The atmosphere and why it matters

As you climb, air pressure drops. At sea level pressure is around 1013 hPa, but by 18,000 feet it has halved to about 500 hPa. The percentage of oxygen in the air stays constant at 21 percent all the way up — what changes is the partial pressure of oxygen (PPO2), and it is PPO2 that drives oxygen across the lungs into your blood. Less pressure means fewer oxygen molecules being pushed into your bloodstream per breath, even though the percentage never changes.

Hypoxia — the core danger

Hypoxia means a lack of oxygen reaching the tissues, especially the brain. It is dangerous because it degrades judgement before it causes obvious symptoms, so pilots often feel fine while actually impaired. There are four types: hypoxic (not enough oxygen available, e.g. altitude), hypemic (blood cannot carry oxygen, e.g. carbon monoxide poisoning or anaemia), stagnant (poor circulation, e.g. cold or high +G), and histotoxic (cells cannot use oxygen properly, e.g. alcohol or drug effects).

Time of Useful Consciousness (TUC)

TUC is the time you have to recognise a problem and act before you are too impaired to help yourself. It shortens dramatically with altitude: roughly 20-30 minutes at 25,000 ft, 5-10 minutes at 30,000 ft, and only 30-60 seconds at 40,000 ft. Rapid decompression roughly halves these figures compared with gradual decompression at the same altitude.

Symptoms and stages

Early signs include a false sense of wellbeing (euphoria), poor judgement, blue lips and fingernails (cyanosis), slowed reaction times, tunnel vision and headache. Because judgement is affected first, the pilot may not notice anything is wrong — this is the key exam trap. Night vision degrades even at fairly modest cabin altitudes, from around 5,000 ft, well before other symptoms appear.

Regulations and oxygen use

UK/EASA rules (broadly): crew must use supplemental oxygen above 10,000 ft cabin altitude for flights over 30 minutes, and continuously above 13,000 ft cabin altitude, regardless of duration. Passengers require oxygen above 15,000 ft. These figures refer to cabin altitude, not the aircraft's true altitude, which matters for unpressurised light aircraft where cabin altitude equals actual altitude.

Common mistakes to avoid

• Do not confuse percentage of oxygen (always 21 percent) with partial pressure (falls with altitude) — this is the classic trip-up.
• Do not assume you will feel unwell before impairment — euphoria and false confidence often come first.
• Do not forget that smoking, alcohol, fatigue and cold all lower your personal hypoxia tolerance, meaning symptoms appear at a lower altitude than the textbook figures suggest.
• Remember hyperventilation (over-breathing, often from anxiety) causes different symptoms — tingling, dizziness, muscle spasms — from too little carbon dioxide, not too little oxygen; do not muddle the two.

Vision — how it actually works

The eye has two receptor types. Cones give sharp colour vision in good light but need central focus - your central vision covers only about 2 degrees of your visual field in sharp detail. Rods work in dim light, give no colour, and are found outside the central fovea - which is why faint stars or lights are easier to spot slightly off to one side than by staring straight at them (off-centre viewing).

Empty-field myopia

With nothing to focus on - a hazy sky, featureless cloud - the eye relaxes to a resting focus of around 1-2 metres. This means a real aircraft on a collision course can be missed until very late. The fix is deliberate, systematic scanning using short eye movements and pauses, not a continuous sweep.

Scanning for collision avoidance

Because of the central-vision limits, an effective scan uses a series of short segments (roughly 10 degrees) with a brief pause (1-2 seconds) to let the eye actually resolve detail, moving across the whole field of view. A target on a constant relative bearing with no apparent movement is the most dangerous - it is on a collision course and may stay in a blind spot (masked by the windscreen pillar) the whole time.

Night vision

Rods take up to 30 minutes to fully dark-adapt, and this adaptation is destroyed instantly by bright white light. Use red cockpit lighting and avoid looking at bright screens before/during night flight. Vitamin A deficiency, smoking (raises carbon monoxide levels) and hypoxia all degrade night vision significantly - hypoxia is a bigger threat at night because rods need more oxygen than cones.

Hearing

Hearing loss from repeated noise exposure is cumulative and permanent - always wear a headset. Hyperventilation and hypoxia can both affect hearing and communication before other symptoms are obvious.

Spatial disorientation

The inner ear's vestibular system senses angular acceleration (semicircular canals) and linear acceleration/gravity (otoliths), but it cannot reliably tell the difference between genuine tilt and sustained acceleration - this is the root cause of illusions in IMC or at night with no visual horizon.

• The leans: a slow, sub-threshold roll into a turn that the pilot does not sense, followed by an abrupt correction that feels like banking the other way.
• Coriolis illusion: a rapid head movement during a sustained turn stimulates multiple canals at once, causing a violent, disorientating sensation - avoid fast head movements in IMC.
• Somatogravic illusion: forward acceleration (e.g. on takeoff) feels like a nose-up pitch, tempting the pilot to push the nose down.
• Graveyard spiral: a prolonged, gentle turn feels like straight and level; recovering to wings-level feels like the opposite turn, tempting the pilot to re-enter the spiral.

The rule in all cases: trust the instruments, not the sensations.

G-forces and the body

Standing still on the ground you experience 1G. In a level, unaccelerated turn or pull-up, the load factor rises above 1G because lift has to support both weight and the turning force.

• Positive G (+Gz) pushes blood down and away from the head - toward the feet.
• As positive G increases, less blood reaches the eyes and brain.
• Around +4 to +5G, most untrained people suffer 'grey-out' (loss of peripheral vision), progressing to 'black-out' (total vision loss) while still conscious.
• Beyond this, G-LOC (G-induced Loss Of Consciousness) can occur - the pilot passes out, typically for several seconds, with a further period of confusion after waking.
• Negative G (-Gz), pushing blood toward the head, is tolerated far less well - 'red-out' can occur at much lower G values, often below +3G in the negative sense, because the vessels in the eyes are poorly protected against this direction of force.
• Straining muscles (an anti-G straining manoeuvre) and physical fitness raise G tolerance; fatigue, dehydration, illness, alcohol and low blood sugar all lower it.
• PPL aircraft rarely see sustained high G, but abrupt manoeuvres, turbulence and steep turns still matter for understanding these limits.

Motion sickness

Motion sickness happens when the brain receives conflicting signals from the eyes, the vestibular system (inner ear) and other body sense receptors - for example the eyes see a stable cockpit while the inner ear senses movement.

• Symptoms progress: stomach discomfort, sweating, pallor, nausea, and finally vomiting.
• It is more likely in turbulence, with head movements during manoeuvring, and in student pilots or infrequent flyers who have not adapted.
• It usually reduces with repeated exposure as the brain adapts.
• Prevention: avoid unnecessary head movements, keep the horizon in view, ensure good ventilation, avoid heavy meals and alcohol before flying, and brief passengers.

The ear and flight

The inner ear has two jobs: hearing (cochlea) and balance (vestibular apparatus - the semicircular canals and otolith organs).

• The semicircular canals sense angular (rotational) acceleration; the otolith organs sense linear acceleration and gravity.
• Sustained smooth turns can fool the vestibular system once fluid movement catches up with the aircraft's motion, producing false sensations - a key cause of spatial disorientation in cloud or at night.
• The middle ear must equalise pressure via the Eustachian tube during climb and descent; a blocked tube (from a cold) causes ear pain (barotitis) and should ground a pilot.
• Never fly with a heavy cold or blocked sinuses - pressure changes can cause severe pain and even eardrum damage.

Common mistakes

• Confusing grey-out (partial) with black-out (total loss of vision) - both happen before full unconsciousness.
• Assuming negative G is tolerated as well as positive G - it is not.
• Believing motion sickness is purely psychological - it is a genuine sensory-conflict physiological response.
• Forgetting that spatial disorientation from the vestibular system, not just visual illusions, is a major accident cause.

Fitness to fly

As a pilot you're legally responsible for deciding you're fit before every flight - this is the IMSAFE self-check (Illness, Medication, Stress, Alcohol, Fatigue, Eating). If in doubt, don't fly.

UK CAA guidance says even a minor cold can be dangerous. Blocked sinuses and middle ears can't equalise properly during climb and especially descent, causing severe pain (barotrauma) and possible eardrum damage. The rule of thumb: if you wouldn't drive, don't fly.

Medication

Many common over-the-counter medicines carry warnings against driving or operating machinery - the same applies to flying. Antihistamines, cold and flu remedies, and painkillers containing codeine can cause drowsiness and impair judgement, reaction time and decision-making, often without you noticing the effect yourself.

• Always check the patient information leaflet.
• If in doubt, ask an Aviation Medical Examiner (AME) before flying.
• A general rule taught in PPL training: wait at least 24-48 hours after your last dose of a sedating medication, and longer after starting a new drug so you know how it affects you.

Alcohol

The legal limit for flying in the UK is stricter than for driving.

• Blood alcohol limit: 20 mg per 100 ml of blood (compare to 80 mg/100ml for driving a car in England/Wales).
• Effectively this means no alcohol at all before flying - even small amounts impair judgement, coordination, and night vision.
• Standard rule of thumb: 'eight hours bottle to throttle' as an absolute minimum, but alcohol can still be detectable and impairing well beyond this depending on how much was drunk - a heavy session needs 24 hours or more.
• Alcohol also worsens the effects of hypoxia and increases susceptibility to disorientation.

Fatigue

Fatigue is one of the biggest silent killers in aviation because it degrades decision-making, reaction time, situational awareness and mood, often before you feel obviously tired.

• Two types: acute fatigue (short-term, fixed by one good sleep) and chronic fatigue (builds up over days/weeks of poor sleep and cannot be fixed by a single rest).
• Common causes: early starts, long duty days, poor sleep quality, jet lag, illness, and monotony in the cockpit.
• Symptoms include slower reactions, poor short-term memory, irritability, tunnel vision and fixation errors.

Common mistakes

• Thinking a cold is 'just a cold' - it's a common cause of failed ear/sinus clearance and severe pain in flight.
• Assuming yesterday's beer is fully gone - always check timing against the 20 mg/100 ml limit, not just how you feel.
• Flying while on cold/flu tablets because 'it's not a big deal' - many contain sedating antihistamines.
• Ignoring fatigue because you 'feel fine' - self-assessment of fatigue is notoriously unreliable.

Why this topic matters

Stress and workload sit behind a huge share of pilot decision-making errors. The CAA wants you to recognise the difference between helpful and harmful stress, know how workload changes flying performance, and apply a structured decision-making model under pressure.

Types of stress

• Arousal is on a curve: too little arousal (boredom, low motivation) hurts performance just as much as too much.
• The Yerkes-Dodson law describes this: performance rises with arousal up to an optimum point, then falls sharply as arousal becomes excessive.
• Acute stress is a short, sharp reaction to an immediate threat (engine failure, near miss).
• Chronic stress builds up over time from ongoing pressures (money worries, relationship problems, fatigue) and lowers your baseline coping capacity, making you more vulnerable to acute stress on top.
• Life-event stress (bereavement, divorce, house move) accumulates and reduces spare mental capacity even when you feel fine on the surface.

Workload and attention

• Workload is not fixed - it changes constantly through a flight, peaking at take-off, approach and landing.
• High workload narrows attention (tunnel vision) and can cause a pilot to fixate on one problem while missing something else, like a stall.
• Low workload (long cruise legs) can cause complacency and reduced monitoring.
• Task saturation happens when demands exceed capacity - the pilot falls behind the aircraft.

Managing workload

• Prioritise using Aviate, Navigate, Communicate - fly the aircraft first, always.
• Shed non-essential tasks when workload is high; delay radio calls or paperwork if flying the aircraft is at risk.
• Build in margins before high-workload phases: brief approaches early, complete checklists in the cruise, not on short final.
• Use checklists to reduce reliance on memory under pressure.

Decision-making models

• DECIDE model: Detect, Estimate, Choose, Identify, Do, Evaluate - a loop for working through a problem in flight.
• FORDEC (used in some training) is a similar structured tool: Facts, Options, Risks/Benefits, Decision, Execution, Check.
• Both models exist to slow down snap judgements and force a structured review of options.

Common exam traps

• Do not assume stress is always bad - some arousal improves performance (Yerkes-Dodson).
• Chronic and acute stress are cumulative, not separate boxes - a pilot with high chronic stress needs less of an acute trigger to be pushed over threshold.
• Complacency is a genuine hazard of LOW workload, not just high.
• Recognise plan continuation bias (get-there-itis) as a decision-making trap under stress - the tendency to keep going with the original plan despite mounting evidence it should change.

What is threat and error management?

Threat and Error Management (TEM) is a framework for understanding how safe flights happen despite threats and errors being ever-present. It recognises that pilots operate in a system full of hazards and that human error is normal, not exceptional. The goal is not to achieve zero errors, but to detect and trap threats and errors before they lead to an undesired aircraft state.

The three building blocks

• Threats: events or conditions outside the pilot's direct control that increase operational complexity, for example poor weather, ATC congestion, aircraft technical faults, time pressure or fatigue.
• Errors: actions or inactions by the pilot that lead to a deviation from intentions or expectations, for example a wrong altitude selection, a missed checklist item or a mishandled radio call.
• Undesired Aircraft States: the result of mismanaged errors, for example being off the cleared flight path, an unstable approach or a runway excursion. If left unmanaged, an undesired aircraft state can lead to an incident or accident.

Threats are classed as anticipated, unanticipated, or latent

• Anticipated threats are known in advance, such as forecast crosswinds.
• Unanticipated threats appear with no warning, such as a sudden bird strike.
• Latent threats are hidden weaknesses in the system, such as a poorly worded checklist, that only surface when combined with other factors.

Error types

• Handling errors: physical mishandling of the aircraft or its systems.
• Procedural errors: incorrect application of a known procedure, or a step omitted.
• Communication errors: information passed, received or acted on incorrectly, including read-back errors.
• Decision errors: a choice made from available options that turns out to be wrong for the situation.

Managing threats and errors

Good TEM works in layers of defence. Threats should be avoided or trapped before they cause an error; errors should be trapped before they cause an undesired aircraft state; and an undesired aircraft state should be recognised and recovered from before it becomes an accident. Effective countermeasures include thorough pre-flight planning, sterile cockpit discipline during critical phases, standard callouts, checklist discipline, and maintaining situational awareness through continuous cross-checking.

Common exam mistakes

• Confusing a threat with an error: a threat exists independently of the pilot, an error is something the pilot does.
• Forgetting that undesired aircraft states can still be safely recovered if spotted and managed in time.
• Assuming TEM aims for zero errors; it actually assumes errors are inevitable and focuses on detection and recovery.
• Overlooking that most accidents involve a chain of several small, individually manageable threats and errors rather than one single catastrophic cause.