American Airlines has revised its procedures after a communications breakdown led an Airbus A330-300 captain to attempt to halt an evacuation, which had been initiated by cabin crew unable to contact the cockpit.
Cabin crew ordered the evacuation after the aircraft quickly filled with smoke, while parked at London Heathrow's stand 307, as flight AA731 prepared for departure to Charlotte on 26 June last year.
While the Air Accidents Investigation Branch traced the smoke emission to an issue with the auxiliary power unit, the inquiry has focused primarily on the evacuation.
Several cabin crew members attempted to call the cockpit, but used the normal interphone call function. The pilots – who were dealing with an unrelated system defect along with other personnel in the cockpit – did not notice the interphone call, possibly because alarms had started sounding.
Investigators suggest that an emergency call to the cockpit "may have been more noticeable" than the normal call. The inquiry points out that interphone handsets differ between aircraft types.
"This lack of standardisation may have been a factor in the [flight attendants'] being unable to initiate an emergency call," it says.
After realising that the smoke was unrelated to the technical issue under discussion in the cockpit, the captain reacted by shutting off the APU bleed. He believed he had isolated the source of the smoke.
But the cabin crew had already commenced an evacuation – in line with procedures when the cockpit could not be contacted – and the captain, rather than making an emergency call to the cabin crew for co-ordination, intervened by using the public-address system in an attempt to halt the evacuation.
Investigators says this led to "some confusion" among the 277 passengers. Around 25 passengers left the aircraft via slides from the rearmost exits. Two other exit doors were also opened, one without a slide activation, but most of the passengers left the aircraft via the airbridge which was still connected.
"Exits at the front of the aircraft were not used, indicating that the [cabin crew] in this part of the aircraft were trying to achieve an 'emergency deplaning' via the [airbridge], even though an evacuation was commanded," says the inquiry.
"This might have been the most appropriate procedure in this situation but it was not a drill familiar to the [crew] and better crew communication was required before using it."
American Airlines established a task force in the aftermath of the event, and implemented several safety measures.
The task force informed pilots that the captain should only halt an evacuation if there is "clear information" that allowing it to continue would cause greater injury.
Analysis of the interphone system confirmed a need to refine communications processes on particular aircraft types.
Enhanced training procedures have also been introduced, including development of a type-specific emergency deplaning drill which relates to aircraft parked at a gate using an airbridge or steps.
American Airlines has stressed that it "prioritises safety in every aspect" of its operation and "intends to benefit from the lessons drawn from this event".
"We are proud of our pilots, flight attendants and our Heathrow-based colleagues who responded quickly, to ensure all customers and crew disembarked flight 731 safely," the carrier says.
American adds that it "constantly evaluates and updates" its training and procedures in order to strengthen its own safety culture and that of the broader air transport industry.