# AD 91-21-10: Leading Edge Slats - Type: Airworthiness directive - Status: Current - Products: Boeing 767-200 Series, 767-300 Series and 767-300F Series - Effective: November 29, 1991 - Superseded by: Nothing supersedes this AD - Product type: Aircraft, Large airplane - Amendment: 39-8057 - Docket: 91-NM-76-AD - Registered aircraft: 402 may be affected, by airframe or engine model (OpenAirDocs) - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/FA0E011AC8F316BD862569610072DB45.0001 - DRS record last changed: May 30, 2024 - Page: https://openairdocs.org/ad/91-21-10 ## The airworthiness directive 91-21-10 BOEING: Amendment 39-8057. Docket No. 91-NM-76-AD. Applicability: 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. Compliance: Required as indicated, unless previously accomplished. To 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: (a) For 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: (1) Within 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. (2) Within 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. (b) For 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. (c) An 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. NOTE: 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. (d) Special 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. (e) The 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. This amendment (39-8057, AD 91-21-10) becomes effective on November 29, 1991. ## Affected data sheets (1) (OpenAirDocs) The type certificate data sheets whose models the AD names. - [TCDS A1NM](https://openairdocs.org/tcds/A1NM): The Boeing Company; names 3 of 5 models: 767-300 Series, 767-200 Series, 767-300F Series ## Registered aircraft it may apply to (OpenAirDocs) Matched by airframe and engine model; the AD's applicability may narrow it by serial number, part number or equipment. - Aircraft: 402 (402 by airframe model, 0 by engine model) - Makers: Boeing 402 Models with the most aircraft: - [Boeing 767-300F](https://openairdocs.org/model/boeing-767-300f): 169 aircraft (airframe model) - [Boeing 767-332](https://openairdocs.org/model/boeing-767-332): 46 aircraft (airframe model) - [Boeing 767-322](https://openairdocs.org/model/boeing-767-322): 36 aircraft (airframe model) - [Boeing 767-323](https://openairdocs.org/model/boeing-767-323): 27 aircraft (airframe model) - [Boeing 767-338](https://openairdocs.org/model/boeing-767-338): 13 aircraft (airframe model) - [Boeing 767-316](https://openairdocs.org/model/boeing-767-316): 12 aircraft (airframe model) - [Boeing 767-346](https://openairdocs.org/model/boeing-767-346): 11 aircraft (airframe model) - [Boeing 767-281](https://openairdocs.org/model/boeing-767-281): 9 aircraft (airframe model) - [Boeing 767-381](https://openairdocs.org/model/boeing-767-381): 8 aircraft (airframe model) - [Boeing 767-232](https://openairdocs.org/model/boeing-767-232): 6 aircraft (airframe model) - [Boeing 767-319](https://openairdocs.org/model/boeing-767-319): 6 aircraft (airframe model) - [Boeing 767-306](https://openairdocs.org/model/boeing-767-306): 5 aircraft (airframe model) - [Boeing 767-224](https://openairdocs.org/model/boeing-767-224): 5 aircraft (airframe model) - [Boeing 767-300](https://openairdocs.org/model/boeing-767-300): 5 aircraft (airframe model) - [Boeing 767-316F](https://openairdocs.org/model/boeing-767-316f): 5 aircraft (airframe model) - [Boeing 767-336](https://openairdocs.org/model/boeing-767-336): 4 aircraft (airframe model) - [Boeing 767-223](https://openairdocs.org/model/boeing-767-223): 3 aircraft (airframe model) - [Boeing 767-231](https://openairdocs.org/model/boeing-767-231): 3 aircraft (airframe model) - [Boeing 767-304](https://openairdocs.org/model/boeing-767-304): 3 aircraft (airframe model) - [Boeing 767-324](https://openairdocs.org/model/boeing-767-324): 3 aircraft (airframe model) All 36 models: https://openairdocs.org/ad/91-21-10/models.md Every aircraft: https://openairdocs.org/ad/91-21-10/aircraft.md Its text cites no advisory circulars or STCs. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.