What happened
On 12 September 2021, an Etihad Airways Boeing 777-30 andER, registration A6-ETP, was operating a return flight from Hazrat Shahjalal International Airport (VGHS) to Abu Dhabi International Airport (OMAA). Prior to this leg, the aircraft had experienced a High-Pressure Turbine Active Clearance Control (HPTACC) status message during its previous sector, which was cleared via engine dry runs in Dhaka.
During the flight over the Muscat FIR, the crew experienced two loud bangs accompanied by moderate airframe vibrations. This was followed by an EICAS alert indicating a loss of thrust on the left engine, with N1 reducing to 40%. The crew initiated the engine severe damage checklist and an engine-out drift down procedure. The commander elected to divert to the original destination, OMAA, and successfully landed the aircraft at 19:59 UTC.
The investigation
An investigation by the UAE Air Accident Investigation Sector (AAIS) focused on the mechanical failure of the left engine. Post-incident inspections revealed significant fuel-in-oil contamination, with laboratory analysis showing that approximately 91% of the engine oil consisted of fuel. This leakage had caused damage to the engine's internal bearings.
Technical examination of the engine by the manufacturer revealed that the accessory gearbox (AGB) hydro-mechanical unit (HMU) ball bearing had failed. The investigation established that the HMU quill shaft had sheared, leading to a leak of fuel into the oil system. The failure was traced to a combination of hard particle contamination in the bearing cage, evidence of handling damage to the outer ring raceway, and assembly damage where the inner ring had migrated and misaligned.
Findings
- The primary cause of the in-flight engine shutdown was the damage to the accessory gearbox (AGB) hydro-mechanical unit (HMU) ball bearing.
- The failure was driven by accumulated hard particle contamination, handling-related deformation, and assembly errors.
- The shearing of the HMU quill shaft allowed fuel to leak into the oil, significantly altering the oil's viscosity and damaging internal components.
- The flight crew operated within their legal flight duty period (FDP) limits, despite an extension of 2 hours and 25 minutes exercised at the commander's discretion.
- The aircraft's maintenance logs showed no technical anomalies or prohibited items that would have prevented dispatch prior to the return leg.