A serious mechanical fault that has occurred seven times in CFM International CFM56-3 turbofans between 1995 and 1997 has been calculated as an acceptable risk by the US and UK aviation authorities, a UK Air Accident Investigation Branch (AAIB) report reveals.
The UK Civil Aviation Authority's acceptance of the AAIB report makes this the first example of the UK adopting a new US Federal Aviation Administration engine failure risk policy, dubbed "hazard ranking".
The AAIB investigation concerned a Boeing 737-500 operated by the Birmingham, UK-based division of Maersk Air, in which the crew had to shut down the starboard power unit following "severe engine damage" caused by a failed high pressure turbine rotor rear shaft seal. The aircraft, flying from Belfast to Birmingham on 23 December, 1997, turned back to Belfast and landed safely.
Despite exhaustive metallurgical tests by the UK Defence Evaluation and Research Unit for the AAIB, the cause has not been determined, except that the breakup was the result of high-cycle fatigue. CFMI partner company General Electric Aircraft Engines says this is reckoned to be associated with high frequency resonance, but no manufacturing quality fault was found.
GE says, however, that, during 1997, a new coating was adopted for the seal tooth and that eddy-current testing for the seals is now standard practice at every shop visit. Since the Maersk event,the failure has not been repeated, the manufacturer says.
The rationale for not taking further action, the UK agencies confirm, is adoption of the FAA engine hazard ranking criteria. Hazards are graded from 1 to 4 in terms of potential consequences, with 4 "severe" and 1 "minor". All the incidents involving the CFM56-3s were graded 1 or 2.
Source: Flight International
