16 January 2002
A recent incident occurred in which the pilot of a Bell 206B helicopter was forced to jettison a slung load and enter autorotation after the Allison 250 C20 engine decelerated to idle during flight.
An investigation identified the engine power turbine governor spring had failed, resulting in an immediate engine deceleration. The spring was found to have failed due to fatigue which initiated from deep pitting on the spring's surface. The pitting, which was found over the entire surface of the spring, appeared to be a result of the manufacturing processes rather than caused by in-service corrosion. Alert Commercial Engine Bulletins (CEBs A-300 / A-1361 / A-73-2069 / A-73-3102 / A-73-4045 / A-172 / A-1312 / A-73-2027) were issued by then Rolls-Royce Allison on 5 October 1998, currently at Revision 2, dated 29 August 2001 to remove and replace governor springs which had been damaged during the manufacturing process.
Based on a detailed review by Honeywell, it was concluded that the governor springs currently in service have a low probability of failure if replaced as per the above-mentioned Alert Commercial Engine Bulletins (CEBs). This spring is also used in a variety of fuel control units and turbine governor models on all Allison 250 series engines.
Transport Canada recommends operators make sure the power turbine governor and fuel control unit springs are removed from service as specified within the compliance section of respective CEBs.
Any defects or further occurrences should be reported by sending a Service Difficulty Report to Transport Canada, Continuing Airworthiness, Ottawa.
For further information, contact a Transport Canada Centre, or Mr. Luc Deniger, Continuing Airworthiness, Ottawa, telephone (613) 952-5385, facsimile (613) 996-9178 or e-mail firstname.lastname@example.org.
For Director, Aircraft Certification
Chief, Continuing Airworthiness