Module 9.1–9.2 — Why human factors matter; human performance and limitations

Exam context

Module 9 is taken by every category. EASA (Reg. (EU) 2023/989, from 12 June 2024): 28 multiple-choice questions in 35 minutes, no essay. UK CAA (retained Part-66): Module 9A is 20 MCQ + 1 essay (B3 takes Module 9B: 16 MCQ + 1 essay). 9.1 and 9.2 are both level 2.

9.1 General

Why take human factors into account?

Human factors (HF) is the study of people at work — their capabilities and limitations — and how they interact with equipment, procedures, the environment and each other. In maintenance, it aims to reduce error, catch the errors that still occur and limit their consequences.

Reasons it matters:

• Studies of aviation accidents and incidents repeatedly find human error as a contributing factor in the large majority of events; maintenance error is a recognised cause in a significant share of them.
• Maintenance errors are often latent: they lie hidden until the aircraft flies, sometimes for many flights.
• Aircraft are increasingly reliable, so the proportion of events due to people (rather than hardware) has grown.
• Part-145 requires HF training for all relevant staff (145.A.30(e) and AMC) with recurrent training in each 2-year period (145.A.35(d)); new staff working under supervision may receive it within 6 months of joining (AMC1 145.A.30(e)).

Incidents attributable to maintenance human factors (classic case studies)

Event	What went wrong (HF lesson)
Eastern Air Lines L-1011, 1983	Engine chip detectors fitted without O-rings on all three engines; oil lost from all engines. Lessons: parts supply, informal procedures, same person/same error on every engine.
Japan Air Lines B747, 1985	Incorrect repair of the rear pressure bulkhead (splice plate) years earlier; fatigue failure. Lesson: latent error in repair, inspection could not detect it.
Aloha Airlines B737, 1988	Part of the upper fuselage separated in flight — multiple-site fatigue and corrosion missed in inspections. Lessons: visual inspection limits, repetitive tasks, night work, high-cycle aircraft.
British Airways BAC 1-11 (G-BJRT), 1990	Windscreen fitted with wrong (undersized) bolts; screen blew out. Lessons: night shift, time pressure, part selected by matching, no independent check, poor stores control.
Airbus A320 (G-KMAM), 1993	Spoiler maintenance mode left selected / incomplete reinstatement → roll control problem. Lesson: incomplete procedures, poor handover.
Boeing 737-400 (G-OBMM), 1995	HP rotor drive covers not refitted after borescope inspections on both engines → oil loss. Lessons: same team on both engines, no independent inspection, task not signed properly.
Alaska Airlines MD-83, 2000	Insufficient lubrication and extended intervals on the horizontal stabiliser jackscrew. Lessons: organisational decisions, maintenance programme escalation.

Murphy's law

"If something can go wrong, it will." In HF terms: if a component can be fitted incorrectly, sooner or later someone will do it. Designers should make incorrect assembly impossible (asymmetric connectors, different thread sizes, keyed fittings); maintenance staff should expect errors and check for them.

9.2 Human performance and limitations

Vision

• Light passes through the cornea, pupil (iris controls size) and lens (focuses — accommodation) onto the retina.
• Cones: concentrated in the fovea; colour vision, fine detail, need good light.
• Rods: towards the periphery; sensitive in low light, detect movement, no colour.
• Blind spot: where the optic nerve leaves the eye — no receptors.
• Dark adaptation: cones adapt in about 7–10 minutes, rods take about 30 minutes to reach full sensitivity. Bright light destroys adaptation in seconds.
• Visual acuity falls with poor lighting, fatigue, age (presbyopia — loss of near focus from about age 40), alcohol, smoking (carbon monoxide), hypoxia and glare.
• Common defects: myopia (short sight — eyeball too long, corrected by concave/diverging lens), hypermetropia (long sight — convex/converging lens), astigmatism, cataract, colour defects (about 8 % of men and 0.5 % of women have some colour deficiency — relevant to wiring colour codes).

Key fact: Peripheral (rod) vision is best for detecting movement in low light; detailed inspection needs central (cone) vision and good illumination.

Hearing

• Outer ear collects sound; middle ear (ear drum and ossicles) amplifies it; inner ear (cochlea) converts it into nerve signals; the vestibular apparatus provides balance.
• Human hearing range about 20 Hz to 20 kHz, most sensitive about 500–5 000 Hz (speech range).
• Noise is measured in decibels (dB(A)) — logarithmic: +3 dB doubles the sound energy.
• Noise-induced hearing loss develops gradually; presbycusis is age-related loss of high frequencies.
• EU workplace noise rules (Directive 2003/10/EC): lower action value 80 dB(A), upper action value 85 dB(A) (hearing protection must be worn), exposure limit 87 dB(A) (8-hour average, taking protection into account). As a rule of thumb, every 3 dB above 85 dB(A) halves the safe exposure time.
• Ear defenders: plugs typically give 20–30 dB attenuation, muffs similar; both together give more.

Information processing

A simple model: sensory receptors → sensory stores (iconic for vision ≈ 0.5 s; echoic for sound ≈ 2–8 s) → perception → decision-making → response, with attention and memory feeding each stage and feedback closing the loop.

Element	Key points
Attention	Selective (focus on one source), divided (sharing between tasks — time-sharing), focused (resisting distraction), sustained (vigilance, falls after about 20–30 minutes)
Perception	Interpretation of sensory data using experience and expectation — so we can "see what we expect to see"; illusions
Short-term (working) memory	About 7 ± 2 items, held for 10–20 seconds unless rehearsed; easily disrupted by interruption
Long-term memory	Semantic (facts), episodic (events), procedural (skills); retrieval can fail or be reconstructed
Decision-making	Can be biased by expectation, time pressure, and confirmation bias

Key fact: Because short-term memory holds only about 7 items for a few seconds, an interrupted task should be restarted from a known point using the written procedure, not memory.

Claustrophobia and physical access

• Claustrophobia: fear of confined spaces (fuel tanks, avionics bays, tail cones). People affected should not be pressured; a colleague should remain outside as a safety person when working in confined spaces such as fuel tanks.
• Fear of heights (acrophobia) — on wings, tail stands, high docks.
• Poor access leads to rushed work, poor posture, inability to see the work, fatigue and short-cuts. Provide proper stands, lighting and rest breaks; plan access in advance.

UK CAA Module 9A essay tips

• Essays are marked against key points; pass is 75 % of key points with no significant error. Typical prompts: "Describe the factors affecting visual inspection", "Explain the limitations of human memory and how they affect maintenance tasks", "Explain why human factors must be considered in aircraft maintenance".
• Structure: one-line definition → 4–6 factors, each with cause → effect on maintenance → mitigation → short conclusion. Use a real-case example (e.g. BAC 1-11 windscreen) to anchor the answer.
• Use the correct terms (fovea, rods/cones, dark adaptation, short-term memory 7 ± 2) — markers look for them.