# AD 77-15-12: Power Turbine Coupling Nut - Type: Airworthiness directive - Status: Current - Products: Rolls-Royce 250-B15A, 250-B15E, 250-B15G, 250-B17, 250-B17B, 250-B17C, 250-B17D, 250-B17E, 250-B17F, 250-B17F/1, 250-B17F/2, 250-C18 and 14 more - Effective: September 23, 1977 - Superseded by: Nothing supersedes this AD - Product type: Engine - Amendment: 39-3037 - Registered aircraft: 1,325 may be affected, by airframe or engine model (OpenAirDocs) - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/CD18C79E55328D9186256E120055AF99.0001 - DRS record last changed: May 30, 2024 - Page: https://openairdocs.org/ad/77-15-12 ## The airworthiness directive 77-15-12 DETROIT DIESEL ALLISON: Amendment 39-2977 as amended by Amendment 39-3037. Applies to 250-B15 and B17 and 250-C18 and C20 series engines equipped with power turbine outer coupling nuts Part Number (P/N) 6846278 as installed in aircraft certificated in all categories. NOTE THAT THIS AMENDED VERSION OF AD 77-15-12 REQUIRES THAT ALL POWER TURBINE COUPLING NUTS, BOTH COATED AND UNCOATED, BE INSPECTED ON A REPETITIVE BASIS. Compliance schedule - Compliance required as follows unless previously accomplished. (a) For engines with turbine sections having more than 400 hours or one calendar year time in service since installation in an aircraft, initial compliance must be accomplished within the next 100 hours or 90 days, whichever occurs first, from the effective date of the original amendment of July 27, 1977. (b) For engines with turbine sections having less than 400 hours and less than one calendar year time in service since installation in an aircraft, initial compliance must be accomplished within 500 hours or one year time in service since installation in an aircraft, whichever occurs first. (c) For each turbine section incorporating an acceptable uncoated or coated coupling nut, as defined by the below referenced service letters, repetitive compliance is required at intervals not to exceed 500 hours or one calendar year time since last inspection, whichever occurs first. To preclude possible engine failure resulting from power turbine coupling nut failure: Inspect the power turbine outer coupling nut P/N 6846278 in accordance with Detroit Diesel Allison Commercial Service Letter Numbers 88 for the C18 series, 1060 for the C20 series, 25 for the B15 series, and 1030 for the B17 series engines. All Service Letters are Revision 3 or later FAA approved revisions. If an unacceptable extent of corrosion, as defined by the Service Letter, is found, the nut shall be replaced before further flight except that the aircraft may be flown in accordance with FAR 21.197 to a base where the repair can be performed. Amendment 39-2977 was effective July 27, 1977. This amendment 39-3037 is effective September 23, 1977, and was effective immediately for all recipients of airmail letters dated August 19, 1977 which contained this amendment. ## Affected data sheets (2) (OpenAirDocs) The type certificate data sheets whose models the AD names. - [TCDS E4CE](https://openairdocs.org/tcds/E4CE): Rolls-Royce Corporation; names 15 of 23 models: 250-C18, 250-C20, 250-C20S, 250-C20C (T63-A-720), 250-C20R/4, 250-C20F, 250-C18A, 250-C20R/2… - [TCDS E10CE](https://openairdocs.org/tcds/E10CE): Rolls-Royce Corporation; names 11 of 11 models: 250-B17F, 250-B17F/1, 250-B17B, 250-B17F/2, 250-B15G, 250-B17E, 250-B17C, 250-B15E… ## 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: 1,325 (0 by airframe model, 1,325 by engine model) - Makers: Bell 656, Hughes Helicopters 147, Mcdonnell Douglas Helicopter 92, MD Helicopters 61, Enstrom Helicopter 60, Hughes 38, Agusta 29, Bell Helicopter Textron 28, Siai-Marchetti 24, Hiller 23 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) - [Rolls-Royce 250-C20J](https://openairdocs.org/engine/rolls-royce-250-c20j): 82 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-C20W](https://openairdocs.org/engine/rolls-royce-250-c20w): 51 aircraft (engine model) - [Rolls-Royce 250-C20](https://openairdocs.org/engine/rolls-royce-250-c20): 42 aircraft (engine model) - [Rolls-Royce 250-C20R/2](https://openairdocs.org/engine/rolls-royce-250-c20r-2): 24 aircraft (engine model) - [Allison 250-C20J](https://openairdocs.org/engine/allison-250-c20j): 16 aircraft (engine model) - [Allison 250-B17 series](https://openairdocs.org/engine/allison-250-b17-series): 14 aircraft (engine model) - [Allison 250-B15G](https://openairdocs.org/engine/allison-250-b15g): 12 aircraft (engine model) - [Rolls-Royce 250-B17F](https://openairdocs.org/engine/rolls-royce-250-b17f): 10 aircraft (engine model) - [Allison 250-C20C](https://openairdocs.org/engine/allison-250-c20c): 9 aircraft (engine model) - [Allison 250-C20R/2](https://openairdocs.org/engine/allison-250-c20r-2): 8 aircraft (engine model) - [Rolls-Royce 250-B17F/2](https://openairdocs.org/engine/rolls-royce-250-b17f-2): 8 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) - [Rolls-Royce 250-C20R/1](https://openairdocs.org/engine/rolls-royce-250-c20r-1): 5 aircraft (engine model) All 38 models: https://openairdocs.org/ad/77-15-12/models.md Every aircraft: https://openairdocs.org/ad/77-15-12/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.