AN AIRWORTHINESS directive (AD) to inspect the high-pressure spool of General Electric CF6 engines is expected to be issued by the US Federal Aviation Administration following recent engine failures on an Egyptair Airbus A300 and a Thai International Airways McDonnell Douglas DC-10.
The AD follows recommendations by the US National Transportation Safety Board and covers inspection of the 3-9 spool of the high-pressure compressor (HPC). Operators will be required to inspect for two main defects in the HPC. Each bore of every disc will have to be scanned for the presence of impurities in the titanium from which they are made.
The presence of foreign particles in the titanium acts like a vacuum and breaks down the strength of the material, leading to the possible start of cracking. Tests will also be conducted to scan for dwell-time fatigue in each disc. The fatigue can occur, when grains of metal became layered during the manufacturing process, rather than being laid down randomly.
GE says that the incidents have occurred in older engines which have HPCs made using billets measuring between 330mm and 405mm diameter. In the 1988-9 period, GE switched to using smaller billets, none of which has so far shown any evidence of weakness.
The AD is therefore likely mostly to affect CF6-50 and -80C2s, made before 1989. As Flight International went to press it was unknown whether the FAA would require one-time inspection of all CF6 engines.
Source: Flight International
