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LESSONS · HUMAN FACTORS · 15 MINUTES · AUSTRALIA AND NZ

Hypoxia and altitude.

Climb, and each breath brings in oxygen at a lower pressure. PHAK's greatest concern is the brain, which is particularly vulnerable to a lack of oxygen. Climb through the effects PHAK describes, in the order it gives them, then see when your country's rule requires supplemental oxygen.

This lesson is general education from the handbooks and the rules. It is not medical advice.

PREDICT FIRST

You climb from sea level to 18,000 ft. What happens to the share of the air that is oxygen?

Thinner air

Hypoxia is a shortage of oxygen. PHAK's chief worry in flight is the brain.

Hypoxia means “reduced oxygen” or “not enough oxygen.” Although any tissue will die if deprived of oxygen long enough, the greatest concern regarding hypoxia during flight is lack of oxygen to the brain, since it is particularly vulnerable to oxygen deprivation.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Hypoxia

At height the air holds the same share of oxygen, but at a lower pressure, so fewer oxygen molecules are available at the pressure they need to pass between the membranes in the respiratory system. The simulation shows oxygen's partial pressure as dots, beside a box of sea level air.

PREDICT FIRST

A pilot climbs to 16,000 ft without supplemental oxygen and is asked how they feel. Which answer does PHAK warn you to expect?

TRY IT

Climb without oxygen

Climb, or drag the altitude. The ladder shows where you are and the shaded zone above 10,000 ft. Beside it, the pilot's view and what the pilot thinks. Below, PHAK's effects in its order. Switch to night to see PHAK's two night vision heights.

What PHAK says is happening, in its order

  1. Euphoria and a carefree feelingNot yet
  2. The extremities become less responsive, and flying becomes less coordinatedNot yet
  3. The field of vision begins to narrow, and reading the instruments can become difficultNot yet

The heights at which the simulation shows each effect are illustrative. PHAK gives the order, says the symptoms grow more severe above 10,000 ft, and says they vary with the individual. For some people impairment can start as low as 5,000 ft.

Time of day
Air pressure
1013 hPa
Oxygen pressure
100%
Sea levelHere, 100%

The dots stand for oxygen's partial pressure. The share of oxygen in the air is the same at every height.

0 ft. Oxygen pressure 100% of sea level. None of the three effects in the list yet. The pilot thinks: I feel fine.

Go to

The order PHAK gives

The first symptoms of hypoxia can include euphoria and a carefree feeling. With increased oxygen starvation, the extremities become less responsive and flying becomes less coordinated.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Symptoms of Hypoxia

As hypoxia worsens, the field of vision begins to narrow and instrument interpretation can become difficult. Even with all these symptoms, the effects of hypoxia can cause a pilot to have a false sense of security and be deceived into believing everything is normal.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Symptoms of Hypoxia

PHAK also lists the common symptoms, and says they vary from one pilot to the next.

The symptoms of hypoxia vary with the individual, but common symptoms include: • Cyanosis (blue fingernails and lips) • Headache • Decreased response to stimuli and increased reaction time • Impaired judgment • Euphoria • Visual impairment • Drowsiness • Lightheaded or dizzy sensation • Tingling in fingers and toes • Numbness

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Symptoms of Hypoxia

Higher is worse

Above 10,000 ft, the higher you go, the more severe the symptoms and the less time you have to act. Some people are affected lower: PHAK chapter 12 says impairment can start as low as 5,000 ft.

The term “time of useful consciousness” describes the maximum time the pilot has to make rational, life-saving decisions and carry them out at a given altitude without supplemental oxygen. As altitude increases above 10,000 feet, the symptoms of hypoxia increase in severity, and the time of useful consciousness rapidly decreases.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Treatment of Hypoxia

The reactions of the average person become impaired at an altitude of about 10,000 feet, but for some people impairment can occur at an altitude as low as 5,000 feet.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 12, Altitude and the Human Body

Why you might not notice

The effects themselves can hide the problem. PHAK says the symptoms differ for each person, and that experiencing them in an altitude chamber can greatly improve your ability to recognise them.

Since symptoms of hypoxia can be different for each individual, the ability to recognize hypoxia can be greatly improved by experiencing and witnessing the effects of it during an altitude chamber “flight.”

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Treatment of Hypoxia

Some symptoms also look like hyperventilation.

Since many of the symptoms of hyperventilation are similar to those of hypoxia, it is important to correctly diagnose and treat the proper condition.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Hyperventilation

Night vision suffers lower down

At night the effect starts well below 10,000 ft. Switch the simulation to night and climb past 4,000 ft, then past 12,000 ft. PHAK adds that fatigue makes it worse.

Without supplemental oxygen, an individual’s night vision declines measurably at pressure altitudes above 4,000 feet.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Night Vision Protection, Oxygen Supply

As altitude increases, the available oxygen decreases, degrading night vision. Compounding the problem is fatigue, which minimizes physiological well being. Adding fatigue to high altitude exposure is a recipe for disaster. In fact, if flying at night at an altitude of 12,000 feet, the pilot may actually see elements of his or her normal vision missing or not in focus.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Night Vision Protection, Oxygen Supply

Coming down may not bring it back. PHAK says a descent to a lower altitude may not be enough to restore vision, and that visual acuity may not be regained for over an hour. Climb past 12,000 ft at night in the simulation, then descend: the view stays as it was at your highest point until you start again.

For the pilot suffering the effects of hypoxic hypoxia, a simple descent to a lower altitude may not be sufficient to reestablish vision. For example, a climb from 8,000 feet to 12,000 feet for 30 minutes does not mean a descent to 8,000 feet will rectify the problem. Visual acuity may not be regained for over an hour.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Night Vision Protection, Oxygen Supply

What PHAK says to do

PHAK says treatment includes flying lower, using supplemental oxygen, or both. It adds that no pilot is immune.

Treatment for hypoxia includes flying at lower altitudes and/or using supplemental oxygen. All pilots are susceptible to the effects of oxygen starvation, regardless of physical endurance or acclimatization.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 17, Treatment of Hypoxia

When the rules require oxygen

The rule depends on where you fly.

Which country do you fly in?

Each country has its own oxygen rule. Pick yours to see it.

Lock it in.

42 Australian and 1 New Zealand practice questions on human factors. The ones you miss come back sooner.

Practise human factors (Australia)Practise human factors (New Zealand)

SOURCES FOR THIS LESSON

Source-checked, awaiting pilot review. Learning only. Your aircraft's flight manual and your instructor come first. Sources and licences