What happened
On 26 June 2016, an Airbus A3/330-323, registration N276AY, was parked at a stand at London Heathrow Airport during the boarding process. While the aircraft was connected to a jetbridge, smoke began to fill the cabin, originating from the ventilation ducts.
As the smoke became dense, cabin crew members, unable to establish immediate contact with the flight deck via the interphone, initiated an emergency evacuation. Some passengers exited the aircraft via the jetbridge at exit 2L, while others used the emergency slides from the aft doors. During the process, passengers opened exits 3L and 3R; however, the slide at 3L failed to deploy because the door had not been armed. One passenger sustained a minor injury during the slide evacuation.
The commander, believing the source of the smoke had been isolated by shutting down the Auxiliary Power Unit (APU), issued a passenger announcement to halt the evacuation. This instruction caused temporary confusion among the crew and passengers, though the evacuation eventually continued via the jetbridge and slides.
The investigation
The AAIB investigation focused on the source of the smoke and the breakdown in communication between the flight deck and cabin crew. Investigators examined the APU and the aircraft's ventilation systems. They also reviewed the crew's response to the smoke, the functionality of the interphone system, and the procedures for evacuations while an aircraft is parked at a gate.
It was established that the flight deck crew was distracted by an ongoing engineering task related to a navigation system defect. This distraction, combined with the simultaneous activation of various cockpit alarms, meant that the crew did not initially notice the cabin crew's attempts to call the flight deck. Furthermore, the investigation found that the crew had not practiced specific 'emergency deplaning' drills for aircraft connected to a jetbridge.
Findings
- The smoke was caused by the failure of the APU load compressor carbon seal, which allowed hot oil to enter the bleed air supply and pyrolyse.
- Metallic debris within the shared oil system had compromised the load compressor bearing, leading to the seal failure.
- A breakdown in communication occurred between the flight deck and cabin crew, as the commander's instruction to stop the evacuation was made without consulting the cabin crew.
- The cabin crew's inability to reach the flight deck was due to the crew being distracted by other cockpit alerts and the interphone call not being sufficiently conspicuous.
- The evacuation was complicated by the fact that the crew had not been specifically trained for unplanned evacuations while the aircraft was parked at a gate using a jetbridge.