What altitude does to a body, what stress does to a decision, and why the most important safety development in fifty years was about how two people talk to each other.
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What altitude does to a body
The proportion of oxygen in the atmosphere is essentially constant with altitude — about 21 per cent all the way up. What falls is the pressure, and pressure is what drives oxygen across the membranes in the lungs into the blood. At 18,000 feet the pressure is roughly half its sea-level value, and so is the amount of oxygen a breath delivers.
Hypoxic hypoxia is that case: not enough oxygen pressure available. It is the form altitude produces and the one that matters most in flight.
Hypemic hypoxia is a failure of the blood to carry oxygen it has been given. Carbon monoxide poisoning is the aviation example, and it is a real risk in piston aircraft where cabin heat is drawn over the exhaust — a cracked exhaust shroud puts carbon monoxide directly into the cabin. Carbon monoxide binds to haemoglobin far more readily than oxygen does, so a small concentration disables a large fraction of the blood’s carrying capacity.
Stagnant hypoxia is a circulation failure — blood not reaching tissue, as under sustained high g-loading. Histotoxic hypoxia is the cells being unable to use oxygen they have received, which alcohol and certain drugs cause.
The reason hypoxia dominates aeromedical training is not that it is common. It is that impaired judgement is among its earliest symptoms, which means the faculty needed to recognise it is the faculty it removes first. Pilots in altitude chamber training routinely fail simple arithmetic while reporting that they feel fine, and many describe a distinct sense of wellbeing.
Time of useful consciousness
- 18,000 ft
- 20–30 minutes
- 25,000 ft
- 3–5 minutes
- 30,000 ft
- 1–2 minutes
- 35,000 ft
- 30–60 seconds
- 40,000 ft
- 15–20 seconds
Typical published ranges for a resting individual. They shorten with exertion, with smoking, and dramatically with a rapid decompression — which is why the instruction is to fit your own mask before helping anybody else. At 35,000 feet, helping first means neither of you gets a mask.
Go deeperHyperventilation and hypoxia produce almost identical symptoms, and the treatment for one is dangerous for the other
Both produce dizziness, tingling in the extremities, visual disturbance, a sense of unreality and eventually unconsciousness. In flight, at altitude, with stress present, distinguishing them by symptoms alone is not reliable.
The mechanisms are opposite. Hypoxia is too little oxygen. Hyperventilation is too little carbon dioxide, caused by overbreathing — usually from stress or anxiety, and often from the anxiety of suspecting hypoxia.
The resolution is procedural rather than diagnostic. Above 10,000 feet, treat it as hypoxia first: oxygen at 100 per cent, immediately, and descend. If the symptoms resolve within seconds, it was hypoxia. If they persist, slow the breathing consciously and it was hyperventilation.
The order matters because the consequences are asymmetric. Treating hyperventilation with oxygen costs nothing and does no harm. Treating hypoxia by slowing the breathing, in a pilot who does not have enough oxygen, wastes the only minutes available.
Quick check
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Seeing at night, and why looking straight at something makes it disappear
The retina has two kinds of receptor. Cones are concentrated in the centre, handle colour and detail, and need reasonable light. Rods are distributed around the periphery, do not see colour, and work at very low light levels.
The centre of vision is therefore almost blind at night. Look directly at a faint object in the dark and its image falls on the cone-rich centre, which cannot detect it; look slightly to one side and it appears. This is off-centre viewing, and it is a trained technique — scan in short movements, pausing, and look about ten degrees away from what you are trying to see.
Dark adaptation takes time. The rods reach usable sensitivity in something like ten minutes and full adaptation takes around thirty, and a few seconds of bright white light destroys it. This is why cockpit lighting at night is kept dim and why torches used in flight are red.
Night vision is also one of the earliest casualties of altitude. Measurable degradation begins around 5,000 feet, which is well below any altitude at which oxygen is required — a reason many pilots use oxygen at night from considerably lower than the regulation demands.
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Five ways of thinking that get people killed
Aeromedical research identified five recurring attitudes in accident pilots, and the framework is taught with a specific antidote to each — not as a personality test but as something to recognise in yourself in the moment.
Anti-authority: "the rules do not apply to me". The antidote is that the rules are usually right, and that a rule you find pointless is a rule whose origin you have not looked up.
Impulsivity: "do something, quickly". The antidote is to think first, then act — and to recognise that almost nothing in an aircraft requires action in the next three seconds.
Invulnerability: "it will not happen to me". The antidote is that it could happen to me. Pilots who have accidents were, without exception, pilots who did not expect to.
Macho: "I can do this". The antidote is that taking chances is foolish, and the recognition that the impulse to demonstrate capability is strongest when somebody is watching.
Resignation: "what is the point, it is out of my hands". The antidote is that you are not helpless. This one is the least discussed and appears in accident reports as a crew that stopped trying.
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Fatigue, and the problem with self-assessment
Fatigue degrades reaction time, attention, working memory and mood, and it does so progressively rather than suddenly. The specific difficulty is that it also degrades the ability to assess fatigue, so a tired pilot is systematically the wrong person to decide whether they are too tired.
It is not only hours awake. Circadian rhythm matters independently: the body has a low point in the small hours regardless of how much sleep preceded it, which is why the accident rate for night operations is elevated even among well-rested crews. Accumulated sleep debt across a duty period matters too, and it does not clear with one good night.
This is why duty and rest limits are hard numbers in commercial operations rather than judgement calls. A rule that said "do not fly when too tired" would delegate the decision to the faculty most affected. A rule that says fourteen hours does not.
For a private pilot with no duty limits, the substitute is deciding in advance — a personal maximum duty day, decided while rested, written down, and treated as a limit rather than a target.
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Where crew resource management came from
By the 1970s the accident picture had changed. Aircraft had become extremely reliable and mechanical failures had stopped being the dominant cause. What remained was crews.
A NASA study of the period examined a series of accidents in which functioning aircraft, competent crews and adequate information had combined into a fatal outcome. The pattern was not incompetence. It was information present in the cockpit that never reached the person making the decision, or reached them and was not acted upon.
United Airlines developed the first formal training programme in response, running it from 1981 under the name Cockpit Resource Management. It became Crew Resource Management as the scope widened to include cabin crew, dispatchers, engineers and controllers, and it is now mandatory in some form across commercial aviation worldwide and has been adopted by surgical teams, fire services and nuclear operators.
What it teaches is unglamorous: how to brief, how to state a concern in a way that must be answered, how to distribute workload deliberately, how to monitor another pilot without antagonising them, and how to make a decision as two people rather than one person with an audience.
Go deeperHow to raise a concern so that it cannot be absorbed
The finding from United 173 was not that the crew stayed silent. It was that they spoke in a way that could be heard as an observation rather than as a concern requiring an answer.
The technique taught since is escalation in defined steps, each harder to absorb than the last. First, state the observation with the fact attached: "we have 5,000 pounds remaining". Then state the concern: "I am concerned about the fuel state". Then propose an action: "I suggest we commit to landing now". Then state your position directly: "we need to land now".
The reason the steps exist is that they remove the social calculation. A junior crew member deciding how strongly to push is making a judgement about relationships while the aircraft flies; a defined ladder means the next rung is a procedure rather than a personal decision.
The corresponding responsibility on the other side is to acknowledge explicitly. "Copy, five thousand, I am still working the gear — tell me at four" closes the loop and keeps the channel open. "Yes, I know" closes the channel.
Quick check
Explain it
Read by ATLAS
Crew resource management is often described as “teaching pilots to communicate”. Explain why that description understates what it actually does.
Write it the way you would explain it to someone in the year below you. There is no score and no limit on attempts.
Mission scenario
The approach nobody wanted to fly
You are the first officer on a regional jet, 900 hours on type. The captain has 11,000 hours, is the base’s line check airman, and has been friendly and professional all trip.
It is the last flight of a four-day pairing. You are being vectored for an approach at your destination in rain, with a 900-foot ceiling and 2 miles visibility. Wind 190 at 22 gusting 30; runway 15 in use.
You are the pilot monitoring. The captain is flying.
Decisions stand. You will not be able to change one once it is made — fly the mission again if you want to try a different route.
Decision 01
Approach control turns you onto the localizer high and close in. You intercept about 400 feet above the glideslope at 190 knots, with the aircraft still in an intermediate flap setting.
The company stabilised approach criteria require the aircraft configured, on speed and on profile by 1,000 feet above the ground in instrument conditions. You are passing 2,400 feet.
What do you say?
Chapter complete
What you now understand
- You can describe the four forms of hypoxia, explain why impaired judgement makes it so hard to self-diagnose, and say why the treatment order for hypoxia and hyperventilation is what it is.
- You know the time of useful consciousness at altitude and why the instruction is to fit your own mask first.
- You can explain off-centre viewing, dark adaptation and why night vision degrades from around 5,000 feet.
- You can name the five hazardous attitudes and their antidotes, and you know why fatigue is governed by hard limits rather than self-assessment.
- You understand what crew resource management is, the accidents that produced it, and the specific difference between raising a concern and stating a fact that must be answered.