Engine Breakup

Editor Joe Escobar highlights the NTSB recommendations released Aug. 28, 2006.

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.

Other failures

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.

Prior action

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.

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