91-21-10 BOEING: Amendment 39-8057. Docket No. 91-NM-76-AD. \n\n\tApplicability: Model 767 series airplanes, Groups 1, 2, and 3, as listed in Boeing Alert Service Bulletin 767-27A0095, Revision 3, dated May 23, 1991; certificated in any category. \n\n\tCompliance: Required as indicated, unless previously accomplished. \n\n\tTo prevent uncommanded asymmetric movement of the leading edge slats due to failure of the No-Back brake combined with disconnect of the torque tube, accomplish the following: \n\n\t(a)\tFor Group 1 and 2 airplanes with leading edge slat offset gearbox No-Back brakes that have not been reworked prior to the effective date of this AD to limit the amount of grease used in the offset gearbox, in accordance with the Accomplishment Instructions, Part V, Offset Gearbox Inspection and Interim Rework, of Boeing Alert Service Bulletin 767-27A0095, Revision 1, dated February 22, 1990; Revision 2, dated January 31, 1991; or Revision 3, dated May 23, 1991: \n\n\t\t(1)\tWithin 2,000 flight hours after the effective date of this AD and thereafter at intervals not to exceed 3,000 flight hours, conduct a torque tube and overtravel stop inspection of the inboard and outboard leading edge slat drive system in accordance with the Accomplishment Instructions, Part I, of Boeing Alert Service Bulletin 767-27A0095, Revision 3, dated May 23, 1991. Repair or replace failed parts at the time specified in, and in accordance with, the instructions provided in, Parts I, II, III, IV, VI, and VII of that service bulletin, as applicable. \n\n\t\t(2)\tWithin 18 months after the effective date of this AD, replace all inboard and outboard leading edge slat offset gearboxes with modified offset gearboxes in accordance with the Accomplishment Instructions, Part VII, of Boeing Alert Service Bulletin 767-27A0095, Revision 3, dated May 23, 1991. Accomplishment of this modification constitutes terminating action for the requirements of paragraph (a)(1) of this AD. \n\n\t(b)\tFor all Group 3airplanes and for those Group 1 and 2 airplanes on which the leading edge slat offset gearbox No-Back brakes have been reworked prior to the effective date of this AD to limit the amount of grease used in the offset gearbox, in accordance with the Accomplishment Instructions, Part V, Offset Gearbox Inspection and Interim Rework, of Boeing Alert Service Bulletin 767-27A0095, Revision 1, dated February 22, 1990; Revision 2, dated January 31, 1991; or Revision 3, dated May 23, 1991: Within 24 months after the effective date of this AD, replace all inboard and outboard leading edge slat offset gearboxes with modified offset gearboxes in accordance with the Accomplishment Instructions, Part VII, of Boeing Alert Service Bulletin 767-27A0095, Revision 3, dated May 23, 1991. \n\n\t(c)\tAn alternative method 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 forwarded through an FAA Principal Maintenance Inspector, who may concur or comment and then send it to the Manager, Seattle ACO. \n\n\t(d)\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\t(e)\tThe modification and inspection requirements shall be done in accordance with Boeing Alert Service Bulletin 767-27A0095, Revision 3, dated May 23, 1991. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR Part 51. Copies may be obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124. Copies may be inspected at the FAA, Northwest Mountain Region, Transport Airplane Directorate, 1601 Lind Avenue S.W., Renton, Washington, or at the Office of the Federal Register, 1100 L Street N.W., Room 8401, Washington,D.C. \n\n\tThis amendment (39-8057, AD 91-21-10) becomes effective on November 29, 1991.