What happened
On 4 November 2023, a Gulfstream 695A, registered VH-HPY, was performing fire zone line scanning north of Mount Isa, Queensland. During the flight at FL280, air traffic control lost contact with the aircraft. When contact was briefly re-established, the pilot's speech appeared slow and delayed, prompting an upgrade to an alert phase. Although the pilot initially reported that the oxygen system was functioning correctly, radio communication eventually ceased entirely.
Shortly after the final transmission, the aircraft entered a descending anticlocklewise turn. As the aircraft reached approximately 10,500 ft, it likely entered an unrecoverable aerodynamic spin. The aircraft struck the ground 55 km south-east of Cloncurry Airport, resulting in 3 fatalities. The impact and a subsequent fire destroyed the aircraft.
The investigation
The investigation focused on the pilot's physiological state and the aircraft's mechanical condition. Evidence from ATC recordings indicated that the pilot exhibited progressive impairment, including misarticulations and slowed responses, which were consistent with altitude hypoxia. These symptoms fluctuated with the aircraft's altitude, worsening as the flight continued at FL280. It is believed the aircraft was actually flying at approximately 29,400 ft, higher than the ADS-B transponder indicated, further increasing the risk of hypoxia.
Investigators also found that the aircraft's power levers had been reduced without a corresponding descent, causing airspeed to decay. During the final stages of the flight, control inputs likely transitioned the aircraft into the fatal spin.
Findings
- The pilot's ability to operate the aircraft was significantly degraded by altitude hypoxia.
- The aircraft's pressurisation system had a known, unresolved, and intermittent defect that prevented it from maintaining a safe cabin altitude.
- There was a normalized practice of operating VH-HPY at altitudes requiring supplemental oxygen without having a suitable supply available on board.
- Management at the operator, AGAIR, failed to exercise effective operational control, allowing the defect to go unrecorded and permitting flights to continue under hazardous conditions.
- Critical safety information regarding the known pressurisation defect was not communicated to air traffic controllers during the emergency phase.