The pilot of a fire surveillance aircraft that collided with terrain near Cloncurry, north‑west Queensland, was almost certainly hypoxic due to a known defect with the aircraft's pressurisation system, the ATSB has found.
The AGAIR‑operated twin turboprop Gulfstream 695A (a variant of the Aero Commander aircraft series) had taken off from Toowoomba to map fire zones near Mount Isa on 4 November 2023, with a pilot and two camera operators on board.
"Over a period of many months, the accident aircraft's pressurisation system was not reliably maintaining the required cabin altitude. 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," ATSB Chief Commissioner Angus Mitchell said.
Early in the accident flight, the pilot had descended from 28,000 ft to 15,000 ft for about 6 minutes, before climbing back to 28,000 ft.
Later, while the aircraft was nearing Cloncurry at 28,000 ft, both power levers were probably reduced, possibly with the intention of undertaking a similar descent profile. This caused the aircraft's speed to decay, before it ultimately entered a steep, descending, anticlockwise turn.
At around 10,500 ft the aircraft transitioned from the steep descent into an unrecoverable aerodynamic spin, until it impacted terrain, fatally injuring all on board.
The transition from steep descent to unrecoverable spin was almost certainly due to pilot control inputs made in an unsuccessful attempt to regain controlled flight.
"The ATSB found the onset of hypoxia during the flight significantly degraded the pilot's ability to safely operate the aircraft, and it is possible that at stages the pilot also experienced some loss of consciousness," Mr Mitchell said.
Air traffic control recordings of the pilot's speech during the accident flight demonstrated significant and progressive impairment, including slowed, stuttering and flat speech, operational mistakes and signs of confusion.
"Hypoxia can occur when an individual is exposed to high altitudes, typically above 10,000 ft, when not enough oxygen is supplied to the blood, tissues and cells for the body and brain to properly function," Mr Mitchell said.
ATSB examination of maintenance documentation and relevant internal correspondence confirmed the aircraft had a long‑term intermittent defect with the pressurisation system. At times this manifested as a reduced maximum attainable cabin differential pressure, exposing the cabin occupants to a relative altitude known to induce hypoxia.
"The intermittent defect was known about by AGAIR senior management, who attempted to have it rectified," Mr 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."
Correspondence and flight data showed the accident pilot had normalised the practice of managing the intermittent pressurisation issue by undertaking short descents to lower altitudes, and by using the aircraft's emergency oxygen system.
"This represented a practice of using a critical safety system designed for emergency use only, in order to continue a commercial activity," Mr Mitchell said.
The ATSB's investigation report details that about 1 hour and 50 minutes into the flight, while the aircraft was in the cruise at 28,000 ft, air traffic control (ATC) lost radio contact with the pilot.
Over the following 30 minutes, ATC made multiple attempts to re‑establish contact with the pilot, including using alternate frequencies and relaying messages via a military aircraft in the vicinity.
ATC eventually contacted the pilot on their mobile phone, and during that brief conversation, the controller noted the pilot's speech was slow and flat. As a result, the aircraft's status was upgraded to 'alert phase' and procedures for a hypoxic pilot emergency were initiated. Contact was again lost with the aircraft, and over the next 13 minutes multiple emergency radio broadcasts were made by ATC and the military aircraft in the vicinity.
During this 'alert phase', ATC phoned AGAIR management to raise concerns about the pilot experiencing hypoxia.
"In this conversation critical safety information about the known intermittent pressurisation defect in the aircraft was not relayed to ATC, had it been done so at that time, the aircraft could have been directed to a lower, safer, altitude when contact was re‑established with the pilot shortly afterwards."
Soon after that call, when contact was made, the pilot confirmed that their oxygen system was operating normally.
Based on this advice from the pilot, and with no knowledge of the aircraft's known pressurisation defect, the possibility of hypoxia was not re‑established a short time later when the pilot mispronounced words and repeated a clearance four times.
The investigation subsequently found there was no guidance in the 'hypoxic pilot emergency checklist' used by Airservices on ceasing an emergency response. This increased the risk during a potential hypoxia scenario of inappropriately downgrading the emergency response.
Since the accident, Airservices Australia has advised that it is in the process of conducting a review of the hypoxia in‑flight emergency response checklist.
Meanwhile, the ATSB has issued a formal safety recommendation to AGAIR to initiate an independent review of its organisational structure and oversight of operational activities to implement ongoing effective operational control by management.
Concluded Mr Mitchell, "This was a tragic and entirely unnecessary accident that underscores the dangers of operational practices which circumvent critical safety defences, and the insidious and deadly potential of altitude hypoxia."
You can find here the final report: Pilot incapacitation, loss of control and collision with terrain involving Gulfstream 695A, VH-HPY, 55 km south-east of Cloncurry Airport, Queensland on 4 November 2023
