Doors on both engines of the British Airways plane in last week's Heathrow emergency landing drama had been left unlatched during maintenance, according to an official accident report.
The unlatching had not been identified before the Oslo-bound Airbus A319 took off with 75 passengers and five crew on May 24, said the report from the Air Accidents Investigation Branch (AAIB).
The fan cowl doors from both engines detached as the aircraft took off, puncturing a fuel pipe on the right engine, the report said.
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 throughout 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 images 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 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 includes the A319.
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 per cent of which occurred during take-off.
On some occasions, significant damage was caused to the aircraft but none of the events resulted in a subsequent fire.
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