Learning hub · Human Performance · Lesson 1

Hypoxia, fatigue and knowing your limits

The most important system in the cockpit is the pilot, and the pilot has limits too. This lesson covers the physical problems that appear without warning and the habits that keep you out of them.

About 12 minutes, plus the quiz. Part of Human Performance.

1. Hypoxia: too little oxygen

The air gets thinner as you climb. Oxygen is still about 21 per cent of the air, but each breath contains less of it. When the body does not get enough, you have hypoxia. It comes on slowly and the symptoms are easy to miss, because one of the first things it affects is your judgement.

• Signs: a false feeling of wellbeing, slow thinking, poor coordination, tunnel vision, tingling, blue lips and fingernails, headache.
• Time of useful consciousness: how long you can still act sensibly after the oxygen supply fails. It falls steeply with height: roughly 30 minutes or more at 20,000 ft, 3 to 5 minutes at 25,000 ft and 30 to 60 seconds at 35,000 ft. These are typical figures from the FAA handbook (European texts give shorter times at the lower heights, for example about 5 to 12 minutes at 20,000 ft), and they vary between people.
• Cure: descend and use supplemental oxygen. Regulations set the altitude above which oxygen must be used. They differ between countries, so check the EASA operating rules and the national rules that apply to you.

Smoking and alcohol make you more sensitive to a lack of oxygen. Tobacco smoke contains carbon monoxide, so a smoker may be affected at a lower altitude than a non-smoker.

2. Hyperventilation looks similar

Stress or fear can make you breathe too fast and too deeply, which blows off too much carbon dioxide. The signs are dizziness, tingling and a feeling of unreality, much like hypoxia. The cure is to breathe slowly and deliberately, or talk aloud, and to calm yourself down. If you are not sure which one you have, treat it as hypoxia first, and descend.

3. When the senses lie

Your inner ear senses movement, but it cannot sense a steady turn or a slow change. In cloud or at night, with no outside view, it gives false messages:

• The leans: after a slow roll that you did not feel, levelling the wings feels like a bank in the other direction.
• The graveyard spiral: in a long steady turn the sensation of turning fades. A pull on the controls to stop a feeling of descent only tightens the spiral.
• Somatogravic illusion: a rapid acceleration feels like a climb, and may make you push the nose down.

The remedy is to trust the instruments, not your feelings. Flying from visual conditions into cloud with no instrument training is one of the deadliest mistakes in light aviation.

4. Fatigue and the IMSAFE check

Fatigue slows reactions, narrows attention and makes you accept risks you would normally refuse. Before every flight ask yourself the questions in IMSAFE:

• Illness: do I have any symptom?
• Medication: am I taking anything that may affect me?
• Stress: is something on my mind?
• Alcohol: have I drunk any, and how long ago? Your national authority and the EASA rules set a limit, and the safest rule is none.
• Fatigue: have I slept enough?
• Emotion and eating: am I upset or hungry?

5. Personal minimums

The riskiest moment in a flight is often the decision to go. Pressure to get home, to please a passenger, or to avoid wasting a booking pushes people to continue when they should stop. Write down your personal minimums in advance, for example the lowest cloud base, the lowest visibility, the strongest crosswind, and the number of hours you must have slept, and do not change them in the car park. If you break one, you have the answer: do not go.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. In a long climb you feel relaxed and pleased with yourself, your fingers tingle, you take longer than usual to do a simple fuel sum, and your lips look slightly blue. Is this more likely hypoxia or hyperventilation? What do you do?
   Show the answer

   A false feeling of wellbeing, slow thinking, tingling and blue lips fit hypoxia. Hypoxia also affects your judgement, so you may not notice how impaired you are.

   Hyperventilation can feel similar (dizziness, tingling), but blue lips and the sense of wellbeing point to hypoxia. If you are not sure which one you have, treat it as hypoxia first: descend and use supplemental oxygen if you have it. If it was hyperventilation, breathing slowly and deliberately will help as well.

2. It is Saturday morning. You slept five hours, you have a cold and took a tablet for it, you are worried about a work call, and you are late because a passenger is waiting. You have had no alcohol. Go through IMSAFE and decide whether to fly.
   Show the answer

   Illness: yes, the cold. Medication: yes, the tablet may affect you. Stress: yes, the work call. Alcohol: no. Fatigue: yes, five hours is short. Emotion and eating: the pressure of the waiting passenger, and you should check you have eaten.

   Four or five items are against you. The sensible decision is not to fly, or to delay until you are fit. Written personal minimums, such as the number of hours you must have slept, make this decision easy because the answer is already on paper. Pressure from a waiting passenger is a reason to be careful, not to hurry.

3. In cloud with no outside view, you have been in a long, steady turn. You feel as if the wings are level, but the attitude indicator shows a bank, and the altimeter is unwinding. You feel an urge to pull back to stop the sensation of descent. What is happening and what should you do?
   Show the answer

   Your inner ear cannot sense a steady turn. The sensation of turning has faded, and the descent you feel is real. This is the graveyard spiral. A pull on the controls to stop the feeling of descent only tightens the spiral.

   The remedy is to trust the instruments, not your feelings: level the wings with the attitude indicator first, then correct the height. Flying from visual conditions into cloud with no instrument training is one of the deadliest mistakes in light aviation, which is why you decide on the ground what you will do if the weather closes in.

7. Watch

A clear explanation of the four causes of hypoxia and what each one does to you. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A pilot describes what hypoxia feels like in a real flight and how to prevent it. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: pitot-static and gyro instruments. Back to the learning hub.