A pilot flying in central Queensland last August suffered a lack of "situational awareness" and the ability to communicate effectively due to a lack of oxygen, a safety report has found.
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The incident resulted in a shake-up of flight protocols at a Mackay-based air charter company.
An Australia Transport Safety Bureau investigation outlined how the pilot became affected by a lack of oxygen as he flew at altitude near Emerald on August 1.
The plane, an unpressurised twin turboprop Reims-Cessna F406, was fitted with an oxygen system for altitudes above 3050 metres when it departed Emerald.
The crew, consisting of the pilot and a navigator, had planned to climb to a flight level of 7315 metres to do some surveying in the area.
The pilot fitted an oxygen mask at about 2440 metres, but, as the plane continued to climb, the pilot, who had an oxygen pulse meter attached to his finger, noticed his blood oxygen level had dropped to 77 per cent.
The ATSB said that was "substantially less" than the 90 per cent or more that would normally be expected.
The pilot, who was aware of the dangers of hypoxia due to oxygen deprivation, alerted the navigator to his plight.
"Without an effective supply of oxygen, the accuracy with which the pilot was controlling the aircraft deteriorated," the ATSB found.
"The navigator recalled encouraging the pilot to maintain control and to descend, a number of times.
"The pilot recalled losing situational awareness, and prompts from the navigator drawing his attention to the attitude of the aircraft."
According to the navigator, the pilot failed to respond to some air traffic control transmissions.
"When he did respond, the responses were non-standard and his speech was slurred," the ATSB found.
"ATC [Air Traffic Control] also noticed that the pilot was not responding normally, also noting that his radio transmissions were slurred. ATC encouraged the pilot to ensure that he was receiving a supply of oxygen by transmitting 'oxygen oxygen oxygen' and clearing the pilot to descend."
Air traffic control declared an "ALERFA" – an alert phase declared when there were concerns about the safety on an aircraft and its occupants – and instructed the pilot to start a descent.
"Despite his apparently hypoxic condition, the pilot was ultimately able to identify that an oxygen supply system fitting had become disconnected," the ATSB found.
"When he reconnected the fitting, he took a number of deep breaths and sensed almost immediate relief."
The pilot said his "confused state" cleared quickly once he had taken some deep breaths, but he still remained on oxygen until he was able to land the plane safely at Emerald.
The flight lasted about 35 minutes, with evidence suggesting the pilot was suffering hypoxia for about 10 minutes, during which time the aircraft was at an altitude of between about 6100 and 7470 metres.
The investigation found the oxygen fitting could be connected in place without being locked.
"The position of the port is such that some concentration and manipulation is required to ensure that the connection is seated correctly," the ATSB said of the operator's report into the incident.
"The operator's report also found that the pilot's oxygen controller was not visible to the pilot or navigator.
"As such, the crew were unable to see indications on the controller that could have alerted them to an oxygen supply problem."
Following the investigation, the ATSB said, the operator had "taken steps in relation to pre-flight oxygen system checks, and crew cross-checks on the performance of each other's oxygen system during flights above 10,000 feet [3050 metres]".
