Engine-covering doors on both engines on the smoke-trailing British Airways plane in last week's Heathrow landing drama had been left unlatched during maintenance, an interim report has found.
The unlatching had not been identified before the Oslo-bound Airbus A319 took off with 75 passengers and five crew on May 24, the report from the Air Accidents Investigation Branch (AAIB) said on Friday.
The fan cowl doors from both engines detached as the aircraft left the runway at Heathrow, puncturing a fuel pipe on the right engine, the report said.
75 passengers were on board at the time of the incident
The detaching also punctured the airframe and some aircraft systems and the flight crew, led by the 50-year-old captain, elected to return to Heathrow.
On the approach to land an external fire developed on the right engine, with the left engine continuing to perform normally tfhroughout the flight.
The report added that the right engine was shut down and the aircraft landed safely. The emergency services quickly attended and extinguished the fire in the right engine.
Passengers and crew evacuated the aircraft via the escape slides, without injury.
The report said: "Subsequent investigation revealed that the fan cowl doors on both engines were left unlatched during maintenance and this was not identified prior to aircraft departure."
The AAIB published a photo of the aircraft taken prior to its pushing back from the stand before take-off.
This was one of a number of photographs showing the fan cowl doors unlatched on both engines.
The report said the aircraft had undergone scheduled maintenance overnight. This required opening the fan cowl doors on both engines to check oil levels.
The report said that plane manufacturer Airbus had recommended airlines strictly adhere to maintenance standards following previous instances of fan cowl door separation on the A320 "family" of planes, which include the A319 in last week's incident.
Procedures for maintenance checks include crouching down to see that the fan cowl doors are closed and latched, the AAIB said.
The report said that last July Airbus said there had been 32 reported fan cowl door detachment events - 80% of which occurred during the take-off phase of flights.
On some occasions, significant damage was caused to the aircraft but none of the events resulted in a subsequent fire.
"The source of ignition that led to the in-flight fire (in last week's BA incident) is still under investigation," the AAIB said.
The AAIB recommended that Airbus notify Airbus A320 family aircraft owners of the BA incident and reiterate "the importance of verifying that the fan cowl doors are latched prior to flight by visually checking the position of the latches".
BA chairman Keith Williams said: "We welcome the publication of the AAIB interim report. We continue to co-operate fully with the investigation team and can confirm that appropriate initial action has already been taken in accordance with the AAIB's safety recommendation to Airbus.
"We regret we are precluded from releasing or discussing any additional details while the AAIB investigation is ongoing.
"We commend the professionalism of the flight crew for the safe landing of the plane and the cabin crew and pilots for its safe evacuation.
"We continue to offer our full support to those customers who were onboard the flight."