What happened
On 31 December 2012, a United Airlines B777-200, registration N206UA, was climbing through approximately 28,000 feet near Incheon International Airport when the aircraft experienced a sudden leftward yaw and a physical jar to the control column. Following a loud sound, the aircraft's Engine Indicating and Crew Alerting System (EICAS) displayed multiple fault messages, including an engine failure warning for the No. 1 engine. The engine's N1 and N2 speeds dropped significantly, and the Integrated Drive Generator (IDG) tripped off.
As the engine flamed out, the flight crew attempted an in-flight restart, but the attempt was unsuccessful. The crew declared an emergency and returned to Incheon International Airport, where the aircraft landed safely at approximately 13:05 KST. There were no injuries to the 246 passengers or the 12 crew members on board.
The investigation
The ARAIB examined the engine's mechanical components, including the Main Gearbox (MGB) and the Electronic Engine Control (EEC) fault memory. Investigators performed a teardown inspection of the MGB and analyzed magnetic chip detectors, which revealed a high volume of metal debris. Analysis of the EEC fault memory confirmed a sequence of events including surge detection, fuel metering valve position faults, and N2 range faults. The investigation also focused on the maintenance history of the engine, specifically the assembly of the PMA drive retaining nut during the most recent shop visit.
Findings
Investigation of the No. 1 engine revealed that the mechanical breakdown of the MB-7 bearing interrupted the transmission of rotational power from the N2 rotor to the MGB. This failure caused the gearbox drive gear shaft to disengage from the layshaft, which subsequently stopped all engine components mounted on the MGB, including the fuel pump, oil pump, and the PMA. This chain of events led to the engine flame-out.
The primary contributing factor was an error during the previous engine assembly. The PMA drive retaining nut had been tightened to a torque level significantly lower than the requirements specified in the Engine Manual (EM). This insufficient torque prevented the nut from properly supporting the MB-7 bearing inner race, leading to the bearing's mechanical failure and subsequent damage to the MGB driveshaft and PMA gears.
Safety action
Following the investigation, the ARAIB issued a safety recommendation to United Airlines to improve quality management procedures within their maintenance program. The goal is to ensure that all assembly procedures, specifically for the PMA drive retaining nut, strictly adhere to the manufacturer's prescribed torque specifications.