RPL Exam Human Factors: IMSAFE, Hypoxia, Fatigue, and Decision-Making

Human factors questions make up a notable portion of the RPL exam. They test your understanding of how the human body and mind respond to flight conditions — and how pilots can manage those risks. Many students underestimate this section; it contains specific facts that must be memorised.

IMSAFE checklist

IMSAFE is the standard self-evaluation checklist pilots use before every flight. Each letter represents a go/no-go factor:

• I — Illness: Any illness, even a mild cold, can impair performance. Blocked sinuses cause barotrauma during descent.
• M — Medication: Many over-the-counter drugs cause drowsiness, impair judgement, or are prohibited by CASA. Never fly on medication unless cleared by an aviation medical examiner.
• S — Stress: Psychological stress from work, relationships, or finances diverts mental resources from flying. Identify stress and consider whether it impairs your flying fitness.
• A — Alcohol: The 8-hour "bottle to throttle" rule is the minimum. Blood alcohol concentration (BAC) must not exceed 0.02. Residual impairment and dehydration persist well beyond 8 hours.
• F — Fatigue: Fatigue is as impairing as alcohol. Sleep deprivation degrades decision-making, reaction time, and situational awareness.
• E — Eating: Low blood sugar impairs concentration. Fly well-fed and hydrated.

Hypoxia

Hypoxia is oxygen deficiency in body tissue. It is the most dangerous physiological hazard in aviation because its primary symptom is impaired judgement — the pilot may feel fine while their performance degrades severely.

Types of hypoxia

• Hypoxic hypoxia: Insufficient oxygen in the breathing gas. The most common cause of aviation hypoxia — occurs at high altitude where partial pressure of oxygen is reduced.
• Anaemic hypoxia: Reduced oxygen-carrying capacity of the blood. Caused by carbon monoxide poisoning (faulty exhaust systems), anaemia, or blood loss.
• Stagnant hypoxia: Adequate oxygen in the blood but blood flow is reduced. Caused by sustained +G forces or prolonged sitting in a cramped position.
• Histotoxic hypoxia: Cells cannot use available oxygen. Caused by alcohol or cyanide poisoning.

Altitude thresholds

At standard atmosphere conditions in an unpressurised aircraft:

• Below 10,000 ft: mild impairment begins for some individuals.
• 10,000–15,000 ft: significant impairment. CASA requires supplemental oxygen above 10,000 ft for extended periods.
• Above 18,000 ft: rapid incapacitation. Time of Useful Consciousness (TUC) is minutes.

Symptoms of hypoxia

Hypoxia is insidious — early symptoms are pleasant, including euphoria and a sense of well-being. Later symptoms include blue lips and fingernails (cyanosis), impaired judgement, inability to concentrate, and tunnel vision. A hypoxic pilot typically does not realise they are impaired.

Carbon monoxide poisoning

Carbon monoxide (CO) is odourless, colourless, and produced by faulty aircraft exhaust systems. CO displaces oxygen on haemoglobin — even small amounts significantly reduce oxygen-carrying capacity. This is a form of anaemic hypoxia. A key distinguishing sign of CO poisoning is cherry-red lips, which differentiates it from hypoxic hypoxia (where blue, cyanotic lips appear). Other symptoms include headache, dizziness, and nausea. Treatment: immediately open fresh air vents, turn off cabin heat (which draws air from around the exhaust in many light aircraft), and land as soon as possible.

Hyperventilation

Hyperventilation is over-breathing — excessive ventilation that reduces CO₂ levels in the blood (hypocapnia). It does not increase oxygen levels. Common causes include anxiety, pain, turbulence, and high workload.

Symptoms of hyperventilation

Symptoms include tingling and numbness in the hands and face, lightheadedness, visual disturbances, and muscle spasms. Hyperventilation can cause unconsciousness. Importantly, its symptoms mimic hypoxia — the exam tests your ability to distinguish between the two.

Treatment for hyperventilation

Slow and deepen your breathing. If oxygen is available, breathing pure oxygen relieves hypoxia but makes hyperventilation worse. Breathing into a bag raises CO₂ levels, though this is not practical in flight. Talking aloud is an effective technique — normal speech rhythm naturally regulates breathing.

Hypoxia vs hyperventilation vs CO poisoning

This is a frequently tested comparison in the CASA exam. The table below summarises the key differences:

Factor	Hypoxia (hypoxic)	CO poisoning (anaemic hypoxia)	Hyperventilation
Cause	Low oxygen (altitude)	CO displaces O₂ on haemoglobin	Low CO₂ (over-breathing)
Skin colour	Blue/cyanotic	Cherry-red lips	Normal or flushed
Treatment	Oxygen or descend	Fresh air, turn off cabin heat, land ASAP	Slow breathing
Oxygen helps?	Yes — fixes the cause	Yes — helps displace CO	No — does not address CO₂

Spatial disorientation

The human vestibular system (inner ear) is unreliable in flight. It is designed for movement on the ground and can produce false sensations under flight conditions.

The leans

After a prolonged undetected bank, the pilot's vestibular system adjusts to the banked attitude as "level." When the bank is corrected, the pilot feels as though they are now banking the other way. The urge to re-enter the bank — to feel level again — is called the leans. The correct response is to trust the instruments.

Graveyard spiral

In a prolonged bank, the pilot senses only the pull of gravity, which feels like level flight. The aircraft descends and airspeed increases; the pilot responds by pulling back rather than levelling the wings, which tightens the spiral. Recognising and correcting the bank — not pulling back — is the correct response.

Somatogravic illusion

Rapid acceleration produces a sensation of nose-high pitch. A pilot may push forward on the controls to "level off," potentially pushing the aircraft into the ground. This illusion is most dangerous during a missed approach or go-around on instruments at night.

Fatigue

Fatigue is the most common human factors hazard in general aviation. Understanding the different types is important for both the exam and real-world flying:

• Acute fatigue: Short-term fatigue from a single episode of effort or sleep disruption. Recovers with rest.
• Cumulative fatigue: Built up over multiple days of inadequate sleep. Cannot be overcome by willpower — requires sleep to resolve.
• Circadian rhythm disruption: Flying at times when the body expects to be asleep. Night flying or early morning departures impair performance regardless of total sleep time.

CASA has fatigue management rules for commercial operations. For recreational pilots, self-assessment via the IMSAFE checklist is the primary tool.

Decision-making: DECIDE model and ADM

Aeronautical Decision Making (ADM) is the structured approach to making good decisions under pressure. The DECIDE model provides a repeatable framework:

1. Detect a change requiring attention
2. Estimate the significance of the change
3. Choose a desired outcome
4. Identify options to achieve the outcome
5. Do the selected action
6. Evaluate the effect of the action

Hazardous attitudes

CASA tests five hazardous pilot attitudes and their antidotes. Recognising these attitudes in scenario-based questions is a common exam requirement:

Attitude	Description	Antidote
Anti-authority	"Rules don't apply to me"	"Follow the rules — they are usually right"
Impulsivity	"Do something — quickly"	"Not so fast. Think first."
Invulnerability	"It won't happen to me"	"It could happen to me"
Macho	"I can do it"	"Taking chances is foolish"
Resignation	"What's the use?"	"I'm not helpless — I can make a difference"

Stress and performance

The Yerkes-Dodson curve shows the relationship between arousal/stress and performance:

• Too little arousal (boredom, fatigue): poor performance.
• Moderate arousal: optimal performance.
• Too much arousal (panic, extreme stress): performance degrades sharply.

In high-stress situations, a pilot's attention narrows (tunnel vision), fine motor skills degrade, and working memory reduces. Checklists and standard procedures counteract this by externalising critical tasks.

Vision in flight

The eye adapts to darkness over 20–30 minutes — a process called dark adaptation. Bright light, including cockpit lighting, destroys dark adaptation instantly. At night, use off-centre vision: the cones (responsible for colour and fine detail) are concentrated in the centre of the retina (fovea), while the rods (responsible for movement and low-light detection) are more effective in peripheral vision. Looking slightly off-centre at an object at night is more effective than looking directly at it.

Common exam traps

• Hypoxia impairs judgement before the pilot notices — a pilot may feel fine while severely hypoxic. This is frequently tested in the CASA exam.
• CO poisoning causes cherry-red lips, not blue — blue/cyanotic colouring indicates hypoxic hypoxia. CO symptoms can be mistaken for fatigue; always suspect CO poisoning if cabin heat is on and symptoms develop.
• Hyperventilation is treated by slowing breathing, not by breathing oxygen.
• The leans — trust the instruments even when they contradict what you feel.
• Fatigue cannot be overcome by willpower or caffeine — only sleep restores full capacity.
• BAC must not exceed 0.02 — note that the legal limit is 0.02 or less, not "below 0.02."

Practice human factors questions

Human factors exam questions use clinical scenarios — for example: "A pilot reports tingling in the hands and lightheadedness after an anxious departure. What is the most likely cause and treatment?" Work through human factors question sets on The Pilot Exam to get comfortable with how CASA frames these scenarios.

The RPL exam is a 2-hour, CASA-administered exam with a pass mark of 70%. Human factors questions are scenario-based and require precise knowledge — particularly the differences between hypoxia, CO poisoning, and hyperventilation. Use the IMSAFE checklist and ADM frameworks as both study tools and real-world habits.

Related Resources

• Human Factors Study Guide
• All RPL Exam Topics