# AD 2001-20-51: Helical Torquemeter Gearshaft Assembly - Type: Emergency airworthiness directive - Status: Historical - Products: 250-B17, 250-B17C, 250-B17D, 250-B17E, 250-B17F, 250-B17F/1, 250-B17F/2, 250-C20, 250-C20B, 250-C20F, 250-C20R, 250-C20R/1 and 3 more - Issued: October 3, 2001 - Issued as: Emergency AD - Superseded by: [AD 2001-24-12](https://openairdocs.org/ad/2001-24-12) - Product type: Engine - Docket: 2001-NE-38-AD - Registered aircraft: 1,193 may be affected, by airframe or engine model (OpenAirDocs) - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/7631ABF455CC457B86256CCB00743F0B.0001 - DRS record last changed: August 7, 2025 - Page: https://openairdocs.org/ad/2001-20-51 ## The airworthiness directive AIRWORTHINESS DIRECTIVES EMERGENCY RULES: 2001-20-51 CITATION: This information is not available. PAGE NUMBER: DOCKET NUMBER: 2001-NE-38-AD AMENDMENT: AD NUMBER: 2001-20-51 SUBJECT HEADING: Rolls-Royce Corporation (formerly Allison Engine Company) Models 250-C20, -C20B, -C20F, -C20R, -C20R/1, -C20R/2, -C20S, and -C20W turboshaft engines, and 250-B17, -B17C, -B17D, -B17E, -B17F, -B17F/1, and -B17F/2 turboprop engines. ACTION: SUMMARY: DATES: ADDRESSES: FOR FURTHER INFORMATION CONTACT: John Tallarovic, Aerospace Engineer, Chicago Aircraft Certification Office, FAA, Small Airplane Directorate, 2300 East Devon Avenue, Des Plaines, IL 60018; telephone (847) 294-8180, fax (847) 294-7834. SUPPLEMENTARY INFORMATION: DATE: October 3, 2001 2001-20-51 Send to all U.S. owners and operators of Rolls-Royce Corporation (formerly Allison Engine Company) models 250-C20, -C20B, -C20F, -C20R, -C20R/1, -C20R/2, -C20S, and -C20W turboshaft engines, and 250-B17, -B17C, -B17D, -B17E, -B17F, -B17F/1, and -B17F/2 turboprop engines. These engines are installed on, but not limited to, Aerospatiale AS355; Agusta A109; A109A, A109C; Bell 206B, 206L, 206LT; Enstrom TH28; McDonnell Douglas 500C, 500D, 500E, 520N; Rogerson-Hiller FH1100; Schweizer TH330; Soloy Conversions Bell 47/47G, Hiller UH-12; American Jet Industries/Cessna 402, 414; and ASTA/GAF Nomad N-22 aircraft. This Emergency Airworthiness Directive (AD) is prompted by a report of uncontained release of power turbine blades and disk fragments caused by engine overspeed, resulting in an uncommanded engine shutdown, engine fire, and damage to the aircraft. This condition is caused by high-cycle fatigue (HCF) failure of the helical torquemeter gearshaft assembly which transmits the power generated by the engine. Investigation has revealed that the HCF failure was due to a preexisting crack. Investigation has also determined that helical torquemeter gearshaft assemblies with low hours of time-since-new (TSN) run a risk of failing, with little or no warning. For the purposes of this emergency AD, any helical torquemeter gearshaft assembly with 100 hours or less TSN is to be replaced with a serviceable helical torquemeter gearshaft assembly, before further flight. All helical torquemeter gearshaft assemblies with greater than 100 hours TSN require no action. This condition, if not corrected, could result in uncontained release of power turbine blades and disk fragments caused by engine overspeed, resulting in an uncommanded engine shutdown, engine fire, and damage to the aircraft. Since an unsafe condition has been identified that is likely to exist or develop on other engines of this same type design, this AD requires replacement of any helical torquemeter gearshaft assembly with 100 hours or less TSN with a serviceable helical torquemeter gearshaft assembly, before further flight. This rule is issued under 49 U.S.C. Section 44701 (formerly section 601 of the Federal Aviation Act of 1958) pursuant to the authority delegated to me by the Administrator, and is effective immediately upon receipt of this emergency AD. REGULATORY TEXT: 2001-20-51 Rolls-Royce Corporation (formerly Allison Engine Company): Docket No. 2001-NE-38-AD Applicability This airworthiness directive (AD) is applicable to Rolls-Royce Corporation (formerly Allison Engine Company) models 250-C20, -C20B, -C20F, -C20R, -C20R/1, -C20R/2, -C20S, and -C20W turboshaft engines, and 250-B17, -B17C, -B17D, -B17E, -B17F, -B17F/1, and -B17F/2 turboprop engines. These engines are installed on, but not limited to, Aerospatiale AS355; Agusta A109; A109A, A109C; Bell 206B, 206L, 206LT; Enstrom TH28; McDonnell Douglas 500C, 500D, 500E, 520N; Rogerson-Hiller FH1100; Schweizer TH330; Soloy Conversions Bell 47/47G, Hiller UH-12; American Jet Industries/Cessna 402, 414; and ASTA/GAF Nomad N-22 aircraft. Note 1: This airworthiness directive (AD) applies to each engine identified in the preceding applicability provision, regardless of whether it has been modified, altered, or repaired in the area subject to the requirements of this AD. For engines that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (d) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and, if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it. Compliance: Required as indicated, unless already done. To prevent uncontained release of power turbine blades and disk fragments caused by engine overspeed, resulting in an uncommanded engine shutdown, engine fire, and damage to the aircraft, do the following: (a) Before further flight, remove helical torquemeter gearshaft assemblies P/N's 23035299 and 23038191 that have accumulated 100 hours or less time-since-new (TSN). Replace with a serviceable helical torquemeter gearshaft assembly. (b) After the receipt of this AD, do not install any helical torquemeter gearshaft assembly P/N 23035299 or 23038191 that has accumulated 100 hours or less TSN. Definition (c) For the purposes of this AD, the following helical torquemeter gearshaft assemblies are considered serviceable parts: (1) P/N's 23035299 and 23038191 that have greater than 100 hours TSN. (2) An assembly with a P/N other than P/N's 23035299 and 23038191. Alternative Methods of Compliance (d) An alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Certification Office. Operators shall submit their requests through an appropriate FAA Maintenance Inspector, who may add comments and then send it to the Manager, Certification Office. Note 2: Information concerning the existence of approved alternative methods of compliance with this airworthiness directive, if any, may be obtained from the Chicago Aircraft Certification Office. Special Flight Permits (e) Special flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the aircraft to a location where the requirements of this AD can be accomplished. (f) Emergency AD 2001-20-51, issued October 3, 2001, becomes effective upon receipt. FOOTER: Issued in Burlington, Massachusetts on October 3, 2001. Mark C. Fulmer, Acting Manager, Engine and Propeller Directorate, Aircraft Certification Service. ## 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 8 of 23 models: 250-C20, 250-C20S, 250-C20F, 250-C20R/2, 250-C20W, 250-C20R, 250-C20R/1, 250-C20B - [TCDS E10CE](https://openairdocs.org/tcds/E10CE): Rolls-Royce Corporation; names 7 of 11 models: 250-B17F, 250-B17F/1, 250-B17F/2, 250-B17E, 250-B17C, 250-B17D, 250-B17 ## 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: 1,193 (0 by airframe model, 1,193 by engine model) - Makers: Bell 557, Hughes Helicopters 147, Mcdonnell Douglas Helicopter 92, Enstrom Helicopter 60, MD Helicopters 60, Hughes 34, Agusta 29, Hiller 23, Bell Helicopter Textron 20, Cessna 18 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-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-B17 series](https://openairdocs.org/engine/allison-250-b17-series): 14 aircraft (engine model) - [Rolls-Royce 250-B17F](https://openairdocs.org/engine/rolls-royce-250-b17f): 10 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-C20R/1](https://openairdocs.org/engine/rolls-royce-250-c20r-1): 5 aircraft (engine model) - [Allison 250-B17D](https://openairdocs.org/engine/allison-250-b17d): 4 aircraft (engine model) - [Rolls-Royce 250-C20R](https://openairdocs.org/engine/rolls-royce-250-c20r): 4 aircraft (engine model) - [Allison 250-B17F/2](https://openairdocs.org/engine/allison-250-b17f-2): 3 aircraft (engine model) - [Allison 250-C20R](https://openairdocs.org/engine/allison-250-c20r): 3 aircraft (engine model) - [Allison 250-C20R/1](https://openairdocs.org/engine/allison-250-c20r-1): 3 aircraft (engine model) All 28 models: https://openairdocs.org/ad/2001-20-51/models.md Every aircraft: https://openairdocs.org/ad/2001-20-51/aircraft.md ## Related documents - [AD 2001-24-12](https://openairdocs.org/ad/2001-24-12): Helical Torquemeter Gearshaft Assembly ยท Rolls-Royce 250-B17, 250-B17C, 250-B17D and 13 more. Effective Dec 19, 2001. Supersedes this AD 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.