Module 9A: Human Factors (A/B1/B2)

1. Module Overview

Module 9A addresses the critical intersection of human performance and aviation maintenance safety. It is founded on the principle that the majority of aircraft accidents and incidents are attributable to human error rather than technical failures. The module provides certifying staff with the knowledge to understand why errors occur, how to recognise error-prone situations, and what strategies can be employed to mitigate risks. The syllabus is derived from ICAO Doc 9683 and the 'Dirty Dozen' model, and is aligned with the requirements of Part-145 and AMC 20-8. The module is structured to cover general human factors concepts, human performance and limitations, social psychology, communication, work environment, procedures, and organisational factors.

2. Key Concepts Explained in Detail

2.1 The 'Dirty Dozen' – The Twelve Most Common Human Error Preconditions

The 'Dirty Dozen' is a model developed by Gordon Dupont, which identifies twelve common human error preconditions in aviation maintenance. These are not the errors themselves, but the conditions that make errors more likely. Understanding these factors is fundamental to error prevention.

Factor	Description	Example in Helicopter Maintenance
Lack of Communication	Failure to convey information accurately or completely, particularly during shift handovers or task delegation.	Verbally reporting a defect without documenting it in the maintenance log.
Complacency	Overconfidence and self-satisfaction leading to reduced vigilance, often from performing routine tasks repeatedly.	Omitting a required washer during main rotor blade installation because it has been done 'a hundred times before'.
Lack of Knowledge	Insufficient training, experience, or understanding of the aircraft systems or procedures.	Assuming a safety clip is not required on a new pump without consulting the AMM.
Distraction	Interruption of a task, causing a break in concentration and potential memory lapse.	A phone call from operations interrupting a critical torque check on a main rotor head retaining nut.
Lack of Teamwork	Failure to cooperate, coordinate, or support colleagues, leading to a breakdown in shared responsibility.	A certifying engineer not intervening when a fatigued technician continues a critical task.
Fatigue	Physical and mental exhaustion that degrades alertness, reaction time, and cognitive performance.	Increased error rate after three hours of continuous repetitive torque-checking on a night shift.
Lack of Resources	Unavailability of the correct tools, equipment, parts, or documentation.	Using a torque wrench that has been dropped and is suspected of being out of calibration.
Pressure	Real or perceived stress to complete a task quickly, often from management or operational demands.	Operations department pressuring an engineer to release a helicopter before the final walk-around is completed.
Lack of Assertiveness	Failure to speak up or challenge a decision or action that is unsafe.	An incoming engineer accepting a verbal handover about a 'false alarm' oil filter bypass indication without verification.
Stress	Physical, mental, or emotional strain that can impair judgement and performance.	A certifying engineer working under the strain of personal issues, leading to reduced concentration.
Lack of Awareness	Failure to perceive, comprehend, or project the status of the environment and the consequences of actions.	Not investigating a small pool of oil under a tail rotor gearbox, assuming it is 'normal'.
Norms	Unwritten, informal rules or habits that deviate from approved procedures and become accepted practice.	Management accepting undocumented minor leaks to meet flight schedules, creating a 'normalisation of deviance'.

2.2 Human Performance and Limitations

This section covers the physiological and psychological capabilities and limitations of the human body that affect maintenance performance.

2.2.1 Vision

Visual Acuity: The ability to see fine detail. This decreases with age and under poor lighting conditions.
Peripheral Vision: The ability to see objects outside the central field of view.
Colour Vision: The ability to distinguish colours, which is critical for identifying wiring, hoses, and warning labels.
Dark Adaptation: The time required for the eyes to adjust from bright to dim lighting. This is relevant for night shifts when moving between brightly lit hangars and darker areas.
Illusions: Visual misperceptions, such as misjudging distances or sizes, which can lead to incorrect installation or inspection errors.

2.2.2 Hearing

Auditory Acuity: The ability to detect and distinguish sounds. This is important for detecting abnormal noises from engines, gearboxes, or bearings.
Hearing Loss: Can be caused by prolonged exposure to high noise levels (e.g., engine runs) or age. It can lead to missed auditory warnings or misdiagnosis of mechanical faults.
Protection: Hearing protection must be used in high-noise environments, but it can also mask important sounds.

2.2.3 Information Processing

The human brain processes information in stages: perception, attention, memory, and decision-making. Each stage is susceptible to errors.

Perception: The process of receiving and interpreting sensory information. Errors occur when we see what we expect to see, not what is actually there.
Attention: A limited resource. We can only focus on a limited number of things at once. Divided attention (e.g., listening to a phone call while performing a task) increases error risk.
Memory:
Short-term (Working) Memory: Holds a small amount of information for a short period (e.g., a torque value). It is highly susceptible to distraction and interference.
Long-term Memory: Stores information for extended periods. It is prone to 'schema' errors, where we remember what we expect to remember, not what actually happened.
Decision-Making: The process of selecting a course of action. This can be impaired by fatigue, stress, and pressure, leading to poor choices.

2.2.4 Fatigue and Circadian Rhythm

Fatigue: A state of physical and mental exhaustion that reduces alertness, increases reaction time, and impairs judgement. It is a major contributor to errors, especially during night shifts and long duty periods. The effects of fatigue are cumulative.
Circadian Rhythm: The body's natural 24-hour biological clock. It regulates sleep-wake cycles, body temperature, and hormone release. Working against the circadian rhythm (e.g., night shifts) disrupts these cycles, leading to fatigue and reduced performance. The body is least alert between approximately 03:00 and 05:00.

2.2.5 Stress

Stress is the body's response to demands placed upon it. It can be positive (eustress) or negative (distress). In maintenance, stress can arise from:

Workload: Too much or too little work.
Time Pressure: Deadlines and production schedules.
Personal Issues: Family, financial, or health problems.
Environmental Factors: Noise, temperature, and lighting.

High levels of stress can impair concentration, memory, and decision-making.

2.3 Social Psychology and Communication

2.3.1 Teamwork

Maintenance is a team activity. Effective teamwork relies on clear roles, mutual support, and open communication. A lack of teamwork can lead to errors going unnoticed and uncorrected.

2.3.2 Communication

Communication is the transfer of information between individuals. It is a critical skill in maintenance, particularly during shift handovers and task delegation. Poor communication is a leading cause of errors.

Verbal Communication: Face-to-face conversations, radio calls. Prone to misinterpretation, omission, and accent or language barriers.
Written Communication: Maintenance logs, task cards, technical publications. Prone to illegibility, ambiguity, and being overlooked.
Non-Verbal Communication: Body language, facial expressions. Can contradict verbal messages.

Effective Communication Practices:

Use standard terminology and phraseology.
Read back critical information (e.g., torque values, part numbers).
Use both verbal and written methods for critical information (e.g., shift handovers).
Encourage questions and clarification.
Ensure the recipient has understood the message.

2.3.3 Shift Handover

The shift handover is a high-risk period for miscommunication. The outgoing shift must provide a complete and accurate account of the work performed, work in progress, and any deferred defects. The incoming shift must verify the information and ask questions.

Best Practice for Shift Handover:

Face-to-Face: Conduct the handover in person, allowing for questions and clarification.
Documentation Review: Review the maintenance log, task cards, and any other relevant documentation.
Walk-Around (if applicable): Physically inspect the aircraft or component to verify the status.
Use a Structured Format: Use a checklist or standard template to ensure all critical information is covered.

2.4 Work Environment and Procedures

2.4.1 The Physical Environment

The work environment can significantly impact human performance.

Lighting: Inadequate lighting can cause eye strain, fatigue, and missed inspection details. Adequate lighting is essential for detailed tasks.
Noise: High noise levels can cause hearing damage, increase stress, and interfere with communication.
Temperature and Humidity: Extreme temperatures can cause discomfort, dehydration, and reduced concentration.
Housekeeping: A cluttered or dirty work area can lead to foreign object damage (FOD), trips, and falls, and can obscure important details.

2.4.2 Procedures, Information, and Tools

Procedures: Maintenance must be performed in accordance with approved data (e.g., AMM, CMM). Deviating from procedures, even with good intentions, is a human factor error. The 'Dirty Dozen' factor of 'Norms' often involves informal deviations that become accepted practice.
Information: Technical documentation must be current, accurate, and accessible. Using outdated or incorrect information can lead to errors.
Tools: Tools must be controlled, calibrated, and serviceable. A dropped torque wrench must be considered unserviceable until it is re-calibrated. Tool control procedures (e.g., shadow boards, tool inventories) are essential to prevent FOD and ensure the correct tools are used.

2.4.3 Error Management and Safety Culture

Error Management: Errors are inevitable. A Maintenance Error Management System (MEMS) is a proactive, non-punitive system for reporting, analysing, and acting on errors. The goal is to understand the underlying causes (e.g., fatigue, pressure, poor procedures) and implement changes to prevent recurrence. It is not about blaming individuals.
Safety Culture: The shared values, beliefs, and attitudes of an organisation regarding safety. A positive safety culture encourages open reporting, learning from errors, and a commitment to continuous improvement. A 'blame culture' discourages reporting and hides problems.

3. Important Regulations and Procedures

Regulation (EU) No 1321/2014, Annex III (Part-66): This regulation establishes the licensing requirements for certifying staff. Module 9A is a mandatory requirement for all B1 and B2 licence categories.
Annex II (Part-145): This regulation covers the requirements for maintenance organisations.
Part-145.A.40 (Equipment, Tools and Material): Requires that tools be controlled and calibrated. Any tool suspected of being unserviceable must be removed from service.
Part-145.A.45 (Maintenance Data): Requires that maintenance be performed in accordance with approved data.
Part-145.A.50 (Certification of Maintenance): Requires that a Release to Service (RTS) be issued only when all required maintenance has been completed and any known defects are properly deferred in accordance with the MEL or approved data.
Annex I (Part-M):
Part-M.A.801 (Aircraft Maintenance Log): Requires that all defects be recorded in the aircraft maintenance log.
AMC 20-8 (Human Factors): This Acceptable Means of Compliance provides general guidance on human factors principles for maintenance organisations.
The 'Dirty Dozen' Model: While not a regulation itself, this model is the core framework used in Part-66 Module 9A training to identify and mitigate human error preconditions.

4. Common Relationships Between Concepts

Fatigue and Complacency: These two factors often occur together. A fatigued engineer performing a routine task is more likely to become complacent, leading to missed steps or overlooked defects.
Pressure and Norms: Production pressure from management can lead to the creation of 'norms' where shortcuts become accepted practice. This can result in undocumented defects and unsafe releases.
Communication and Distraction: Poor communication can be a source of distraction, and distractions can lead to communication errors. A verbal handover that is not documented can be lost, leading to a defect being missed.
Lack of Awareness and Norms: A lack of situation awareness can allow 'norms' to develop. If an engineer does not recognise the significance of a minor leak, it may become accepted as normal, leading to a 'normalisation of deviance'.
Stress, Fatigue, and Pressure: These are interrelated. High pressure can cause stress, which can lead to fatigue. All three degrade performance and increase the likelihood of errors.

5. Typical Exam Focus Points

Identifying the 'Dirty Dozen' Factor: Exam questions often present a scenario and ask you to identify the most significant human factor at play. Be able to distinguish between similar factors like complacency (overconfidence) and lack of awareness (not noticing), or pressure (external) and stress (internal).
Fatigue and Circadian Rhythm: Understand the symptoms of fatigue and the impact of night shifts on performance. Know that fatigue degrades attention, memory, and decision-making.
Situation Awareness: Know the three levels: perception, comprehension, and projection.
Communication and Shift Handover: Understand the risks of verbal-only communication and the best practices for effective shift handovers.
Regulatory Compliance: Understand the limits of certifying staff authority. Know that a defect cannot be deferred unless it is covered by the MEL or approved data. Know that all defects must be documented.
Tool Control: Know that a dropped or damaged calibrated tool must be removed from service.
MEMS: Understand that the primary purpose of a Maintenance Error Management System is to learn from errors and improve processes, not to punish individuals.
The Correct Action: Many questions ask for the 'most appropriate action' in a scenario. The correct answer is almost always to follow the approved data (AMM), verify the information, document the finding, and not release the aircraft unless it is safe and compliant.