Three tragic deaths, one preventable cause: would you have spotted the symptoms?
- Dr. Rob Massera | DAME
- Jun 30
- 4 min read
What happened to VH-HPY?
On November 4th 2023, VH-HPY a Gulfstream 695A twin turboprop, took off from Toowoomba Queensland en-route to a fire reconnaissance operation north of Mount Isa, with a single pilot and two camera operators onboard. The aircraft had a known pressurisation system defect, resulting in a cabin pressure-altitude of 19,000 feet. Despite this, the pilot flew the aircraft between altitudes of 15,000 to 28,000 feet for more than two hours. ATC lost radio contact for more than 30 mins, & when communication was re-established ATC observed the pilot’s speech was “slow & flat”. However, despite the pilot’s reassurance of normal operations, the aircraft gradually entered an increasingly steep descending turn, before impacting the ground south-east of Cloncurry - killing all onboard.
The ATSB investigation concluded that the pilot was incapacitated by acute altitude hypoxia. Although rare & well documented, it has been the cause of several fatal accidents in Australia.
Understanding Altitude Hypoxia: an insidious threat to Pilots and Flight Safety
With increasing altitude, pilots are exposed to a hypobaric (low pressure), hypoxic (low oxygen) environment. While the percentage of oxygen in the air doesn’t actually drop, its partial pressure does, making it harder for the body to extract. Without cabin pressurisation or supplemental oxygen, the brain and body are gradually starved of the oxygen needed to function - resulting in acute altitude hypoxia - with 4 stages:
Indifferent stage (MSL to 5,000 ft) - virtually no compensatory physiological responses or performance decrements in a healthy pilot.
Complete compensatory stage (5,000 to 10,000 ft) - physiological compensation with near normal performance in a healthy pilot - however night vision is decreased by 10% to 30% at 5,000 to 10,000 ft respectively & novel tasks, like emergency procedures, may be impaired.
Partial compensatory stage (10,000 to 20,000 ft) - incomplete physiological compensation & impaired performance - slow, flat or delayed speech is an early sign, lack of awareness, slow thinking & reaction times, task fixation or repetition.
Critical stage (above 20,000 ft) | All senses fail, unconsciousness occurs with little/no warning - no external stimuli, such as alarms or radio-comms, can help a pilot suffering from severe altitude hypoxia.
Q: What are the symptoms & signs of altitude hypoxia?
Symptoms & signs are insidious & include: Slow, flat or delayed speech, light-headedness, headache, irritability, dizziness, confusion, impaired judgment, task-fixation, slowed reaction times, tingling of the mouth/fingers, increased breathing rate, impaired peripheral/night vision and eventually unconsciousness - often without the pilot realising they’re affected! However, each pilot’s symptoms are highly idiosyncratic, meaning that with altitude chamber training pilots can experience their unique symptoms, learn to recognise the early phases of hypoxia & take corrective action before becoming significantly impaired.
Q: How long does a pilot with hypoxia have to take corrective action?
Time of Useful Consciousness (TUC) is defined as the period of time a pilot is able to perform corrective actions under hypoxic conditions in flight. Above 10,000 feet, a pilot’s cognitive and motor performance begins to degrade; at 15,000 to 18,000 feet, signs & symptoms of hypoxia occur - a pilot may only have 20 to 30 minutes of useful consciousness; while at 28,000 feet the TUC is just 2 to 3 minutes!
Furthermore, while all pilots are susceptible to the effects of hypoxia, health factors can increase susceptibility (even at lower altitudes) - such as: illness or pre-existing medical conditions, smoking/vaping, excessive alcohol/hangover, fatigue, cold-stress or physical exertion at altitude.
“The great danger of altitude hypoxia is its insidious onset - causing subtle yet increasing impairment…and eventually…total incapacitation.”
Q: What countermeasures can pilots take?
Firstly, pilots should maintain better than average fitness, proactive management of pre-existing health conditions & NEVER smoke. Secondly, pilots should either undertake altitude-chamber training or education about altitude hypoxia & its early symptoms. Thirdly, pilots must ensure functioning pressurisation systems or oxygen supplies (if piloting unpressurised aircraft above 10,000 ft) - prior to departure. Finally, in the event of early symptoms or decompression, immediate descent to 10,000 feet or the use of supplemental oxygen.
The vital role of aviation medicine
With basic knowledge of acute altitude hypoxia, sadly the risks, signs & symptoms were patently clear. The pilot of VH-HPY spent considerable time in the partial compensatory & critical stages of acute altitude hypoxia; radio contact was lost, likely due to mental-slowness, inattention or task-fixation; the air-traffic controller noted the pilot had slow, flat speech; the pilot reassured ATC, yet was clearly unaware he was hypoxic & medically impaired.
This “entirely preventable” accident highlights why aviation medicine is important & more than just “box ticking” - it is a foundation of flight safety. It ensures pilots are medically fit-to-fly, understand their physiological limits & the countermeasures they can take. Understanding, recognising, and mitigating the risks of hypoxia can literally mean the difference between life and death for pilots, crew & passengers.
Q: Got more questions?
Contact us: info@aerokare.com.au
References & links to more information:
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