Dutch investigators have disclosed that they are investigating an incident on board a Transavia Boeing 737-800 in which a pilot left alone in the cockpit fell asleep.
The aircraft (PH-HZF) had been in the vicinity of the Greek island of Crete during a flight on 20 September 2012.
Investigators from the Dutch Safety Board have revealed, in a quarterly incident review, that the captain left the cockpit for the lavatory 2.5h into the flight.
When he returned he used the intercom to call for the cockpit door to be opened. But the safety board says there was "no reaction".
The captain informed the cabin crew and, when he was able to gain access the cockpit, he discovered that the first officer was sleeping.
While the incident had "no direct influence" on operations the airline reported the event to the safety board, which has classified the occurrence as serious.
Transavia declines to detail its internal procedures and will not comment on the incident.