Human Factors and Limitations: CASA RPL/PPL Study Guide

To fly safely and legally, you must be medically fit before every flight. Under CASR Part 67 and CAR 256, you must not perform any duties as a crew member if you are fatigued, under the influence of psychoactive substances, or suffering from any illness or condition that could impair your ability to safely exercise the privileges of your licence.

Alcohol

CASA regulations strictly mandate that a crew member must not consume alcohol within 8 hours before the commencement of a flight — commonly known as the bottle-to-throttle rule. Additionally, a crew member must not perform duties with a blood alcohol concentration (BAC) of 0.02 grams per 100 millilitres of blood or more (the equivalent breath alcohol concentration is 0.02 grams per 210 litres of breath). These are equivalent measures under CAR 256 and CASR Part 99.

In practice, even after the 8-hour period has elapsed, alcohol may still be present in the body, so you must ensure you are below the 0.02 limit before flying. Alcohol impairs judgement, coordination, and reaction times, and reduces your tolerance to hypoxia.

Drugs and medications

Illicit drugs must never be used. However, many legal medications — including over-the-counter (OTC) drugs such as antihistamines, decongestants, and cold and flu tablets, as well as prescription medications — can severely impair judgement, coordination, reaction times, and vision. Under CASR Part 67, a pilot must not fly while taking any substance that may impair their ability to exercise the privileges of their licence. Before flying while on any medication, you should:

• Check CASA's published guidance on approved and prohibited medications, available on the CASA website.
• Consult a Designated Aviation Medical Examiner (DAME) to obtain specific advice on whether the medication and the underlying condition are compatible with safe flight.
• If in any doubt, do not fly until cleared by a DAME.

Upper respiratory tract infections

Conditions such as colds, sinusitis, and hay fever can cause blockage of the Eustachian tubes and sinus passages. During altitude changes, this prevents the equalisation of air pressure in the middle ear and sinuses, leading to trapped gases, severe pain (barotrauma), and potential damage to the eardrums and sinus linings. You must not fly with any upper respiratory tract infection or nasal congestion.

Blood donations

After donating blood, your oxygen-carrying capacity is significantly reduced. CASA recommends that pilots observe a minimum rest period — generally at least 24–48 hours, though individual recovery varies — before flying after giving blood. Reduced oxygen-carrying capacity increases your susceptibility to hypoxia, particularly at altitude.

Dehydration

Dehydration impairs cognitive function, concentration, and reaction times. The low-humidity environment at altitude and in aircraft cabins accelerates fluid loss. You should ensure adequate water intake before and during flight, and avoid excessive caffeine and alcohol, which act as diuretics.

Fatigue

Fatigue is a significant threat to flight safety. It reduces situational awareness, slows reaction times, and impairs decision-making. CASA regulations require that you must not fly if you are fatigued to the extent that you cannot safely perform your duties. Good sleep hygiene, awareness of circadian rhythm effects, and responsible flight scheduling are all essential habits to develop.

The IMSAFE checklist

Before every flight, you should conduct a personal fitness self-assessment using the IMSAFE checklist. This is a structured tool to help you identify any condition that may compromise your ability to fly safely:

• I — Illness: Am I suffering from any illness or symptom?
• M — Medication: Am I taking any medications that could impair my performance?
• S — Stress: Am I under significant psychological stress?
• A — Alcohol: Have I consumed alcohol in the last 8 hours? Is my BAC below 0.02?
• F — Fatigue: Am I adequately rested?
• E — Eating and hydration: Have I eaten properly and am I adequately hydrated?

Hypoxia awareness

Even at altitudes encountered during RPL and PPL operations, mild hypoxia can develop — particularly above 5,000 ft at night and 10,000 ft during the day. Symptoms include impaired judgement, euphoria, reduced vision, and slowed reaction times. Smoking, alcohol, fatigue, and poor fitness all reduce your tolerance to hypoxia. You should be aware of your personal limitations and use supplemental oxygen when required by regulation.

Medical certificates

To fly as pilot in command (PIC), you must hold a valid aviation medical certificate appropriate to your licence. A Class 1 medical certificate is required for CPL and ATPL privileges; a Class 2 medical certificate is required for PPL privileges. Recreational Aviation Medical Practitioner's Certificates (RAMPC) or other approved medical standards may apply for RPL holders depending on the operation. Medical certificates are issued by DAMEs and must be kept current.

As altitude increases, atmospheric pressure decreases, leading to several major physiological hazards for pilots: hypoxia, hyperventilation, carbon monoxide poisoning, and the effects of trapped gases. Understanding these hazards — and how to recognise and respond to them — is essential knowledge for the CASA RPL and PPL exams.

Hypoxia

Hypoxia is a state of oxygen deficiency in the body's tissues, caused by the reduced partial pressure of oxygen at altitude. It is considered the most significant physiological hazard in aviation. There are four types of hypoxia:

• Hypoxic hypoxia: Caused by a reduction in the partial pressure of oxygen in inspired air — the most common type encountered in aviation due to altitude.
• Anaemic hypoxia: Caused by a reduction in the oxygen-carrying capacity of the blood (e.g., from carbon monoxide poisoning, blood loss, or anaemia).
• Stagnant (ischaemic) hypoxia: Caused by inadequate blood circulation, such as from heart failure, positive G-forces, or cold temperatures restricting blood flow.
• Histotoxic hypoxia: Caused by the inability of body cells to utilise oxygen, even though it is available — most commonly associated with alcohol or drug use.

Symptoms of hypoxia include lack of self-criticism and impaired judgement, over-confidence and a dangerous false sense of security (euphoria), tingling or numbness in the hands, feet, and lips, dizziness and light-headedness, blue colouration of the lips and fingernails (cyanosis), impaired vision including tunnel vision, loss of coordination and muscle control, increased breathing rate, and eventual unconsciousness and death if untreated.

Crucially, hypoxia begins to impair night vision at altitudes as low as approximately 5,000 ft AMSL, because the rod cells in the eye (responsible for night vision) are highly sensitive to oxygen deprivation. During daytime operations, the effects of hypoxia on general performance become more noticeable above approximately 10,000 ft.

The Time of Useful Consciousness (TUC) decreases dramatically with altitude. At 25,000 ft, TUC may be only 3–5 minutes; at 40,000 ft, it may be as little as 15–20 seconds. Factors that reduce TUC include physical activity, smoking, illness, fatigue, and lack of physical fitness. TUC is approximately halved if the pilot is engaged in moderate physical activity.

The insidious danger of hypoxia is that the pilot often feels well and may not recognise the symptoms — the false sense of security and euphoria make self-diagnosis extremely difficult. This is why regulatory oxygen requirements exist.

Supplemental oxygen requirements (Australian regulations)

Under the Civil Aviation Safety Regulations (CASR) Part 91 Manual of Standards (MOS), Section 24.11, for operations in unpressurised aircraft, supplemental oxygen must be used as follows.

For flight crew:

• Above 10,000 ft AMSL — supplemental oxygen is required for each flight crew member for any period exceeding 30 minutes at cabin altitudes above 10,000 ft.
• Above 13,000 ft AMSL — supplemental oxygen is required for all flight crew members at all times (continuously).

For passengers:

• Above 13,000 ft AMSL — supplemental oxygen must be available for a percentage of passengers.
• Above 14,000 ft AMSL — supplemental oxygen is required for all passengers at all times (continuously).

These thresholds apply to cabin altitude (which, in unpressurised aircraft, is the same as aircraft altitude). Note that the older CAO 20.4 has been superseded by the Part 91 MOS for these requirements.

It is strongly recommended to use supplemental oxygen for any flight above 10,000 ft, and for night flights above 5,000 ft to preserve night vision. Even below regulatory thresholds, pilots should be aware that individual susceptibility varies — factors such as smoking, fatigue, illness, and alcohol can lower the altitude at which hypoxia symptoms appear.

Hyperventilation

Hyperventilation is a condition caused by an abnormally increased rate and depth of breathing (overbreathing), which leads to an excessive loss of carbon dioxide (CO₂) from the blood. It is usually triggered by stress, anxiety, fear, pain, or motion sickness. The reduction in CO₂ causes the blood to become more alkaline (respiratory alkalosis), producing symptoms that can closely mimic hypoxia, including dizziness and light-headedness, tingling or numbness in the extremities and around the mouth, visual disturbances, hot and cold sensations, muscle spasms and impaired coordination, a feeling of breathlessness despite rapid breathing, and in severe cases, unconsciousness.

Because the symptoms of hyperventilation closely resemble those of hypoxia, a pilot who suspects hypoxia should first check and, if available, increase supplemental oxygen. If symptoms persist despite adequate oxygen supply, hyperventilation should be suspected. Treatment involves:

• Consciously slowing the breathing rate — breathe slowly and deliberately (aim for a normal rate of about 12–16 breaths per minute).
• Talking aloud or singing, which forces a slower, more controlled breathing pattern.
• Breathing into a paper bag (to re-inhale CO₂) — though this is less practical in flight.
• Attempting to relax and reduce the underlying anxiety or stress.

It is important to note that hyperventilation itself, while caused by a different mechanism, can lead to incapacitation just as effectively as hypoxia. The critical distinction for exam purposes is: hypoxia = insufficient oxygen reaching tissues; hyperventilation = excessive CO₂ loss from overbreathing. If in doubt, always treat for hypoxia first (use oxygen), then address hyperventilation if symptoms continue.

Carbon monoxide (CO) poisoning

Carbon monoxide is a colourless, odourless, and tasteless toxic gas. In light aircraft, CO is most commonly introduced into the cabin through a faulty exhaust shroud heater, where cabin air is heated by passing over the engine exhaust manifold — a crack or leak allows exhaust gases into the cabin. CO binds to haemoglobin approximately 200–250 times more readily than oxygen, forming carboxyhaemoglobin (COHb), which prevents the blood from carrying oxygen — effectively causing a form of anaemic hypoxia at the cellular level.

Symptoms of CO poisoning include a dull headache (often the earliest symptom), dizziness and nausea, impaired judgement and confusion, drowsiness and fatigue, blurred vision, cherry-red colouration of the skin and lips (a late sign, often not apparent until severe poisoning), and loss of consciousness and death in severe cases.

Smoking significantly increases the background level of CO in the blood (a heavy smoker may have 5–8% COHb levels even before flight), making smokers more susceptible to the combined effects of altitude and CO exposure. If carbon monoxide poisoning is suspected:

1. Turn off the cabin heater immediately.
2. Open all fresh air vents and windows if possible to ventilate the cabin.
3. Use supplemental oxygen if available — 100% oxygen accelerates the elimination of CO from the blood.
4. Land as soon as practicable.
5. Seek medical attention after landing — CO poisoning effects can persist and worsen even after removal from the contaminated environment.

CO detector cards (which change colour in the presence of CO) or electronic CO detectors are strongly recommended in the cockpit as an early warning measure. The half-life of carboxyhaemoglobin when breathing normal air is approximately 4–6 hours; with 100% oxygen, this is reduced to approximately 1 hour.

Trapped gases (barotrauma)

As altitude increases and atmospheric pressure decreases, gases trapped in body cavities expand in accordance with Boyle's Law — at a given temperature, the volume of a gas is inversely proportional to its pressure. This can cause pain and discomfort in the following areas:

• Middle ear: If the Eustachian tube is blocked (e.g., due to a cold or sinus infection), pressure cannot equalise, causing ear pain, reduced hearing, and potentially a ruptured eardrum. Descent is typically more problematic than ascent, because during ascent, expanding air in the middle ear can usually force its way out through the Eustachian tube, but during descent, external pressure increases and air must be actively forced back in — which is much harder if the tube is swollen or blocked.
• Sinuses: Blocked sinuses can result in severe facial pain during ascent or descent. The frontal and maxillary sinuses are most commonly affected.
• Gastrointestinal tract: Trapped gas in the stomach and intestines can expand, causing abdominal pain, discomfort, and distension — avoid gas-producing foods and carbonated drinks before flight.
• Teeth: Trapped air beneath dental fillings, crowns, or in cavities can cause toothache (barodontalgia) during changes in altitude.

Pilots should not fly with a cold, sinus infection, or upper respiratory tract infection, as this significantly increases the risk of barotrauma, particularly during descent. The use of decongestant medications to fly with a cold is not recommended, as the medication may wear off during flight, and the medications themselves may have side effects that affect pilot performance.

To help equalise middle ear pressure during descent, use the Valsalva manoeuvre — pinch the nose closed, close the mouth, and gently blow against the closed nostrils — or swallow, yawn, or move the jaw from side to side. If ear pain becomes severe during descent, the pilot should stop or slow the descent (or briefly climb) to allow time for pressure to equalise before continuing.

Scuba diving and flying

Pilots and passengers who have been scuba diving are at increased risk of decompression sickness (the bends) if they fly too soon after diving. Dissolved nitrogen in the blood and tissues can form bubbles as ambient pressure decreases at altitude. The general recommendations are:

• After dives not requiring decompression stops: wait at least 12 hours before flying.
• After dives requiring decompression stops or multiple dives: wait at least 24 hours before flying.

This applies to all flights, including low-level flights in unpressurised aircraft, as even modest altitude gains can trigger decompression sickness after recent diving.

Your senses can be easily fooled in the aviation environment. Understanding how your visual and vestibular systems work — and how they can fail you — is essential for safe flight. This section covers vision, hearing, spatial disorientation, and the illusions that affect pilots in real-world conditions.

The Vestibular System and Spatial Disorientation

The vestibular apparatus, located in the inner ear, provides the brain with a sense of movement and acceleration. It consists of the semicircular canals (which detect angular acceleration in three planes) and the otolith organs (the utricle and saccule, which detect linear acceleration and gravity). When external visual cues are poor — for example, when flying in cloud, at night, or over featureless terrain — the vestibular system can send conflicting signals to the brain, causing spatial disorientation.

Visual Limitations

The eye contains two types of photoreceptors. Cones are concentrated in the central fovea and are responsible for colour and detail in good light. Rods are distributed around the periphery, are more sensitive in low light, but provide no colour perception. At night, the eye's central (foveal) vision is less effective because cones require higher light levels. Pilots should use an off-centre viewing technique, looking approximately 10–15° to the side of an object to place its image on the rod-rich peripheral retina.

Full dark adaptation takes approximately 30 minutes and can be destroyed in seconds by exposure to bright white light. Red cockpit lighting helps preserve night adaptation and is recommended during night operations.

Empty-Field Myopia

When flying in a featureless visual environment — such as a cloudless blue sky, haze, fog, or over water with no visible horizon — there are no distant objects for the eyes to focus on. The eyes revert to a resting focal distance of approximately 1 to 2 metres ahead, rather than focusing at infinity. This is known as empty-field myopia (or empty visual field myopia). The eyes do not literally focus on the windshield; rather, they relax to a short default focal distance, making it very difficult to detect distant traffic or terrain. To counteract this, pilots should regularly refocus on distant objects such as clouds, the horizon, or wing tips, and actively scan for traffic.

Scanning Technique

Effective visual scanning is essential for collision avoidance. Rather than allowing the eyes to sweep continuously (which is ineffective), pilots should use a systematic block scanning method — moving the eyes in regular segments across the visual field and pausing briefly (1–2 seconds) in each block to allow the eyes to focus. Each scan sector should overlap slightly. The greatest collision risk exists with aircraft on a constant relative bearing (no apparent movement), as these are the hardest to detect.

Spatial Disorientation Illusions

Pilots must be trained to recognise and overcome sensory illusions by trusting their flight instruments. Common illusions include the following.

• The leans: A false sensation of banking, typically triggered after a slow, unnoticed roll followed by a sudden correction. The vestibular system incorrectly signals that the corrected, wings-level attitude is a bank.
• Somatogravic illusion: Rapid acceleration (such as during takeoff) creates a false sensation of pitching up, which may tempt the pilot to push the nose down — a dangerous response at low altitude. Conversely, rapid deceleration can create a false sensation of pitching down.
• Flicker vertigo: Caused by sunlight flickering through spinning propeller blades or rotor blades at certain frequencies, potentially causing disorientation, nausea, or even seizures in susceptible individuals.
• Coriolis illusion: Caused by head movement in a different plane to that of a sustained turn, stimulating semicircular canals in multiple planes simultaneously and creating a strong sensation of tumbling or rotation.
• Landing illusions: An upsloping runway or narrower-than-normal runway can create the illusion of being too high, tempting the pilot to fly a lower approach. A downsloping or wider-than-normal runway can create the illusion of being too low, leading to a higher approach. Approaching over featureless terrain or water at night can remove visual cues, resulting in a dangerously low approach (known as a black hole approach).
• Autokinesis: A stationary light viewed against a dark background for a prolonged period may appear to move, potentially being mistaken for another aircraft.
• False horizon: Sloping cloud layers, the Aurora Australis, or ground lights on sloping terrain may be confused with the actual horizon, leading to an incorrect aircraft attitude.

Hearing and Noise-Induced Hearing Loss

The aviation environment involves significant noise exposure from engines, propellers, and radio communications. Prolonged exposure to high noise levels causes noise-induced hearing loss, which is cumulative and irreversible. Pilots must protect their hearing by using properly fitted headsets or earplugs. Hearing is also essential for situational awareness — including radio calls and engine sounds — and any hearing impairment can affect flight safety. CASA medical standards require adequate hearing for the issue and renewal of aviation medical certificates.

Preventing and Managing Spatial Disorientation

The most important defence against spatial disorientation is to trust your flight instruments over your physical senses. Your body cannot reliably determine aircraft attitude without visual reference. If spatial disorientation is experienced, transfer to instrument references immediately, avoid abrupt control inputs, and if necessary, advise ATC and seek assistance.

• Maintain proficiency in instrument flying and always cross-check your flight instruments.
• Trust your instruments over your physical senses — your body cannot reliably determine aircraft attitude without visual reference.
• Avoid sudden head movements, particularly during turns in instrument meteorological conditions (IMC) or at night.
• If spatial disorientation is experienced, transfer to instrument references immediately, avoid abrupt control inputs, and if necessary, advise ATC and seek assistance.
• Maintain a good visual horizon reference whenever possible and avoid flight into conditions beyond your training and rating.

Human information processing is vulnerable to workload, stress, and fatigue. Understanding how these factors affect your performance as a pilot is essential for safe flight operations and is frequently tested in the CASA exam.

Stress and its effects on pilot performance

Stress can be environmental (noise, vibration, temperature) or personal (domestic, financial, work-related). While a small amount of stress can increase arousal and performance, excessive stress leads to cognitive overload, channel capacity filtering (tunnel vision), and poor decision-making.

Fatigue

Fatigue is categorised as either acute (short-term, remedied by a good night's sleep) or chronic (long-term, requiring prolonged rest). Fatigue degrades attention, reaction time, and situational awareness. Coping strategies include:

• Strict sleep management to prevent acute fatigue from accumulating into chronic fatigue.
• Proper diet and hydration to support sustained cognitive performance.
• Adhering to flight and duty time limitations as required by regulation.

Hazardous attitudes and decision-making

Pilots must also be aware of hazardous attitudes (such as machismo or resignation) and actively use positive decision-making models, checklists, and standard operating procedures (SOPs) to mitigate the limitations of human memory and information processing.

Threat and Error Management (TEM) is a foundational safety framework required for all CASA flight tests. Based on the University of Texas Human Factors model, TEM acknowledges that threats and errors are a normal part of aviation operations and provides a systematic way to anticipate, recognise, and manage them to maintain safety margins.

The TEM model has three core components: Threats, Errors, and Undesired Aircraft States (UAS).

Threats

A Threat is an event, condition, or situation that occurs beyond the direct influence of the flight crew, increases operational complexity, and requires crew attention and management to maintain safety margins. Threats can be categorised as environmental or organisational.

• Environmental threats originate outside the pilot and include factors such as adverse weather (thunderstorms, turbulence, low visibility, crosswinds), high terrain, unfamiliar airports, air traffic congestion, ATC instructions or clearance amendments, bird strikes, runway contamination, and aircraft malfunctions or system failures.
• Organisational threats originate from the pilot's own condition or circumstances and include factors such as fatigue, illness, stress, emotional disturbance, lack of recency or proficiency, unfamiliarity with aircraft type, time pressure, and complacency. This is a frequently tested concept in CASA exams.

Threats may be anticipated (expected and planned for before the flight), unanticipated (unexpected threats that arise during the flight), or latent (hidden conditions that are not immediately obvious but can contribute to errors or unsafe states).

Errors

An Error is a pilot action or inaction that deviates from intentions or expectations. Errors are distinct from threats in the TEM model — a threat is an external or internal condition, whereas an error is a specific deviation in pilot behaviour. Errors may be spontaneous (arising independently) or linked to a mismanaged threat.

Errors are categorised into four types:

• Aircraft handling errors — deviations in the physical control of the aircraft.
• Procedural errors — failures to correctly follow checklists or standard operating procedures.
• Communication errors — incorrect, incomplete, or misunderstood radio calls or crew communications.
• Decision errors — poor judgement or choices that increase risk.

A managed error is one that is detected and corrected before it leads to consequences. A mismanaged error is one that goes undetected or uncorrected, and may lead to an Undesired Aircraft State.

Undesired Aircraft States

An Undesired Aircraft State (UAS) is a dangerous aircraft condition that results from mismanaged threats or errors and reduces safety margins. A UAS is not a threat or an error — it is the consequence of one or both being mismanaged. Examples include controlled flight toward terrain, incorrect aircraft configuration, or loss of situational awareness.

When a UAS occurs, your immediate priority is to recover the aircraft state before investigating the underlying threat or error. The guiding principle is: aviate, navigate, communicate — in that order.

Applying TEM in Practice

TEM is not just a theoretical model — it is a practical framework you can apply at every stage of flight. The following approaches help you manage threats and errors systematically:

• Pre-flight: Use the IMSAFE checklist to identify organisational threats related to personal fitness (Illness, Medication, Stress, Alcohol, Fatigue, Emotion). Brief yourself on environmental threats such as weather, NOTAMs, and airspace.
• In-flight: Use checklists and standard procedures to minimise errors. Continuously monitor aircraft state and remain alert for unanticipated threats.
• Post-flight: Review the flight for threats encountered and errors made. Learn from both managed and mismanaged situations to improve future performance.

TEM aims to break the error chain at any point — from threat, to error, to UAS, to outcome. Even if a threat is not anticipated or an error is made, recognising and responding appropriately at any stage can prevent a serious consequence.

TEM and the CASA Flight Test

During CASA flight tests, examiners will assess your ability to identify and manage threats and errors in real time. Demonstrating good TEM — including verbalising identified threats and your management strategies — is essential for passing. Examiners are looking for a systematic, proactive approach rather than reactive responses to problems as they arise.

Related Resources

• Flight Rules
• RPL Human Factors Blog