UK investigators have determined that a false fire alarm on a Korean Air Boeing 747-400F was probably caused by raised humidity levels from a cargo of cattle.
The aircraft (HL-7601) had been operating from Chicago to Brussels on 17 January with 390 cows herded onto the main deck.
But it diverted to London Heathrow after a detector in the aft main deck transmitted a fire warning to the pilots while the jet was cruising at 34,000ft over the Irish Sea.
In a bulletin on the incident the Air Accidents Investigation Branch says the crew "suspected that the warning was false", after making a visual check, but nevertheless conducted a rapid descent and executed a controlled depressurisation.
Emergency services at Heathrow discovered "no evidence" of smoke or fire, the AAIB says, adding that the crew believed "higher than normal" levels of humidity from the cattle had triggered the warning. There were no injuries among the three crew members on board.