Hypoxia symptoms reported during helicopter operations below 10,000 ft: a retrospective survey

During routine aviation medicine training, rotary-wing aircrew are instructed that the impact of hypoxia on them from flying in unpressurized cabins up to 10,000 ft (3048 m) above mean sea level (AMSL) is relatively small and has few implications for aviation safety. Such reassurance is based on data derived from experiments conducted on resting subjects and may not reflect the true impact of hypoxia in aircrew engaged in operational tasks.

A survey listing common symptoms of hypoxia was distributed to Australian Army helicopter aircrew who had operated at altitudes up to 10,000 ft AMSL.

There were 53 surveys that were returned (71% response), representing 25 loadmasters, 23 pilots, and 5 aircrewman technicians. All respondents were Australian Army aircrew. One or more symptoms consistent with hypoxia were reported by 86.6% of non-pilot aircrew and 60.9% of pilots. 60% of non-pilot aircrew reported four or more symptoms, compared with only 17% of pilots. The most commonly reported symptoms were difficulty with calculations (45%), feeling light-headed (38%), delayed reaction time (38%), and mental confusion (36%). Loadmasters reported more symptoms (mean 5.4) than pilots (mean 2.2) (p < 0.001). From the narratives provided (n = 21), aircrew experienced potentially operationally significant symptoms at a mean altitude of 8462 ft (2579 m).

The helicopter aircrew surveyed reported symptoms consistent with hypoxia at altitudes within the so-called physiological zone; loadmasters reported more effects than pilots. It may be inappropriate to emphasize the benign nature of the physiological zone during aviation medicine training of a non-resting population such as helicopter aircrew.