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Hypoxia is a condition in which a shortage of oxygen in the air being breathed affects the oxygen saturation of haemoglobin in the blood. Once such a shortage becomes significant, loss of consciousness quickly follows. In the context of aircraft in flight, the onset can be sudden or gradual. Sudden Onset may require a rapid and instinctive response by aircrew whereas gradual onset is a matter of awareness so that an appropriate response can be made before incapacitation occurs.
The Medical Background
Adequate oxygen saturation of haemoglobin is vital to human function. Although ambient air pressure reduces with increasing altitude and, as a direct consequence of this, the partial pressure of oxygen reduces too, oxygen saturation of haemoglobin is initially little affected. Between the surface and 10,000 feet altitude, even though air pressure decreases by 25%, the saturation of haemoglobin with oxygen only declines from about 98% to 90% which makes little difference to most human functions; the exception to this is a gradual onset of significant deterioration in night vision sensitivity, which has been found to reduce by 30% by 10,000 feet altitude. However, above 10,000 feet altitude, the amount of haemoglobin in the blood begins to decrease much more rapidly, much faster than air pressure which continues to decrease at a similar rate. By 20,000 feet altitude, the concentration of haemoglobin in the blood is only 65% saturation and at these levels, normal human function is materially interrupted and the effects are cumulative over time. At higher altitudes, the effects worsen quickly.
The Technical Response
Aircraft which routinely operate above 10,000 feet altitude are pressurised to keep the aircraft cabin no higher than the equivalent of 8000 feet altitude at any actual altitude up to the prescribed AFM maximum operating altitude. The partial pressure of oxygen is equivalent to the prevailing "Cabin Altitude". The existence of an air pressure inside the aircraft pressure hull, which is never less than that outside it, implies the existence of a pressure differential between outside and inside the aircraft. Aircraft pressurisation systems operate automatically and monitoring correct operation by reference to cabin altitude, cabin rate of climb and descent, and differential pressure is the only routine requirement.
The possibility of Hypoxia arises in two very different ways:
- Sudden loss of normal cabin pressurisation at high altitude as a result of explosive decompression - usually resulting from structural failure.
- Gradual and progressive onset during flight above 10,000 feet altitude in the absence of normal pressurisation. This can arise either by climb above 10,000 feet without the pressurisation system functioning, or because it is functioning incorrectly.
Defences - Sudden Onset
The Time of Useful Consciousness may be very short.
- For Aircrew - appropriate training which ensures the instinctive response of immediate oxygen mask donning if the obvious signs of sudden decompression occur and, in the case of the pilots, ensures that there is a sequential response so that control of the aircraft is maintained. The shortest available response times before consciousness will be lost are at high altitude in small aircraft.
- For Passengers - attention to the pre-departure cabin safety briefing, and recall if required since cabin crew will not be able to assist if sudden decompression occurs
Defences - Gradual Onset
The early symptoms of hypoxia do not include either discomfort or pain and may be more obvious to an observer than to the affected person. An increased rate and depth of breathing may be noticed but beyond that, a whole range of effects may apply which are dependant on the individual. The onset symptoms for Hypoxia are almost identical to those of Hyperventilation and it is important not to assume that they are due to Hyperventilation; Hypoxia is immediately life-threatening and should always be considered as the cause of these symptoms.
Strict adherence by the flight crew to SOP checks of pressurisation status, which usually provide warning of any abnormalities before automatic system warnings are generated. If pressurisation warnings or cautions are generated, the response prescribed in the QRH must follow. When such responses are executed immediately then this may preclude a need for fight crew to don oxygen masks or for passenger oxygen masks to drop (this usually occurs at 14000 feet altitude although some aircraft may need this to be selected manually).
Accidents & Incidents
- Two examples of the gradual onset case:
- Time of Useful Consciousness
- Crew Incapacitation
- Explosive Depressurisation
- Emergency Depressurisation
- Aircraft Pressurisation Systems
Related OGHFA Material
- Hypoxia (OGHFA BN)
- Stress and Stress Management (OGHFA BN)
- Well-Being (OGHFA BN)
- Lifestyle and Adverse Performance Effects (OGHFA BN)
- "Lessons Learned from Transport Airplane Accidents": Pressurization / Decompression Failures