The US National Transportation Safety Board says that an undiagnosed sleep apnea condition for the captain and crew work schedules were both factors in two pilots falling asleep during a flight operated by Mesa's Hawaii inter-island subsidiary Go in February 2008.

The crew was operating a Bombardier CRJ200 on a 40min flight from Honolulu to Hilo. But the aircraft flew 48km (26nm) beyond Hilo at cruising altitude. The captain and first officer later reported to Mesa management that they fell asleep during the flight.

"The fact that both pilots fell asleep during the mid-morning hours, a time of day normally associated with wakefulness and rising alertness, indicates that both pilots were fatigued," the NTSB says.

In its final report issued, NTSB unsurprisingly says the probable cause for the aircraft veering off course was the captain and first officer inadvertently falling asleep during the cruise phase of flight.

After the incident, the captain - who told the NTSB that he "dozed off" - was diagnosed with severe obstructive sleep apnea. But the board also says that symptoms such as snoring and risk factors of obesity were present before the flight.

"This condition likely caused him to experience chronic daytime fatigue and contributed to his falling asleep during the incident flight," the board says.

The board also stresses that the day of the flight was the third in a row that both pilots started duty at 05:40, which caused the crew to get less sleep than needed and increased daytime fatigue.

The NTSB estimates that the first officer accumulated a sleep debt of 1h 15min to 2h 45min in the 72h before the incident. The board cites a 1998 NASA study that concluded early report times make it more difficult for crews to obtain adequate sleep.

Another study conducted in the same year by the NATO Research and Technology Organisation highlighted by the board concluded that pilots reporting before 06:00 had a significant shorter total sleep time, impaired sleep quality and impaired performance both pre-flight and at the top of descent.

Legislation is proposed requiring the US Federal Aviation Administration to update and implement new flight and duty time rules for pilots, and for airlines to create fatigue risk management systems.

Source: Flight International