(CNN) — (CNN) -- Cruising at 30,000 feet, pilots snoozed in the cockpit of a 300-passenger airliner en route to Britain last August, UK aviation authorities told CNN on Thursday.
A spokesman for the UK's Civil Aviation Authority tells CNN that the Airbus A330 incident occurred while the aircraft was operating on autopilot on a long-distance flight. The CAA wouldn't reveal any other details of the flight, including the airline, its route or its destination airport.
The August 13 incident appears to be the result of bad scheduling by the airline, said the spokesman, Richard Taylor. The pilots reported having only five hours of sleep over two nights "due to longer duty period with insufficient opportunity to sleep," the CAA report states. "Both crew rested for 20 minute rotations and fell asleep."
The aircraft apparently arrived at its destination safely. It's the first British incident of its kind in two years, said Taylor. He said it's unlikely the pilots will be disciplined.
Fatigue ranks among the most sensitive issues in the pilot community. Rules surrounding sleep and relief crews vary from country to country.
Jim McAusian, general secretary of the British Pilots Association, used the incident to blast the CAA's record on pilot fatigue in a statement Thursday, accusing it of being "far too complacent about the levels of tiredness among British pilots and failing to acknowledge the scale of the underreported problem."
Next week, the European Parliament is expected to vote on new EU rules regulating pilot flying hours. McAusian said the proposed rules will increase "tiredness among pilots and the risk of dangerous incidents."
In the U.S., flights longer than eight hours require a relief pilot on board to take over so pilots can take a break to sleep. If the flight is scheduled to last more than 12 hours, an additional relief pilot must be added.
Last February, the Dutch airline Transavia said it had launched an investigation after a Boeing 737 pilot was locked out of the cockpit and his first officer was later found asleep at the controls. The sleeping pilot had been left alone at the controls while his co-pilot took a bathroom break.
The incident took place in September 2012, when the airliner was flying from Greece to the Netherlands, a top Dutch safety investigation agency said. The 737 landed safely in Amsterdam as scheduled.
Fatigue played a role in the 2009 crash of Colgan Air Flight 3407 near Buffalo, New York, which killed 50 people. The National Transportation Safety Board report on the crash cited pilot error as a factor, adding, "the pilots' performance was likely impaired because of fatigue, but the extent of their impairment and the degree to which it contributed to the performance deficiencies that occurred during the flight cannot be conclusively determined."
Also in 2009, Northwest Flight 188 -- an Airbus A320 carrying 144 people and five crew members -- flew past the Minneapolis-St. Paul International Airport. One of the pilots told CNN that no one was sleeping on the flight deck at the time. The two pilots "became distracted by a conversation," according to a report by the National Transportation Safety Board.