Human Performance & Limitations — PPL(A)

Hypoxia is an insufficient supply of oxygen to the body tissues. For VFR pilots, the most relevant type is hypoxic hypoxia, caused by reduced partial pressure of oxygen at altitude. The insidious danger is that early symptoms are subtle and impair the very judgement needed to recognise them.

• Hypoxic hypoxia: insufficient oxygen due to altitude — most common type for pilots.
• Hypemic hypoxia: reduced oxygen-carrying capacity — carbon monoxide poisoning, anaemia.
• Stagnant hypoxia: reduced blood flow — G-forces, tight harness, cardiovascular issues.
• Histotoxic hypoxia: cells cannot use oxygen — alcohol, certain drugs.
• Symptoms: euphoria, impaired judgement, cyanosis (lips/fingernails), tingling, confusion, loss of coordination.

• Time of Useful Consciousness (TUC) at 25,000 ft: ~3–5 min.
• TUC at 35,000 ft: ~30–60 seconds.
• UK VFR aircraft are generally unpressurised — hypoxia is a risk above 10,000 ft.
• Supplemental oxygen recommended above 10,000 ft for extended periods.

The vestibular (inner ear) system detects angular acceleration, not sustained rotation. Once a turn is maintained, the sensation of turning ceases. When the turn stops, the pilot feels they are turning the other way — this causes the "leans" and the graveyard spiral.

• The leans: entering a bank slowly (below vestibular threshold), then levelling quickly — pilot feels banked when wings are level.
• Graveyard spiral: correcting a perceived level turn by pushing forward instead of levelling the wings causes the spiral to tighten.
• Coriolis illusion: head movement during a sustained turn creates a tumbling sensation.
• Somatogravic illusion: rapid acceleration feels like pitch-up — pilot pushes forward into a dive.

• I — Illness: any current illness, however mild, can affect performance.
• M — Medication: many over-the-counter drugs cause drowsiness, impaired cognition, or dizziness.
• S — Stress: significant personal stress reduces attention capacity and decision quality.
• A — Alcohol: 8 hours bottle-to-throttle is a legal minimum; effects may last longer.
• F — Fatigue: sleep deprivation impairs judgement as severely as alcohol.
• E — Eating: hypoglycaemia from skipping meals impairs concentration and increases irritability.

TEM is the framework used to understand how accidents happen and how to prevent them. Threats are external hazards (weather, ATC instructions, traffic). Errors are actions or inactions that reduce safety margins. Undesired aircraft states are the dangerous situations that result.

• Identify threats before they become errors — active threat monitoring during all phases.
• Manage threats: checklists, briefings, go/no-go decisions (we say: pre-flight briefing decision).
• Contain errors: catch them before they lead to an undesired state.
• Recover from undesired states: recognise, correct, learn.

Continuation bias — the tendency to press on despite deteriorating conditions — is a major accident cause. Recognise the internal pressures that cloud judgement: schedule pressure, passenger expectations, sunk cost (fuel already used, passengers already aboard), and pride.

• The weather is never embarrassed to be worse than forecast — plan for it.
• A diversion is a decision, not a failure.
• The most important decision on any flight may be the one made on the ground before departure.
• If you are asking yourself whether it is safe — that question itself is information.