A pilot fell asleep at the controls as his plane flew over Brisbane and the Gold Coast for 40 minutes before he woke up, an investigation has found.

The Air Transport Safety Bureau said the Cessna 208B Caravan aircraft was about 53 kilometres from Redcliffe airport when the pilot became uncontactable.

The plane, which had departed Cairns on the afternoon of July 2, 2020, overflew the Redcliffe runway and continued on towards Brisbane Airport.

The ATSB said the pilot of a Royal Flying Doctor Service Beechcraft B200 King Air taking off from the larger airport tried to intercept the Cessna.

The second pilot approached the Cessna to try to trigger its traffic alert and collision system, but the other pilot remained unresponsive.

The smaller plane continued for 111 kilometres before the pilot woke and spoke to air traffic controllers.

The path taken by the sleeping pilot, who overflew Redcliffe Airport before landing at Coolangatta. Image: ATSB

He had been asleep for about 40 minutes, according to the ATSB report.

The pilot safely landed at the Gold Coast Airport not long after, without incident.

The ATSB probe said the pilot probably fell asleep because he was fatigued due to inadequate sleep in the lead-up to the flight.

The investigation found he had suffered mild hypoxia as he had been using only intermittent oxygen despite flying above 11,000 feet.

“This likely exacerbated the pilot’s existing fatigue and contributed to the pilot falling asleep,” ATSB acting transport safety director Kerri Hughes said.

She said the incident demonstrated the importance of pilots monitoring their own health and wellbeing before flying.

“To ensure that they are well rested and adequately nourished, especially when conducting single pilot operations,” Ms Hughes said.

She said a common symptom of hypoxia was a loss of consciousness, but it was not typical for people to wake up when they were still at the same altitude without oxygen.

Ms Hughes said information from medical specialists indicated the pilot had not lost consciousness solely because of mild hypoxia.

“Rather, the pilot had fallen asleep likely due to a combination of fatigue and mild hypoxia, possibly exacerbated by dehydration and diet,” she added.

The ATSB said pilots must use continuous oxygen when flying above 10,000 feet in unpressurised aircraft.

-AAP