What happened
On 13 June 1999, a Boeing 737-4Y0, registration G-OBMM, was parked at London Heathrow Airport for passenger embarkation. During pre-departure checks, the crew activated the auxiliary power unit (APU) to provide air conditioning and electrical power. Shortly after the APU began supplying power to the aircraft, cabin crew reported a whining noise originating from the rear of the aircraft.
Within minutes, the APU failed, and a fire warning was triggered. A ground engineer observed smoke and flames exiting the APU exhaust. In response, the crew executed the AP/U fire drill, alerted the Airfield Fire Service, and evacuated the approximately 30 to 40 passengers through the forward exit. There were no fatalities and no injuries to the crew or passengers.
The investigation
Initial inspections by a maintenance engineer found no structural damage to the airframe but noted burnt electrical wiring looms. During a subsequent scheduled maintenance check, the APU was removed from the aircraft, revealing extensive internal destruction.
Detailed examination of the unit showed that the compressor rotor could not be turned. Investigators found metal fragments within the compressor inlet and evidence of heavy damage to the 1st stage compressor impellers. A strip examination at an overhaul facility revealed that the APU rotor shaft and accessory gearbox components had seized. The investigation also uncovered that the front bearing of the APU rotor had completely disintegrated, and the turbine section had suffered severe heat distress, with melted turbine blade tips.
Findings
- The APU suffered massive damage to its 1st stage compressors, which released metal fragments into the system.
- These fragments caused secondary damage to the 2nd stage compressor, leading to a loss of airflow and a subsequent severe turbine overheat.
- The loss of mass in the compressor created a significant rotor imbalance, resulting in intense vibrations that caused the complete disintegration of the front rotor bearing.
- While foreign object ingestion was considered, the investigation also noted that a failure in the front bearing—potentially caused by arc-burning related to a previously repaired high-tension (HT) igniter lead—could have initiated the sequence of events.