More than half of pilots have fallen asleep on the flight deck, according to a survey by UK pilots' union Balpa.
And of the 56 per cent who admitted nodding off, as many as 29 per cent said they awoke to find the other pilot asleep.
The findings come after it emerged both pilots on an Airbus passenger plane were asleep at the same time with the UK-operated aircraft flying on autopilot.
One of the pilots indicated in a report to the UK's Civil Aviation Authority (CAA) that the pair nodded off after both had only five hours sleep in the previous two nights.
Details of the August 13 incident come as UK pilots' organisation Balpa is unhappy at proposed European changes to flight-time regulations.
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Of the 500 commercial pilots polled for the survey, 43 per cent said they believed their abilities had been compromised at least once a month in the last six months by tiredness, with 84 per cent saying it has been compromised at some stage during the past six months.
Also, 31 per cent did not believe their airline had a culture that lent itself to reporting tiredness concerns, with only half (51 per cent) saying they believed their airline chief executive would back them if they refused to fly because of tiredness.
Unprompted, 49 per cent said pilot tiredness was the biggest threat to flight safety - three times more than any other threat.
The poll results come ahead of a European Parliament vote on new EU rules on pilot flying hours next Monday.
