United B772 near Seoul on Dec 31st 2012, engine shut down in flight

Last Update: February 14, 2015 / 22:59:47 GMT/Zulu time

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Photo of United N206UA, Boeing 777-200 — United N206UA, Boeing 777-200 (Photo credit: Jun Seita / Flickr / License: CC by)

Incident Facts

Date of incident
Dec 31, 2012

Classification
Incident

Cause
Engine shut down in flight

Airline
United

Flight number
UA-890

Departure
Seoul, South Korea

Destination
Tokyo Narita, Japan

Aircraft Registration
N206UA

Aircraft Type
Boeing 777-200

ICAO Type Designator
B772

A United Boeing 777-200, registration N206UA performing flight UA-890 from Seoul (South Korea) to Tokyo Narita (Japan), was climbing through FL250 out of Seoul when the crew received an engine failure indication for the left hand engine (PW4090), shut the engine down, declared emergency and returned to Seoul for a safe landing.

The flight was cancelled.

The NTSB rated the occurrence an incident reporting that a post flight inspection revealed metal fragments in the left engine's chip detector. The South Korean Aviation and Railway Accident Investigation Board (ARAIB) opened an investigation.

South Korea's ARAIB have released their final report concluding the probable cause of the serious incident was:

The Aviation and Railway Accident Investigation Board determines the causes of this serious incident as follows:

The mechanical breakdown of the MB-7 bearing, which supports the front part of the gearbox drive gear shaft at the No. 1 engine MGB, stopped the transmission of the N2 rotor's rotational power to the MGB through the layshaft, which shut down all engine components mounted on the MGB, thereby resulting in the IFSD.

Contributing to this serious incident were as follows:

During the last engine assembly, when the MB-7 bearing was mounted on the MGB driveshaft, the PMA drive retaining nut was tightened with a less torque than that prescribed by the EM, which caused its failure to support the MB-7 bearing inner race, thereby resulting in the mechanical damage of the MB-7 bearing.

The ARAIB reported based on the captain's (52, ATPL, 20,150 hours total, 4,100 hours in command on type) testimony, that the first officer (38, ATPL, 14,782 hours total, 5,782 hours on type) was flying manually. When the aircraft climbed through FL280 the aircraft yawed to the left associated with shaking on the control columns, the left hand engine's N1 and N2 showed 20% and 0%, the IDG tripped off and the EICAS displayed ENG FAIL L. The crew worked the related checklists, attempted a restart of the engine to no avail, the captain assumed duties of pilot flying and returned the aircraft for a safe landing on runway 33L at Incheon Airport.

Immediately after the return a visual inspection of the engine revealed the N2 shaft had seized. The magnetic chip detectors in the main gearbox bearing and the engine oil tank showed substantial amount of metallic particles. An attachment bolt on the permanent magnet alternator (PMA) was fractured and was missing its head.

The left engine was removed from the airframe and sent for further analysis. The ggearbox drive gear shaft was found moved forward from its assembled position thus disengaging the drive gear shaft and layshaft. The PMA unit was removed from the main gearbox bearing, all PMA gearshaft teeth were found severly damaged and missing material. PMA rotor, stator, shaft roller bearings were in good condition however.

Main Bearing-7 rollers and cage were found liberated and distributed throughout the main gearbox case, severly deformed cage fragments were found inside the gearbox. Main Bearing-8 however remained intact.

The ARAIB analysed that during examination of the engine the break away torque of the PMA drive retaining nut was extremely lower than specified even though the breakaway torque could not be measured during disassembly. The ARAIB analysed: "It is highly likely that out of three assembly procedures above, the procedure 3 was implemented with a significantly lower torque than that prescribed by the EM, or that the procedure 3 was omitted after the retaining nut was loosened in the procedure 2." and continued: "If the torque is insufficient when the PMA drive retaining nut is assembled into the MGB drivesahft, the retaining nut's clamping force in the axial direction will reduce, which causes not only the material transfer or galling/fretting between the MGB driveshaft and the PMA drive spur gear, but also axial vibration due to the rotation of the MB-7 bearing inner race, thereby damaging the whole MB-7 bearing."