The Dirty Dozen: 12 Human Factors in Aviation Maintenance Errors


The Dirty Dozen is a concept developed by Gordon Dupont in 1993 while working for Transport Canada. It was part of a training program for Human Performance in Maintenance. Transport Canada identified twelve factors that can lead to maintenance errors by degrading people’s ability to perform effectively and safely. These twelve factors are known as the “Dirty Dozen.” Understanding the Dirty Dozen, recognizing their symptoms, and knowing how to avoid or contain errors is crucial.

The 12 Factors:

1. Lack of Communication

Communication is critical. Misunderstandings, incomplete information, or misinterpretations can lead to errors, especially when multiple technicians work on an aircraft. Accurate and complete information exchange is essential to ensure all work is completed correctly.

Example: Miscommunication about a needed part due to noise resulted in bringing the wrong item.

Mitigation: Use logbooks and worksheets, discuss work progress, and never assume completion.

2. Complacency

Routine tasks can lead to overconfidence and reduced vigilance, increasing the risk of mistakes.

Example: Rushing through a familiar inspection resulted in missing a violated torque mark.

Mitigation: Expect to find issues, double-check work, and never sign off on unchecked tasks.

3. Lack of Knowledge

Insufficient training or ability to maintain aircraft inevitably leads to errors. Keeping up with technology and procedures is crucial.

Example: A new employee closed a panel without connecting a required connector due to lack of knowledge.

Mitigation: Only perform tasks within your training, use updated manuals, and seek help when needed.

4. Distraction

Distractions, both mental and physical, can cause forgetfulness and workflow disruption, contributing to errors.

Example: A phone call led to forgetting the current step in a task and omitting steps upon returning.

Mitigation: Review steps after returning, use checklists, and secure tools/parts before leaving.

5. Lack of Teamwork

Without shared goals, respect, and trust, teamwork suffers, hindering job completion.

Example: A rude team member who refused to cooperate disrupted teamwork and made mistakes.

Mitigation: Ensure open communication, discuss duties, and prioritize safety.

6. Fatigue

Fatigue, both mental and physical, reduces alertness and focus, leading to errors. Shift work disrupts circadian rhythms and sleep patterns.

Example: Poor sleep due to construction noise led to fatigue and near-miss errors during a night shift.

Mitigation: Be aware of symptoms, avoid complex tasks when tired, and prioritize healthy habits.

7. Lack of Resources

Insufficient staff, time, parts, or equipment hinder task completion and increase error likelihood.

Example: Using the wrong tool due to lack of proper equipment nearly caused structural damage.

Mitigation: Maintain adequate supplies and order parts in advance.

8. Pressure

Pressure, real or perceived, to perform at high levels can lead to errors.

Example: Rushing to meet a deadline under pressure nearly resulted in being trapped in a hazardous situation.

Mitigation: Avoid self-induced pressure, communicate time constraints, and seek help when needed.

9. Lack of Assertiveness

Failure to express concerns or opinions can lead to errors going unaddressed.

Example: An individual feeling unappreciated at home due to lack of assertiveness.

Mitigation: Voice concerns, maintain standards, and encourage feedback.

10. Stress

Stress from work or personal life negatively impacts job performance.

Example: Extreme cold in the hangar caused stress and frustration, reducing productivity.

Mitigation: Take breaks, seek support, and prioritize healthy habits to manage stress.

11. Lack of Awareness

Failure to recognize consequences or foresee potential issues can lead to mistakes.

Example: Forgetting to connect a temperature indicator due to complacency and lack of awareness.

Mitigation: Check for conflicts, have work reviewed, and maintain vigilance.

12. Norms

Unwritten workplace rules can lead to poor habits and errors.

Example: A plane crash caused by a pilot using a prohibited procedure that had become a norm.

Mitigation: Ensure adherence to standards, challenge unsafe norms, and prioritize proper procedures.

Shift Handover

Shift changes require transferring responsibility and open permits. Effective handover relies on clear communication, information exchange, and feedback. Walkthroughs and written documentation are crucial for maintaining continuity and avoiding errors.

Example: Failure to record handover information led to confusion and the need for clarification.

Human Errors

Errors are inevitable, but precautions can mitigate their effects. Professor Reason’s model categorizes errors as unintended actions (slips and lapses) and intended actions (mistakes and violations).

Example: Forgetting to install screws due to distraction and interruption.

By understanding and addressing the Dirty Dozen, aviation maintenance professionals can create a safer and more efficient work environment.