22 July 1999
A Hughes Model 369 helicopter powered by an Allison 250-C18 engine experienced an engine failure due to fuel starvation and the unsuccessful autorotation resulted in three fatalities.
Engine failure was attributed to progressive wear on the splines of the No. 1 element of the engine driven fuel pump from the continued operation of chromed drive shaft splines mating against non-chromed spur gear splines. This wear eventually led to spline disengagement and the inability of the pump to deliver fuel to the engine. The wear effect was exacerbated by the wear debris jamming open the No. 1 check valve.
Allison and Sundstrand recognized the potential problem and revised the Overhaul Manual in 1983 to delete the non-chromed spur drive gears. Allison also revised Alert Commercial Engine Bulletins CEB-A-161 (C18 series) and CEB-A-1051 (C20 series) in 1989 to address spline wear, with compliance to be within 100 hr time in service (TIS) but no later than 1 November 1989.
The failed fuel pump was last overhauled in 1976 and was on an engine that was out of service from 1987 to 1992, when it was purchased by the operator that experienced the accident. The overhaul period (2250 hr) for the pump is based on TIS with no calendar limit and the pump had only accumulated approximately 1000 hr TIS at time of purchase; hence, the pump was not overhauled and was not due for overhaul for approximately another 300 hr TIS when the accident occurred six years later.
Transport Canada recommends that operators of Allison 250-C18 and -C20 engine models with dual element fuel pump assemblies identified by Allison part numbers (P/N) 6877719 and 6876803 or Pesco/Sundstrand P/N 024731-112 and 024731-113 review their maintenance records to ensure that low utilization fuel pumps that have not been overhauled since 1983 are in compliance with Allison Alert CEB -A-161 or CEB-A-1051.
Any defects or further occurrences should be reported by sending a Service Difficulty Report to Transport Canada, Continuing Airworthiness, Ottawa
For more information, contact a Transport Canada Centre or Mr. Ramon A. Raoux, Continuing Airworthiness, Ottawa, telephone (613) 952-4365, facsimile (613) 996-9178 or e-mail email@example.com.
For Director, National Aircraft Certification
Chief, Continuing Airworthiness