# AD 79-21-01 R1: Third Stage Turbine Wheel Blade - Type: Airworthiness directive - Status: Current - Products: Rolls-Royce 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B and 250-C20C (T63-A-720) - Effective: June 3, 1981 - Superseded by: Nothing supersedes this AD - Product type: Engine - Amendment: 39-4123 - Registered aircraft: 1,074 may be affected, by airframe or engine model (OpenAirDocs) - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/9A93808E1A4A25BE862568270069B958.0001 - DRS record last changed: May 30, 2024 - Page: https://openairdocs.org/ad/79-21-01-R1 ## The airworthiness directive 79-21-01 R1 DETROIT DIESEL ALLISON: Amendment 39-3585 as amended by Amendment 39-4123. Applies to 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) engines equipped with third stage turbine wheel Part Numbers 6898551, 6898567, 6898733, 6898743, 6898753, 6898763, 6899406, 6899415, 6899416, 6899417, 6899418, 6899419, 6896863, 6898823 and 6899364 installed in aircraft certificated in all categories. Compliance required as follows unless previously accomplished. (a) For engines that have previously experienced a hot start beyond the established limits, compliance must be accomplished prior to further flight. (b) For engines that experience a hot start beyond the established limits after the effective date of this AD, compliance must be accomplished prior to further flight, except that the aircraft may be flown in accordance with FAR 21.197 to a base where the removal can be performed. To preclude possible engine power loss resulting from third stage turbine wheel blade separation, remove the turbine wheels from service if the following temperature-time limits are exceeded and install an FAA Approved part number turbine wheel. Temperature Range Time Limit 810 degrees - 927 degrees C. 10 seconds maximum (1490 degrees - 1700 degrees F.) Over 927 degrees C. 0 seconds (1700 degrees F.) The turbine wheels may not be reworked and reinstalled after a hot start. NOTE: Time at temperature limits is not additive and may be repeated without restriction. (Detroit Diesel Allison Commercial Service Letter 1051, Rev. 3, for the 250-B17 series engines and Commercial Service Letter 1084, Rev. 3, for the 250-C20 series engines also pertain to this subject.) Amendment 39-3585 became effective October 16, 1979. This amendment 39-4123 becomes effective June 3, 1981. ## Affected data sheets (2) (OpenAirDocs) The type certificate data sheets whose models the AD names. - [TCDS E10CE](https://openairdocs.org/tcds/E10CE): Rolls-Royce Corporation; names 3 of 11 models: 250-B17B, 250-B17C, 250-B17 - [TCDS E4CE](https://openairdocs.org/tcds/E4CE): Rolls-Royce Corporation; names 3 of 23 models: 250-C20, 250-C20C (T63-A-720), 250-C20B ## 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: 1,074 (0 by airframe model, 1,074 by engine model) - Makers: Bell 565, Hughes Helicopters 145, Mcdonnell Douglas Helicopter 79, MD Helicopters 44, Hughes 35, Hiller 23, Agusta 21, Bell Helicopter Textron 20, Messerschmitt-Bolkow-Blohm 13, Aerospatiale 12 Models with the most aircraft: - [Allison 250-C20 series](https://openairdocs.org/engine/allison-250-c20-series): 530 aircraft (engine model) - [Rolls-Royce 250-C20B](https://openairdocs.org/engine/rolls-royce-250-c20b): 215 aircraft (engine model) - [Allison 250-C20B](https://openairdocs.org/engine/allison-250-c20b): 113 aircraft (engine model) - [Allison 250 series](https://openairdocs.org/engine/allison-250-series): 79 aircraft (engine model) - [Allison 250-C20](https://openairdocs.org/engine/allison-250-c20): 54 aircraft (engine model) - [Rolls-Royce 250-C20](https://openairdocs.org/engine/rolls-royce-250-c20): 42 aircraft (engine model) - [Allison 250-B17 series](https://openairdocs.org/engine/allison-250-b17-series): 14 aircraft (engine model) - [Allison 250-C20C](https://openairdocs.org/engine/allison-250-c20c): 9 aircraft (engine model) - [Allison 250-B17C](https://openairdocs.org/engine/allison-250-b17c): 6 aircraft (engine model) - [Rolls-Royce 250-B17C](https://openairdocs.org/engine/rolls-royce-250-b17c): 6 aircraft (engine model) - [Rolls-Royce 250-C20C](https://openairdocs.org/engine/rolls-royce-250-c20c): 6 aircraft (engine model) All 13 models: https://openairdocs.org/ad/79-21-01-R1/models.md Every aircraft: https://openairdocs.org/ad/79-21-01-R1/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.