A pilot fell asleep during cruise and flown his aircraft 111km beyond the intended destination airport before he woke up and contact the Air Traffic Control.
The incident which happened in July 2020 was investigated by the Australian Transport Safety Bureau (ATSB).
The pilot was operating a Cessna 208B Caravan performing a ferry flight from Cairns to Redcliffe, Australia.
The aircraft was cruising at 10,000ft when the pilot was met with unforecasted icing conditions with low visibility and requested to climb to 11,000ft. There after the pilot started to use supplemental oxygen intermittently.
It is mandatory for the flight crew to use supplemental oxygen continuously for any period in excess of 30 minutes above 10,000ft.
The ATC attempted to contact the pilot 90 km from the destination however there was no response. After several failed attempts, ATC requested nearby aircraft to assist in contacting the aircraft.
A pilot of another aircraft flew close to the Caravan to trigger the traffic alert and collision system to wake him up to which there was no response.
Ultimately after 40 minutes, the pilot woke up and contacted ATC. By then the Caravan was 111 Km southeast of the intended destination airport. The pilot continued for a safe landing.
According to the investigation, the pilot most likely fell asleep due to fatigue exacerbated by mild hypoxia from the intermittent use of supplemental oxygen.
The report also highlighted that it is very unlikely that a pilot may become unconscious due to mild hypoxia alone.
It is also not common for a person suffering from hypoxia to regain consciousness, while still operating at the same altitude and without additional oxygen.