A Queensland pilot who flew past his destination had fallen asleep for 40 minutes at the controls of his aircraft, an investigation from the Australian Transport Safety Bureau (ATSB) has found.
The pilot was flying a Cessna 208B Caravan on a repositioning flight aircraft from Cairns, to Redcliffe in July 2020.
While cruising at 10,000 feet the pilot encountered unforecast icing conditions and poor visibility due to cloud and climbed to 11,000 feet and began using the aircraft's supplementary oxygen system intermittently. Pilots are required to continuously use an oxygen supply when flying unpressurised aircraft, such as the Caravan, above 10,000 feet.
The pilot flew past his destination and was unresponsive to air traffic control calls for 40 minutes, the investigation revealed.
"Fatigue was likely exacerbated by mild hypoxia, contributing to the pilot falling asleep; pilots need to monitor their health and wellbeing to ensure they are well rested before flight," the report said.
When the aircraft was about 53km north-west of Sunshine Coast Airport, air traffic control unsuccessfully attempted to contact the pilot regarding his planned descent into Redcliffe.
Following repeated calls to the pilot, ATC enlisted the assistance of pilots in nearby aircraft to contact the Caravan pilot, who was seen to overfly Redcliffe and track towards Brisbane.
The pilot of a Royal Flying Doctor Service Beechcraft B200 King Air aircraft departing Brisbane was asked by ATC to intercept and contact the Caravan pilot, but their initial efforts were unsuccessful. The King Air pilot then dipped their wings and approached the Cessna in an attempt to trigger its traffic alert and collision system (TCAS), but the pilot remained unresponsive.
At 5:35 pm (local time), after 40 minutes without contact and when the aircraft was about 111km south-east of the intended destination, the pilot woke and ATC communications were re-established. The pilot was then instructed to land at Gold Coast Airport, which he safely did just after 6pm.
"The ATSB found that the pilot was likely experiencing a level of fatigue due to inadequate sleep the night before and leading up to the incident," ATSB Acting Transport Safety Director, Kerri Hughes, said.
"Further, operating at 11,000 feet with intermittent use of supplemental oxygen likely resulted in the pilot experiencing mild hypoxia. This likely exacerbated the pilot's existing fatigue and contributed to the pilot falling asleep."
Hughes said most people generally underestimate their level of fatigue and tend to overestimate their abilities.
"This incident emphasises the importance of pilots monitoring their own health and wellbeing, to ensure that they are well-rested and adequately nourished, especially when conducting single pilot operations."
Hughes said the investigation carefully considered the role of hypoxia in the incident.
"Although a common symptom of hypoxia is loss of consciousness, it is not typical for someone experiencing hypoxia to regain consciousness, while still operating at the same altitude and without additional oxygen," she said.
"Therefore, from the information obtained by a medical specialist engaged by the ATSB and from studies conducted on mild hypoxia at moderate altitudes, the ATSB determined that it was unlikely that the pilot had lost consciousness solely due to mild hypoxia.
"Rather, the pilot had fallen asleep likely due to a combination of fatigue and mild hypoxia, possibly exacerbated by dehydration and diet."