What happened
On 26 December 2009, an Asiana Airlines B777-2/200ER, registration HL7500, was operating a flight from Los Angeles toward Incheon when the aircraft experienced a significant power loss. While cruising at approximately 36,000 feet, roughly 900 nautical miles west of Anchorage, the left engine experienced a sudden shutdown.
During the event, the N1 and N2 indicators dropped to zero, and the Exhaust Gas Temperature (EGT) fell abruptly. The aircraft experienced a leftward tilt, and the cockpit crew received a master caution alert along with an "ENG Fail L" message. After attempting to restart the engine without success, the captain declared an emergency and diverted the aircraft to Anchorage International Airport, where a safe landing was completed on runway 07R. There were no injuries among the 318 people on board.
The investigation
The ARAIB conducted a detailed examination of the left engine, a Pratt & Whitney PW4090, which was sent to the manufacturer for precision disassembly. Investigators analyzed flight data recorder information, which showed no prior signs of vibration or mechanical defect before the shutdown.
Upon ground inspection, investigators found no external damage to the engine inlet or exhaust ports. However, a significant quantity of metal debris was discovered during a check of the magnetic chip detector in the main gearbox. Subsequent laboratory analysis of the angle gearbox revealed that the AB-1 bearing retaining lock washer had been damaged. This failure allowed the retaining nut to loosen, leading to extreme friction, heat, and the eventual destruction of the bearing cage and balls. The investigation also examined the towershaft splines, which had crumbled due to the mechanical instability.
Findings
- The primary cause of the engine shutdown was the crumbling of the towershaft bottom splines within the engine angle gearbox.
- This mechanical failure interrupted power delivery to the main gearbox, causing the engine's accessory parts to lose function.
- The failure originated from the damage of a bearing retaining lock washer, which led to a loosened retaining nut.
- High-velocity rotation and subsequent friction generated enough heat to reach the fusion point of the metal, causing the bearing cage and balls to deform and fuse.
- There were no contributing factors related to pilot performance, aircraft weight and balance, or meteorological conditions.