Last June, American Airlines mechanics were performing a high-power engine run on a Boeing 767. The pilots of the previous flight had reported that the left engine lagged behind the right engine by about 2 percent during the climb from 36,000 to 38,000 feet. The mechanics performed a series of troubleshooting procedures including two rapid excursions with the power levers from idle to maximum power and back to idle. It was after the engine reached maximum power for the second time and was decelerating through 95 percent N1 that it experienced a severe failure. The high-pressure turbine (HPT) stage 1 disk on the GE CF6-80A failed, hurling chunks of metal as it tore apart.
From inside the cockpit, the mechanics heard a loud explosion followed by a fire under the left wing and fuselage aft of the wing. They shut down the engines, discharged a fire bottle into the left engine nacelle, and evacuated the aircraft. The fire continued until it was extinguished by airport fire department personnel.
When the HPT stage 1 disk ruptured, it completely split the engine, with the fan, booster, high pressure compressor, and combustor hanging from the forward-engine mount and the low pressure turbine and exhaust hanging from the rear-engine mount. The ruptured disk broke into three pieces approximately equal-sized pieces as well as a smaller triangular piece and several smaller fragments. One piece of the disk bounced off the ground before penetrating the aircraft. It then severed the left-hand keel beam and partially severed the right-hand keel beam before exiting the airplane and lodging itself in the number two engine's exhaust duct. A second piece of the disk was imbedded in an air duct in the plane. A third piece of the disk was found 2,500 feet away from the airplane against an airport perimeter fence and crossed two active runways and taxiways. The fourth triangular-shaped piece was found embedded in the engine pylon.
In addition to the chunk of the HPT disk that imbedded in the exhaust duct, the inboard side of the number two engine was peppered with holes and impact marks from debris from the number one engine. There were some holes in the right-hand fuel tanks (although the fuel didn't ignite like the left-hand side did). Despite all the flying engine parts and subsequent fire, none of the three mechanics onboard or the fourth on the ground were injured in the incident.
On Sept. 22, 2000, a US Airways Boeing 767-2B7 (ER) CF6-80C2B2 number one engine experienced an uncontained failure of the HPT stage 1 disk during a high-power maintenance run at Philadelphia, PA. The failure resulted in a fire under the left wing of the aircraft. Although the aircraft sustained substantial damage, no maintenance personnel were injured.
In another incident, an in-flight failure occurred to an Air New Zealand 767. On Dec. 8, 2002, while climbing through 11,000 feet, the number one engine experienced an uncontained HPT stage 1 disk failure. A section of the disk's rim and web separated and after penetrating the engine's case and nacelle, damaged the left hand wing leading edge. The pilots landed the aircraft safely, and none of the 10 crew members or 190 passengers was injured.
On May 18, 2001, the FAA issued AD 2001-10-07 requiring initial and repetitive inspections of the CF6-80C2 HPT disk. This included a double etch and fluorescent penetrant inspection and eddy current inspection called out in GE Alert Service Bulletin ASB 72-A1026 Revision 1. In January 2003 (after the Air New Zealand event), the FAA issued AD 2003-01-05 that mandated the enhanced visual inspection and eddy current inspection of the HPT stage 1 disks of CF6-80A series engines in accordance with GE service bulletin CF6-SB 72-0779. In February 2004, the FAA issued AD 2004-04-07 which superseded both AD 2001-10-07 and 2003-01-05. This AD retained the initial and repetitive inspections required in the previous ADs as well as required the modification of HPT stage 1 disks by chamfering the blade slot bottom aft corners.
On Aug. 28, the NTSB issued the following recommendations:
(A-06-60) Urgent. 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-06-61) Urgent. Require that all CF6-80A and -80C2 high pressure turbine (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-06-62) Revise the engine-related airworthiness directive process to ensure that the compliance timelines are appropriately established.
(A-06-63) Require a design review of CF6-80 series high pressure turbine (HPT) stage 1 disks that incorporate chamfered blade slot bottom aft corners that includes a stress analysis and finite element model emphasizing 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.
On another topic, when the NTSB went to review the flight data recorder (FDR) and cockpit voice recorder (CVR) it noted that although playback of the FDR provided data of the engine failure event as well as the engine performance problem reported by the flight crew of the previous flight, there was nothing on the CVR concerning the maintenance run. This was due to the common practice of maintenance personnel pulling the CVR circuit breaker prior to a maintenance run.
The NTSB has advocated disabling CVRs as soon as possible after landing after a reportable incident or accident has occurred to preserve data that could be used to assist with the event investigation. Due to the recent incident the NTSB is recommending the following:
(A-06-64) Require that maintenance personnel ensure that an aircraft's cockpit voice recorder (CVR) is operating before conducting any engine ground tests. If an airplane 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.
This article is based on the NTSB Aug. 28 report. These are NTSB recommendations and not regulatory. To read the full report, visit www.ntsb.gov. Additional images of the June engine failure can be viewed on www.amtonline.com.