UK investigators have confirmed that fan cowl doors were left unlatched after maintenance before a British Airways Airbus A319 suffered loss of the cowls on take-off from London Heathrow on 24 May.
Loss of the doors from both International Aero Engines V2500s during lift-off from runway 27L punctured a fuel pipe in the right-hand engine, the UK Air Accidents Investigation Branch states.
The airframe and some aircraft systems also sustained damage.
After the pilots elected to return to Heathrow an external fire developed on the right-hand engine during approach. The crew shut down this engine.
But the left-hand engine continued to perform "normally" throughout the flight, the AAIB says in an update to the inquiry.
This information contradicts earlier statements from the US National Transportation Safety Board, which had indicated that the shutdown and the fire affected different engines.
"Subsequent investigation revealed that the fan cowl door on both engines were left unlatched during maintenance," the AAIB says. "This was not identified prior to aircraft departure."
It adds that while the pilots had been unaware of the loss of the doors they were confronted with engine thrust-control degradation, a "significant" fuel leak, and the loss of the yellow hydraulic system.
Subsequent examination revealed that the detached cowl doors had damaged inboard leading-edge slats, the fuselage skin on both sides, and inboard flaps. The left horizontal stabiliser's leading-edge was also damaged, as were some of the aircraft's fairings.
Debris had struck the left main landing-gear door and damaged a hydraulic brake pipe.
The crew declared a "pan" call and stated an intention to return to Heathrow once they had assessed the situation. But after the fire started, the pilots upgraded the call to a Mayday.
Both fire bottles were discharged and the right-hand engine was shut down, but the fire persisted. The source of ignition for the fire is still under investigation.
Maintenance had been carried out on the aircraft overnight, to check the integrated drive generator oil levels, and this required the cowl doors to be opened. Photographs of the jet prior to pushback, says the AAIB, shows the doors were unlatched on both engines.
Loss of unlatched fan cowl doors has been a long-running issue for the A320 family, and the inquiry is recommending that Airbus reiterates to A320 operators the importance of visually verifying that the doors are latched before flight.
British Airways chief Keith Williams says the carrier has already taken "appropriate initial action" in response to the investigation, but declines to comment further while the probe is continuing.
"We commend the professionalism of the flight crew for the safe landing of the [aircraft] and the cabin crew and pilots for its safe evacuation," he says. "We continue to offer our full support to those customers who were on board the flight."