What happened
On 9 May 2008, during pre-startup checks at Manchester Airport, the crew of a Boeing 767-324, registration G-OOBL, detected a burning smell originating from the rear of the cabin. Upon investigation, the commander decided to evacuate the passengers. During the disembarkation process, an engineer alerted the commander that the aircraft appeared to be on fire and advised the immediate shutdown of all electrical power. In response, the commander issued a mayday call to Air Traffic Control, deactivated all aircraft power, and exited the aircraft via the standard emergency route.
There were no fatalities and no injuries resulting from the incident.
The investigation
Technical examinations focused on the potable water system's air compressor, which is responsible for pressurizing the aircraft' and is located beneath the rear cabin floor. Investigators found that the electric motor driving the compressor had undergone severe overheating, with the protective paint discoloured by temperatures reaching at least 125°C. The internal components of the motor were so heavily damaged that the rotor had been separated from its core.
Crucially, the investigation determined that the thermal cut-out switch, designed to protect the motor from overheating, had failed. One of the three sets of electrical contact points within the switch had become welded together, which kept the electrical supply to the compressor motor constantly active. This prevented the system from automatically disconnecting power even as the motor reached dangerous temperatures.
Findings
- The primary cause of the overheating was a failure of the unloader check valve, which allowed moisture or liquid to enter the air compressor bellows.
- The unloader check valve had failed in the open position, which caused the compressor to lock up and overheat.
- The thermal protection switch failed to trip because its electrical contacts had fused in the 'on' position, allowing a continuous supply of electricity to the motor.
- The compressor motor experienced extreme heat, resulting in the physical separation of the rotor from the motor core.