US safety board urges FAA to toughen up airworthiness directive in wake of uncontained failure on American 767
The US National Transportation Safety Board (NTSB) wants the Federal Aviation Administration to rethink its recently issued airworthiness directive (AD) over General Electric CF6-80 engines and withdraw some from service immediately. The agency is urging inspections at much earlier intervals of as little as 3,000 cycles for -80A, -80C2 and even -80E1 engines.
The manufacturer admits that even with the FAA AD unamended, "there are 600 engines that need inspections or rework by the end of 2008. It's a logistical challenge."
The FAA AD, issued following an American Airlines Boeing 767-200 uncontained engine failure so serious that it wrote the aircraft off during a 2 June ground run at Los Angeles International, mandated the inspection and reworking when necessary of the high-pressure turbine (HPT) first stage disks on CF6-80 engines, starting at 6,900 cycles in some cases.
But the NTSB says the FAA should - "on an urgent basis" - mandate the same action if the disks have been "in service for more than 3,000 cycles since new or since the last inspection. This would not permit disks to remain in service without inspection beyond the earliest known number of cycles at which cracks have been detected or failure has occurred." Investigations revealed the latest disk rupture was the result of a rim-to-bore radial fracture that originated at a small dent found at the bottom of a blade slot. The disk had accumulated 9,186 cycles in service (48,429h), with 5,814 cycles remaining for the disk's life limit of 15,000 cycles.
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The NTSB says all affected engines should be "immediately removed from service for inspection and rework in accordance with the [GE] service bulletins". The safety board also urges a design review of the -80 series HPT first stage disks "that incorporate chamfered blade slot bottom aft corners that includes a stress analysis and finite element model to determine whether sufficient material property margin exists to ensure that cracks do not occur". It also suggests that the aircraft's cockpit voice recorder should be running during any engine ground tests.
It is not yet known if the FAA will adapt the current AD or consider issuing a revised directive as a result of the NTSB's probe, particularly as this is believed to be one of the first instances of this type where the FAA has taken action before the NTSB has issued its report. The FAA has responded that it "will not hesitate to take further action if additional information from the investigation warrants it".
There is speculation that the urgent recommendations of the NTSB were prompted by the scale of the damage to the 767 and the fact that both engines were affected. However, sources close to the investigation say the overall level of destruction was made far worse by the ricocheting of debris off the ground and that an in-flight failure, like that experienced on an Air New Zealand 767 in 2002, would be unlikely to have wrought so much damage.
GE says: "We feel the FAA AD is very aggressive, and we don't disagree with it. The actions are a result of a long collaborative effort. We feel we have the right plan in place to ensure the safety of the fleet."
Source: Flight International
