The pilot of a Gulfstream 695 Twin Commander that crashed near Cloncurry in November 2023 was almost certainly hypoxic according to an ATSB investigation report published today.
VH-HPY was on a mission from Toowoomba to photograph fire zones near Mount Isa when the aircraft entered an uncontrolled rapid descent and spin south-east of Cloncurry. The pilot and two camera operators on board did not survive the crash.
According to the ATSB, the pilot became hypoxic because of a known defect with the aircraft's pressurisation system, which had led to operator AGAIR developing techniques to deal with the problem.
"Over a period of many months, the accident aircraft’s pressurisation system was not reliably maintaining the required cabin altitude," ATSB Chief Commissioner Angus Mitchell said.
"This led some company pilots to employ a variety of actions in the aircraft to manage the potential and deadly effects of hypoxia, including at times briefly descending to lower altitudes, and improperly using emergency oxygen systems."
Earlier in the flight, the pilot had requested a descent from FL280 to FL150, which was held for six minutes before climbing back to FL280.
ATC had detected the possibility of hypoxia after they lost contact with HPY, and after several failed attempts to re-establish contact, resorted to ringing the pilot on their mobile phone. Controllers reported the pilot's speech as "slow and flat", prompting them to declare an alert phase on the aircraft and initiate hypoxia emergency procedures.
After re-establishing radio contact with HPY, ATC downgraded the alert, then canceled the emergency procedures. The pilot confirmed the pressurisation system was operating normally, and ATC cleared the aircraft to undertake line scanning the fire zone.
However, the pilot's ability to communicate on the radio worsened.
"Over the following 4 minutes, the pilot repeated the clearance from ATC four times, seeming uncertain about the status of the clearance," the ATSB found. "The radio recordings during this period indicate that the pilot’s rate and volume of speech had substantially lowered from earlier communications and was worsening.
"The pilot’s final radio transmission displayed the slowest speaking rate of all their communications during the flight and contained stuttering and operational mistakes."
ATSB investigators also determined that both power levers had been rolled-back without a descent being started, which caused the airspeed to decay. The aircraft then entered a descending anti-clockwise turn with an increasing rate of descent, and despite apparent control inputs made at around 10,500 ft, the aircraft changed from a high‑speed descent to an unrecoverable spin.
“The intermittent defect was known about by AGAIR senior management, who attempted to have it rectified,” Mitchell said.
“However, they did not formally record the defect, communicate it to the safety manager, undertake a formal risk assessment of it, or provide explicit procedures to pilots for managing it.
“Instead, AGAIR management personnel participated in and encouraged the practice of continuing operations in the aircraft at a cabin altitude of 19,000 ft, and as such required the use of oxygen, without access to a suitable oxygen supply.”
Investigators found there was no guidance in the "hypoxic pilot emergency checklist" used by Airservices Australia on canceling an emergency response, which the ATSB says increased the risk of inappropriately downgrading the emergency response during a potential hypoxia scenario.
Airservices Australia is conducting a review of the hypoxia in‑flight emergency response checklist, and the ATSB has issued AGAIR with a formal safety recommendation to conduct an independent review of its organisational structure and oversight of operations.
The full report is on the ATSB website.