Hypoxia and Aviation

A passel of recent headlines (here, here and here, for instance) have highlighted a persistent hypoxia problem facing pilots of jet aircraft in both the Air Force and the Navy. These or similar episodes, designated “physiological episodes”, are blamed for the deaths of four Naval aviators over the past several years.

“Mountain sickness”, that is, the effects of altitude, were first written about in western literature in the 16th century, most particularly in a description of the syndrome by Father Jose de Acosta, who, in 1590, published his observations on the effects of altitude on men and animals in the Andes mountains of Peru. The British scientist Robert Boyle was the first to identify a vital factor in air that was lacking at altitude. Joseph Priestly identified that vital factor in 1774, and Antoine Lavoissier named it “oxygen” in 1777.

There the matter lay until men started going up in balloons, although apparently the first recorded “altitude-related” hypoxic deaths resulted when three men (two of whom died) were subjected to a simulated altitude of 28,000 feet in a pressure chamber developed by French physiologist Paul Bert, in 1875. As a result of these and other experiments, Bert was able to show that the breathing of supplemental oxygen could prevent the physiological ill effects of altitude.

With the advent of fixed wing aviation, and in particular, military aviation in World War I, the main thrusts of aviation medical research involved the physical safety of pilots (restraining apparatus and the like), and dealing with the cold of altitude. The use of supplemental oxygen apparently was a given, and both gaseous and liquid O2 were used.

Aviation medicine research between the wars depended on a few unsung stalwarts who responded to queries concerning the physiological effects – now including loss of consciousness while pulling gs – from the aviators and engineers who designed ever more capable aircraft. That said, developing oxygen delivery systems for aircraft expected to operate at high altitudes for hours at a time – bombers – was a critical matter. The advent of high performance jet aircraft in the late 1940s led to significant improvements in aircraft oxygen systems.

Physiologists now break the effects of hypoxia out in “stages”, viz.,


  1. INDIFFERENT STAGE – The only adverse effect is on dark adaptation.
  2. COMPENSATORY STAGE – Physiological compensations provide some defense against hypoxia so that the effects are reduced unless the exposure is prolonged or unless exercise is undertaken. Respiration may increase in depth or slightly in rate, and the pulse rate, the systolic blood pressure, the rate of circulation, and the cardiac output increases.
  3. DISTURBANCE STAGE – In this stage the physiological compensations do not provide adequate oxygen for the tissues.
    Subjective symptoms may include fatigue, lassitude (state of exhaustion), somnolence (drowsiness, sleepiness), dizziness, headache, breathlessness, and euphoria.
    Objective symptoms include:
        Special Senses – Both the peripheral and central vision are impaired and visual acuity is diminished. 
        Extraocular muscles are weak and incoordinate
     – Touch and pain are diminished or lost. Hearing is one of the last senses to be impaired or lost.
        Mental Processes – Intellectual impairment is an early sign and makes it improbable for the individual to comprehend his own disability. Thinking is slow. Calculations are unreliable.
    Memory is faulty. Judgment is poor. Reaction time is delayed.
        Personality Traits – There may be a release of basic personality traits and emotions as with alcoholic intoxication (euphoria, elation, pugnaciousness, overconfidence, or moroseness).
    Hyperventilation Syndrome
     – Over-breathing due to excitement or stress. Cyanosis – Blue discoloration of the skin.
  4. Critical Stage – In the critical stage consciousness is lost. Death follows shortly.

Source: http://www.mountainflying.com/pages/mountain-flying/hypoxia.html, accessed 31 July 2017

Which brings us to today. Modern jet combat aircraft – including the T-45 trainer, Navy F/A-18s and E/A-18s, and all F-35s – have very sophisticated on-board oxygen generation systems. Engineers’ attention is now being directed to this commonality, the “OBOGS”, among the aircraft where the “hypoxia-like” episodes have happened. Another factor seems to be altitude: all reported episodes appear to be have occurred above 25,000 feet, and engineers now suspect a malfunction in the oxygen metering device in these systems at these altitudes.

Until the engineers find a solution to the problem, the services are limiting operations in OBOGS-bearing aircraft to below 25,000 feet, introducing oxygen monitoring devices and beefed up supplemental oxygen supplies, and giving aviators additional training in how to recognize symptoms of hypoxia before they reach the “disturbance” stage.

(c)2017 Thomas L Snyder

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