Jan 23, 2023

Many aviation incidents and accidents are officially attributed to a pilot’s inability to detect and react to various physiological changes which results in their skills, judgement and abilities becoming compromised.

When operating at high altitudes there are a number of physiological problems that can affect pilot safety and, therefore, the safety of the crew, passengers and, ultimately, the aircraft. Hypoxia is one of these and in fact the most commonly encountered problem pilots must contend with at high altitudes.

What Is Hypoxia?

In short, hypoxia is when there is not enough oxygen circulating in the blood, tissues and cells to maintain normal bodily functions. As humans we are not designed to exist at the altitudes aeroplanes fly at, our lungs do not have the capacity nor the ability to function at their best in the high altitude low density oxygen environments.

What Makes Hypoxia So Dangerous For Pilots?

Other than the obvious issues with bodily function, the primary danger of hypoxia in an aviation setting is due to the fact that generally, people suffering from the condition do not necessarily feel its onset or realise it is occurring, which means its effects (the impairment of cognitive and physical performance) can go unnoticed for some time.

It also pays to note that an individual’s ability to withstand the effects of hypoxia may differ considerably from others in the early stages, meaning some people on board may be more seriously affected than others and also affected differently.

This delay in recognising symptoms can result in the aircraft becoming unsafe or out of the pilot’s control, lessening the chances of recovery or correction before an accident occurs. It can also mean others have a difficult time convincing them there is a problem.

What Causes Hypoxia In Aviation?

Between the earth’s surface and altitudes of 10,000 feet, the oxygen levels of a healthy person are not affected too much, however, at altitudes above 10,000 feet, the amount of oxygen in the blood decreases at a much more rapid pace. This is essentially because the lungs cannot transfer oxygen from the air into the blood as effectively under reduced air pressure conditions. Therefore, the higher the altitude, the more quickly the effects worsen.

Hypoxia during flight can occur for a number of reasons, some of the most common causes in aviation are malfunctioning oxygen equipment or pressurisation systems, flying non-pressurised aircraft above 10,000 feet without supplemental oxygen, rapid rates of ascent or descent, extreme cabin temperatures and unexpected rapid decompression during flight. In addition, a person’s existing medical and physiological issues affecting blood circulation or the quantity of oxygen carried in the blood can also be a factor.

Types Of Hypoxia

There are four general types of hypoxia; hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia.

Hypoxic Hypoxia. Sometimes referred to as “altitude hypoxia”, hypoxic hypoxia is the most common form encountered in aviation. Hypoxic hypoxia occurs when there is insufficient amounts of oxygen available to breathe in from the air meaning the lungs struggle to transfer enough oxygen to the blood. This form of hypoxia most commonly occurs in unpressurised aircraft flying at altitudes above 10,000 feet or in the case of oxygen system failure.

Hypemic Hypoxia. A result of the blood’s reduced ability to carry oxygen, hypemic hypoxia is usually caused by low haemoglobin content due to things like exposure to too much carbon monoxide (excess carbon monoxide in the body greatly reduces the amount of haemoglobin available to carry oxygen around the body). During flight, CO poisoning is likely a result of equipment malfunction and cockpit contamination.

Hypokinetic Hypoxia. For this one there is enough oxygen in the blood however existing circulatory system problems (e.g. heart failure) or by blood pooling in the lower limbs under high g-force manoeuvres prevents this oxygen from getting where it needs to be. Stagnant hypoxia in aviation most often occurs from rapid decompression during flight or when operating in very cold weather conditions without adequate cabin heating.

Histotoxic Hypoxia. This form of hypoxia is the impaired ability to absorb oxygen due to introduced substances such as alcohol, narcotics and cyanide poison. There is plenty of oxygen in the blood and its getting to the cells but the body itself is rejecting it due to the toxins present that effectively prohibit that cell from taking oxygen from the blood.

What Are The Signs And Symptoms Of Hypoxia?

Because the initial signs and symptoms of hypoxia can be difficult to detect and vary between individuals, it is essential for all aviation personnel to know what to look out for and how to react in order to maintain the ongoing safety of the flight.

These are the 13 most common signs and symptoms of hypoxia experienced by pilots.

  1. Headache
  2. Changes in vision
  3. Unexplained tiredness and fatigue
  4. Confusion
  5. Euphoria
  6. Inability to concentrate
  7. Blueness of the lips and fingertips (Cyanosis)
  8. Pounding in the ears
  9. Impaired decision-making
  10. Breathlessness
  11. Excessive yawning
  12. Impaired cognitive performance and motor skills
  13. Loss of consciousness

Hypoxia In Action

The variation in the nature of hypoxic symptoms and their sudden or gradual onset means it can be difficult for a pilot to recognise whether his or her actions are becoming hazardous. In practice, this could mean pilots are slow to react, have a tendency to repeat an action without realising that they just completed that action moments before, exercise poor judgement and struggle with relatively easy physical movements, and become unnaturally uncoordinated.

Pilots may also often have trouble concentrating when suffering from hypoxia which may be evident when attempting to read instruments, respond to instructions, or follow through with an action. Delayed or seemingly dulled communication responses can be an indication something is amiss as can changes in a pilot’s voice.

Mood disturbances are also common and can include anything from deep sorrow, uncontrollable laughing, to nervous behaviour and aggressiveness. These uncontrollable mood changes can result in the appearance of being drunk, fights breaking out among crew members or being incapacitated with grief and depression – all leaving the individual unable to perform the tasks needed to continue to operate the aircraft safely.

Some good examples of hypoxic behaviour in pilots include a ‘nothing can be done’ response to problems that arise and carrying out actions the complete opposite of what should be done. Because of the types of symptoms described above, it can be difficult for air traffic controllers or other people to convince the pilot there is actually a problem making recovery actions even more difficult to achieve.

Preventing Hypoxia

There are two general methods for preventing hypoxia in aviation. One involves pressurising the aircraft cabin (artificially increasing the air pressure). The other method is to increase the breathable oxygen in the air using a supplemental oxygen generation system (wearing an oxygen mask).

Cabin pressurisation controls the cabin barometric pressure or the inner-outer pressure difference by utilising the incoming and outgoing air flows. Aircraft pressurisation is controlled by the aircraft’s onboard electronic systems enabling crew and passengers to travel at high altitudes safely and comfortably.

Supplemental oxygenation or O2 devices are generally only used in emergencies involving cabin pressurisation failure, or sometimes can be used to enhance night flying conditions and situations where pilots are operating at or near 10,000 feet altitudes.

Insufficient training or experience in high-altitude physiology will lead pilots into unsafe operating conditions. High altitude-training is an important factor in learning to fly that will ensure you know what to look for if hypoxia occurs — interested in flight training in New Zealand? Get in touch with the Southern Wings Flight School for more information.

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