Dutch airline Transavia has launched an investigation after one of its pilots returned from the toilet to find his co-pilot asleep at the controls.
The incident occurred on a Boeing 737 flight to Crete in September, but was made public by the Dutch Safety Board (OVV) on 30 January in a report published on its website, reports Fox News.
According to The Australian, the report read: "After two-and-a-half hours in the air the captain of the Dutch-registered plane left the cockpit to go to the toilet.
"A little later he wanted to return to the cockpit. When he used the intercom to call the first officer to open the door he got no reaction.
"When he managed to get into the cockpit, he found the first officer asleep."
The OVV said it would decide what action to take after it has received a report from Transavia, a subsidiary of Air France-KLM.
Meanwhile, nearly half of Europe's commercial airline pilots say they have fallen asleep while in control of an aircraft, says a disturbing report.
A study of 6,000 pilots, carried out by the European Cockpit Association (ECA) in November 2012, says that between 43 and 54 per cent had drifted off "involuntarily" while flying. And one third of these claimed they had woken up to find their colleagues were also asleep.
The ECA is campaigning to highlight the safety issues caused by long flying hours. It says that fatigue among pilots is a common, dangerous and under-reported phenomenon in Europe.
The results of the ECA study come at a critical time, just as the European Aviation Safety Agency published proposals for new rules on flight and duty times and rest requirements for pilots.
Under the new rules, pilots could be expected to land commercial jets after being awake for stretches of up to 22 hours.
The proposals have been criticised by pilot organisations including the ECA, which says they will cause "significant consequences" for passengers.
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