A Korean Air pilot performed an emergency landing when a fire warning light came on in his cargo plane - but no blaze was found on board.
The Daily Mirror reports that the pilot later said the alarm could have been triggered by extra humidity generated by the 390 cows on board.
The alarm was set off over Bristol and the emergency landing at Heathrow was one of 88 mayday calls reported to the Civil Aviation Authority last year.
A mayday landing is the highest level of emergency and is only used in the most urgent cases.
Low fuel levels, suspected bombs and even crew members getting food poisoning were other reasons for using the international distress signal last year, the report said.
The Daily Mail reports that a spokesman for the CAA said it was a pilot's discretion to call a mayday based on an assessment of a given situation on board the aircraft.
'It could be the result of some technical failure to the aircraft or a passenger being unwell,' he said.
'The pilot will declare the mayday to air traffic control and communicate the nature of the situation.
'The air traffic controller may then give the aircraft a priority landing and scramble airport fire and rescue service or medical assistance if required.'
After extensive checks on the aircraft, the pilot said he believed the presence of the cattle led to a high level of humidity on the plane.
Cows produce high levels of methane gas, the second most significant heat-trapping emission.
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