Recommendations to FAA

Citation

(2007), "Recommendations to FAA", Aircraft Engineering and Aerospace Technology, Vol. 79 No. 1. https://doi.org/10.1108/aeat.2007.12779aab.020

Recommendations to FAA

Recommendations to FAA

Require that all CF6-80A and -80C2 high pressure turbine (HPT) stage 1 disks and applicable -80E1 HPT stage 1 disks that have more than 3,000 cycles since new (CSN) and have not been reworked in accordance with General Electric Service Bulletins (SB) 72-0788 or 72-1089 or have not yet been inspected in accordance with SB 72-0779 or Alert Service Bulletin 72-A1026 be immediately removed from service for inspection and rework in accordance with these SBs. Those CF6-80A and - 80C2 HPT stage 1 disks and applicable - 80E1 HPT stage 1 disks that have fewer than 3,000 CSN and have not been reworked or inspected in accordance with these SBs can remain in service until reaching the 3,000 CSN threshold, at which time they should also be removed from service for inspection and rework (A-06-60) Urgent.

Require that all CF6-80A and -80C2 HPT stage 1 disks and applicable -80E1 HPT stage 1 disks that have not been reworked in accordance with General Electric Service Bulletins (SB) 72-0788 or 72-1089 but have been inspected in accordance with SB 72-0779 or Alert Service Bulletin (ASB) 72-A1026 and have more than 3,000 cycles since the inspection be immediately removed from service for reinspection and rework in accordance with these SBs. Those CF6-80A and -80C2 HPT stage 1 disks and applicable -80E1 HPT stage 1 disks that have not been reworked in accordance with SBs 72-0788 or 72- 1089 but have been inspected in accordance with SB 72-0779 or ASB 72-A1026 and have fewer than 3,000 cycles since the inspection can remain in service until reaching the 3,000 cycles-since-inspection threshold, at which time they should also be removed from service for reinspection and rework (A-06-61) Urgent.

Revise the engine-related airworthiness directive process to ensure that the compliance timelines are appropriately established (A-06-62).

Require a design review of CF6-80 series HPT stage 1 disks that incorporate chamfered blade slot bottom aft corners that includes a stress analysis and finite element model emphasising the blade slot bottom aft corner to determine whether sufficient material property margin exists to ensure that cracks do not occur. If the design review of chamfered HPT stage 1 disks finds that this design does not provide sufficient material property margin, then a redesign or material change should be implemented (A-06-63).

Require that maintenance personnel ensure that an aircraft's cockpit voice recorder (CVR) is operating before conducting any engine ground tests. If an aeroplane has been involved in a reportable event, the incident CVR should be removed to preserve the event data and any subsequent ground test should be delayed until a suitable replacement CVR can be installed in the aircraft (A-06-64).

The NTSB has also issued five recommendations to the FAA stemming from an ongoing investigation of an uncontained engine failure on an airliner in Los Angeles. Two of the recommendations are classified “Urgent” by the Safety Board.

The incident occurred on 2 June 2006, when the high-pressure turbine (HPT) stage 1 disk in the left engine, a General Electric CF6-80A, on an American Airlines B-767, failed during a maintenance ground run at Los Angeles International Airport (LAX). Debris from the failed engine punctured the aeroplane's left and right wing fuel tanks; leaking fuel ignited and damaged the wing and fuselage. Pieces of the ruptured disk also penetrated the fuselage and the right engine, and another, found about 2,500 feet from the aeroplane against an airport perimeter fence, had crossed two active runways and taxiways. The three maintenance personnel working on board as well as another on the ground were not injured.

“We were fortunate that there were no fatalities or injuries in this serious incident” said NTSB chairman Mark V. Rosenker. “We need to take every precaution and move rapidly to avoid something similar happening again”.

Post-incident metallurgical examination revealed that the 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 examination also revealed two other similar cracks on the disk. 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.

The Board is aware that, as a result of the LAX incident, the FAA has issued an airworthiness directive with a schedule for maintenance – removal, inspection, and reworking – of CF6-80 series HPT stage 1 disks beginning at 6,900 cycles.

The Safety Board, however, is proposing on an urgent basis that the FAA require that the disks be immediately removed for maintenance if they have been in service for more than 3,000 cycles since new or since the last inspection. This significantly more stringent standard 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.