Coverings on both engines had been left unlatched after routine maintenance, on the British Airways plane in last week's Heathrow landing drama an interim report by the UK Air Accidents Investigation Branch (AIIB) has found.
ITV News reporter Paul Davies:
The engine doors punctured the airframe and some aircraft systems after coming loose and the flight crew decided to return to Heathrow.
On the approach to land an external fire developed on the right engine, although the left engine performing normally throughout the flight.
The emergency services attended and extinguished the fire after the plane had safely landed.
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 report said the BA plane had undergone scheduled maintenance the night before the incident, which required opening the fan cowl doors on both engines to check oil levels.
The report noted that the 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.
British Airways plane surrounded by emergency vehicles after it had to make an emergency landing at Heathrow. Credit: Steve Parsons/PA Wire
The report said that last July Airbus said there had been 32 reported fan cowl door detachments.
On some occasions, significant damage was caused to the aircraft but none of those events resulted in a fire.
"The source of ignition that led to the in-flight fire is still under investigation," the AAIB said.
Transport Secretary Patrick McLoughlin said the AAIB report on the Heathrow emergency landing "contains serious findings" and the aviation industry "must act immediately to take the appropriate safety action and ensure that all lessons are learnt from what has happened".
Earlier this week BA said that it will not pay compensation to passengers after flights were delayed and some cancelled after the emergency landing.