action with respect to this AD is required. If the cadmium plating is not adequate, or if corrosion exists in the bolt hole, prior to further flight, conduct an eddy current inspection of the bolt hole for cracks in accordance with the Bulletin. \n\n\tB.\tIf cadmium plating is not adequate and no corrosion or cracks are found during the inspection required by paragraph A. of this AD, within the next 1,000 flight cycles, cadmium plate the affected bolt holes in accordance with the Bulletin, and conduct the following inspections of the affected track in accordance with the Bulletin until the cadmium plating is restored: \n\n\t\t1.\tPerform a close visual inspection of each side of the track, at the lower chord, for cracks emanating from the forward four fail-safe bar bolt holes within 50 flight-cycles after the inspection required by paragraph A. of this AD, and repeat thereafter at intervals not to exceed 50 flight cycles. \n\n\t\t2.\tPerform an eddy current inspection for cracks of the bolt holes within 250 flight-cycles after the inspection required by paragraph A. of this AD, and repeat thereafter at intervals not to exceed 250 flight cycles. \n\n\t\t3.\tPrior to each flight on which a fifth engine is to be carried, conduct a close visual inspection of each side of the track, at the lower chord, for cracks emanating from the forward four fail-safe bar bolt holes. \n\n\tC.\tIf cracks are found in any flap track during the inspections required by this AD, prior to further flight, replace the affected flap track with a serviceable track. \n\n\tD.\tIf corrosion, but no cracking, is found during the inspections required by paragraph A. of this AD, rework the affected bolt holes within the next 1,000 flight cycles in accordance with the Bulletin, and conduct the following inspections of the affected track in accordance with the Bulletin until the rework is accomplished: \n\n\t\t1.\tPerform a close visual inspection of each side of the track, at the lower chord, for cracks emanating from the forward four fail-safe bar bolt holes within 50 flight-cycles after the inspection required by paragraph A. of this AD, and repeat thereafter at intervals not to exceed 50 flight cycles. \n\n\t\t2.\tPerform an eddy current inspection for cracks of all the bolt holes found to have corrosion, within 250 flight-cycles after the inspection required by paragraph A. of this AD, and repeat thereafter at intervals not to exceed 250 flight cycles. \n\n\t\t3.\tPrior to each flight on which a fifth engine is to be carried, conduct a close visual inspection of each side of the track, at the lower chord, for cracks emanating from the forward four fail-safe bar bolt holes. \n\n\tE.\tTerminating action for this AD consists of any of the following: \n\n\t\t1.\tDetermining that the affected flap track bolt holes have adequate cadmium plating and no corrosion as described in paragraph A., of this AD; or \n\n\t\t2.\tApplication of cadmium plating to bolt holes wherethe cadmium plating was found to be inadequate as required by paragraph B., of this AD, or \n\n\t\t3.\tRework of corroded bolt holes as required by paragraph D., of this AD. \n\n\tF.\tIf any cracks are found as a result of the inspections required by this AD, within 15 days after discovery, report the following to the Manager of the Seattle Aircraft Certification Office, ANM- 100S, 1601 Lind Avenue, S.W., Renton, Washington 98055-4056: \n\n\t\t1.\tSize of the crack. \n\n\t\t2.\tLocation of the crack.\n \n\t\t3.\tNumber of cycles on the airplane. \n\n\tG.\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, Northwest Mountain Region. \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.H.\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 (39-6884, AD 91-03-17) becomes effective on March 11, 1991.