What happened
On 3 September 2005, a Boeing 737-528, registration G-GFFE, was parked at Stand 110 in the North Terminal of London Gatwick Airport. The aircraft was undergoing refueling and pre-departure preparations with the flight and cabin crew on board, though passengers had not yet boarded. During the ground operation, the commander instructed the co-pilot to start the Auxiliary Power Unit (APU).
Shortly after the APU was started, the aircraft's lights extinguished, and the unit automatically shut down. The commander, noticing a commotion at the rear of the aircraft, observed ground staff clearing hot metal fragments from the stand and adjacent taxiway. An inspection of the APU access door revealed that the unit's external casing had bulged and partially split. The failure caused the ejection of hot turbine debris through the jet pipe, which spread in a fan-shaped pattern across the rear of the stand and the width of the adjacent taxiway, extending approximately 90 metres behind the aircraft.
The investigation
The AAIB investigation focused on the internal failure of the APU. Examination of the unit showed that the turbine wheel had split into two halves, causing significant damage to the exhaust duct and the APU's external casing. While the containment ring successfully prevented the debris from exiting in-plane, the high-energy rotating components were able to exit through the exhaust duct.
Detailed analysis of the turbine wheel revealed that the fracture was caused by internal oxide inclusions within the casting. These aluminium-magnesium oxide films created areas of poor adhesion within the metal structure, leading to fatigue crack growth and eventual failure under operational stresses. The investigation also noted that this was one of nine similar turbine failures involving this specific type of component between 1999 and 2006.
Findings
- The APU failure was caused by the separation of the cast inflow turbine wheel into two halves.
- The root cause was the presence of aluminium-magnesium oxide inclusions within the core of the turbine casting.
- These inclusions led to fatigue crack initiation and subsequent rapid failure under normal working stresses.
- The containment structure of the APU functioned as designed, preventing any in-plane departure of debris.
- No injuries were sustained by the crew or ground personnel during the event.