Two pilots flying a commercial airliner filled with passengers were both asleep at the controls last month, leaving the jet on autopilot for an unknown amount of time, British aviation authorities said.
Authorities have not released the names of the pilots or the airline, but disclosed that the incident took place in August aboard an Airbus A330, capable of carrying up to 350 passengers, operated by a British-based airline.
The pilots came clean to authorities on their own, reporting the incident to their airline and the U.K.'s Civil Aviation Authority.
The pilots said they had received little sleep over two nights due to busy flight schedules and planned to rotate taking 20-minute naps, but they discovered they had both fallen asleep at the same time.
A pilot reported "both members of flight crew had only 5 hrs sleep in two nights due to longer duty period with insufficient opportunity to sleep," according to a report by the CAA.
The incident sheds new light on pilot fatigue and international concerns that flight crews receive insufficient sleep to perform their jobs safely and effectively.
The U.K. has rules governing how much sleep pilots need before they are permitted to operate an aircraft, but a CAA spokesman told ABC News it was unclear how much sleep the pilots really received because of changes in time zones when flying internationally.