The Australian Transport Safety Bureau (ATSB) said in an investigation report released today that mild hypoxia likely played a part in an incident when the pilot of a Cessna Caravan fell asleep at the controls in July last year.
The pilot was ferrying C208B VH-DQP at 11,000 feet from Cairns to Redcliffe when it overflew its destination and failed to respond to ATC radio calls for 40 minutes. An RFDS King Air, FlyDoc 425, was sent to intercept the aircraft east of Brisbane, where the RFDS crew tried to trigger the Caravan's TCAS, but the pilot remained unresponsive.
After flying 60 nm past Redcliffe, the pilot regained consciousness, but appeared confused in communications with ATC. Eventually, ATC guided the pilot to land the aircraft at Gold Coast Airport.
The pilot had earlier requested a climb to 11,000 feet from 10,000 due cloud, and had begun to use the Caravan's supplemental oxygen intermittently, whereas pilots of unpressurised aircraft flying above 10,000 are required to be on oxygen constantly.
“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.”
ATSB investigators also concluded that the loss of consciousness was not caused by the hypoxia alone, but worked in conjunction with fatigue, dehydration and diet issues.
"Most people generally underestimate their level of fatigue and tend to overestimate their abilities," the ATSB states in their Safety Message. "The 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.
"Further, it demonstrates that, although mild hypoxia is not known to impair complex cognition it has been found to increase fatigue and decrease vigour. Symptoms of hypoxia can begin very subtly at lower altitudes and can also begin to show below 10,000 ft for people who are smokers, unfit, or fighting off an illness."
The full report is on the ATSB website.