Investigation Summary
The flight proceeded normally until the aircraft reached its assigned cruising altitude of FL250. When asked to confirm his altitude at 10:33 UTC, the pilot provided a brief, slurred response, which was the final transmission from the aircraft. For the next eight minutes, open-microphone transmissions recorded the pilot's breathing becoming labored and eventually silent, while no sounds of passenger activity were detected. The aircraft, likely under the control of the autopilot in heading and pitch-hold modes, continued to climb and fly on a northeasterly heading, overflying its destination and crossing the Northern Territory into Queensland. Intercepting aircraft reported that the cockpit and cabin appeared dark and motionless. As the left wing fuel tanks neared exhaustion, the engine likely suffered momentary power losses, causing the aircraft to yaw and roll. The aircraft entered a descent and turned 90 degrees before impacting the ground at approximately 240 knots. The investigation was hampered by the extreme fragmentation of the wreckage and the remote, hot environment of the crash site, which limited the effectiveness of autopsies. While toxic fumes like carbon monoxide were ruled out, the ATSB could not determine why the aircraft failed to pressurize or why the supplemental oxygen system was not utilized by the occupants.
Final Conclusions
The Australian Transport Safety Bureau (ATSB) concluded that the pilot and passengers were incapacitated by hypobaric hypoxia due to the aircraft being unpressurized. The investigation identified two significant factors: the aircraft was probably unpressurized for a significant portion of the flight for undetermined reasons, and the occupants did not receive supplemental oxygen. The report noted that the lack of an aural high-cabin-altitude warning may have contributed to the pilot's failure to recognize the depressurization before becoming incapacitated.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
Correctly licensed and medically fit for the flight.
Systems & Failure Modes
Pressurization System
Uses bleed air from the engines to maintain a breathable atmosphere; the investigation could not determine if the failure was due to mechanical malfunction or incorrect switch selection.
Autopilot
Maintained the aircraft's heading and pitch after the crew became incapacitated, allowing it to fly until fuel exhaustion.
Supplemental Oxygen
A tank-based system with masks for the crew and drop-down masks for passengers; it was not successfully deployed or used.
Interesting Facts
- 01The aircraft flew for approximately five hours and covered over 2,800 kilometers from its origin.
- 02The pilot's final words were 'Sierra Kilo Charlie—um—standby.'
- 03The aircraft was not equipped with a high cabin altitude aural warning device, as it was not required by regulations at the time.
- 04Post-impact fire and high-speed disintegration left the wreckage in a highly fragmented state.
- 05Toxicological tests ruled out carbon monoxide and hydrogen cyanide poisoning.
Safety Actions & Advisories
Cabin Altitude Warning Enhancements
The ATSB recommended that CASA require visual alerts to operate when cabin altitude exceeds 10,000 feet (rather than 12,500 feet) and mandate the addition of an aural warning to accompany the visual alert.