Ramon Lopez/WASHINGTON, DC
THE US NATIONAL Transportation Safety Board (NTSB) has called for an airworthiness directive (AD) to be issued requiring fatigue-crack checks on General Electric CF6 engine high-pressure compressor (HPC) spools.
The US Federal Aviation Administration says that an AD is imminent - only the inspection intervals have yet to be determined. This could affect 1,200 aircraft, including the Airbus A300/A310, Boeing 747 and 767 and McDonnell Douglas DC-10 and MD-11.
GE's big turbofan has been found to have rotor-spool cracks caused by hard-alpha inclusions (titanium-alloy local anomalies during manufacture) and dwell-time fatigue, according to the NTSB. Dwell-time fatigue occurs when the peak cyclic load is sustained at a relatively low temperature
Although hard-alpha cracking inspections of CF6 HPC stage 3-9 rotor spools are routine, the NTSB comments that "...portions of the spool that are not easily viewed, such as the web areas, may not have received an adequate inspection".
On 10 April, an Egyptair A300B4 crew aborted take-off from Cairo after one of the CF6-50C2s sustained an uncontained HPC failure. The cause was found to be a fatigue fracture in a disk because of a hard-alpha inclusion on the aft side of the disk web.
The NTSB says that GE has recorded three additional separations in CF6 engines caused by fatigue cracking. On 10 August, GE issued a service bulletin recommending improved fluorescent-penetrant inspections.
The Safety Board also wants the FAA, to revise a previous AD, involving CF6-50s, -80As, and -80C2s to require inspections every 4,000 cycles of spools, which have been solution-heat-treated after welding.
Dwell-time fatigue is believed to have caused HPC failure on a CF6-50C-powered DC-10 during take-off from Bangkok on 11 May.
Source: Flight International
