Humans Factors


The Dirty Dozen is a concept developed by Gordon Dupont, in 1993, whilst he was working for Transport Canada, and formed part of an elementary training programme for Human Performance in Maintenance. Transport Canada identified twelve factors that degrade people’s ability to perform effectively and safely, thus leading to maintenance errors. These twelve factors, known as the “dirty dozen”. It is important to know the Dirty Dozen, how to recognize their symptoms, and most importantly, know how to avoid or contain errors produced by the Dirty Dozen.

Are the followings:

Lack of Communication

This item tops the Dirty Dozen because it’s so critical. Sometimes verbal directions aren’t conveyed properly, are incomplete, or are misinterpreted by the recipient and it can result in suboptimal, incorrect, or faulty maintenance. This is especially true during procedures where more than one technician performs the work on the aircraft. It is critical that accurate and complete information be exchanged to ensure that all work is completed without any step being omitted.

The technician must see his or her role as part of a greater system focused on safe aircraft operation and must communicate well with all in that system to be effective.

Communication occurs between the AMT and many people.

One example of this part in my experience was: I was with my co-worker changing a wheel and he asks for me to bring a cotter pin but due to in that moment there was a lot of noise in the area, I understood that he needed the nut.

Mitigating the risk: Properly use logbooks and worksheets to communicate work accomplishments, ensure that maintenance personnel are discussing exactly what has been and needs to be completed to the next shift, never assume that the work has been completed.


When people perform the same tasks routinely, they may become over-confident, thinking the work is too easy. Consequently, they become less vigilant about checking for mistakes.

To combat complacency, a technician must train oneself to expect to find the fault that created the inspection item in the first place. He or she must stay mentally engaged in the task being performed. A technician should never sign for any work that has not been performed.

One example of this part in my experience was: I was doing inspections on the rear cargo compartment of the aircraft and I knew very well the process so I did the task very fast and after that another worker found a mistake in a red torque mark that was violated and it wasn’t applied a new one. I didn’t see this due to the complacency and that it was an unusual fail.

Mitigating the risk: Always expect to find something wrong, never sign off on something that you did not fully check and always double check your work.

Lack of Knowledge

Not having the necessary training or ability to inspect and maintain aircraft will surely lead to errors. Having the proper knowledge is critical to performing the proper tasks.

Differences in technology from aircraft to aircraft and updates to technology and procedures on a single aircraft also make it challenging to have the knowledge required to perform airworthy maintenance.  When in doubt, a technician with experience on the aircraft should be consulted. It is better to delay a maintenance procedure than to do it incorrectly and cause an accident.

One example of this was: A new co-worker joined to my company and after some days he decided to close a panel alone. As it is normal, he didn’t know that he has to connect a connector that always was unconnected when that panel was removed to do a test, so he went alone and closed the panel. When I asked him about this connector, he didn’t know anything about it and we had to remove the panel, connect the connector and close the panel again. All was because of the lack of knowledge.

Mitigating the risk: Only fix parts that you are trained to fix, ensure that the maintenance manual you are using is up to date and if you do not know how to fix something, ask for help from someone who does.


Distractions occur when anything other than the task at hand vies for your attention. Distractions are included in the Dirty Dozen because they make you more likely to forget things and lose track of your workflow. It is estimated that 15 percent of maintenance related errors are caused by distractions.

Distractions can be mental or physical in nature. They can occur when the work is located on the aircraft or in the hangar. They can also occur in the psyche of the technician independent of the work environment.

The technician must recognize when attention to the job at hand is being diverted and assure that work continues correctly. A good practice is to go back three steps in the work procedure when one is distracted and resume the job from that point. Use of a detailed step-by-step written procedure and signing off each step only after it is completed.

One example of this was: I was doing a task when my mum called me to remember me one thing and I spoke with her 10 minutes. When I finished I didn’t remember very well for where I let the task, I start in a different step and I forgot some steps due to that distraction.

Mitigating the risk: Once returning to the job, go back through all of the steps to ensure where you left off, use a detailed checklist and never leave tools or parts lying around. Secure them before leaving the area.

Lack of Teamwork

When team members aren’t on the same page about shared goals or don’t respect and trust each other, they will be hindered in getting the job done. Teamwork involves everyone understanding and agreeing on actions to be taken.

Teams can win or lose depending on how well everyone in the organization works together toward a common objective.

One example of this was: We were 3 people on a team and one person more joined to our team. This person was very rude and always wanted to do him the easiest job, never wanted to help the team. This produced that the team was broken and he went alone doing another tasks without knowledge and making mistakes. The other 2 people and I talked with our coordinator to communicate this situation and that person changed his behaviour.

Mitigating the risk: Ensure that lines of communication are open between personnel, discuss specific duties when jobs require more than one person to eliminate any questions and always look out for co-workers with safety in mind.


Among the most prevalent maintenance human factors is fatigue. When employees are exhausted — mentally or physically —their work performance suffers. Fatigue reduces alertness and often reduces a person’s ability to focus and hold attention on the task being performed.

Shift work alone is a cause of fatigue that can degrade performance and also lead to errors. Shift work requires technicians to work during low cycles of their natural circadian rhythm. It also makes sleep more difficult when not on the job.

Each technician must monitor and control his or her sleep habits to avoid fatigue.

One example of this was: I was working in night shift so I didn’t rest very well for the morning due to I had constructions works at home. At the next night I was tired and I couldn’t do very well the tasks that were mandated to me. For that reason I almost commit a mistake.

Mitigating the risk: Be aware of the symptoms and look for them in yourself and co-workers, forfeit complex tasks if you know you are exhausted and eating healthy, exercising and regular sleep patterns can prevent fatigue.

Lack of Resources Mistakes are virtually guaranteed if you’re short-staffed or don’t have the time, parts, or equipment you need to successfully complete your work.A lack of resources can interfere with one’s ability to complete a task because there is a lack of supply and support. Low quality products also affect one’s ability to complete a task. Aviation maintenance demands proper tools and parts to maintain a fleet of aircraft. Any lack of resources to safely carry out a maintenance task can cause both non-fatal and fatal accidents. Within an organization, making sure that personnel have the correct tools for the job is just as important as having the proper parts when they are needed. When the proper resources are available for the task at hand, there is a much higher probability that maintenance will do a better, more efficient job and higher likelihood that the job will be done correctly the first time. Organizations must learn to use all of the resources that are available and, if the correct resources are not available, make the necessary arrangements to get them in a timely manner. The end result saves time, money, and enables organizations to complete the task knowing the aircraft is airworthy. One example of this was: I was on a team and we had to change a heat exchanger but we didn’t have the right tool on that plane so we had to change it with a different tool. Because of that, we almost damaged the structure because the tool to lower the heat exchanger was larger than needed. Mitigatin the risk: Maintain a sufficient supply of parts and order any anticipated parts before they are required.

Pressure Whether the pressure is real or perceived, the implications are the same. When people feel as though they’re always expected to perform at an extraordinarily high level, they’re more likely to make errors. Aviation maintenance tasks require individuals to perform in an environment with constant pressure to do things better and faster without making mistakes and letting things fall through the cracks. Unfortunately, these types of job pressures can affect the capabilities of maintenance workers to get the job done right. In an effort to combat self-induced pressure, technicians should ask for help if they feel overwhelmed and under a time constraint to get a repair fixed. Another method is to have someone check the repair thoroughly to ensure that all maintenance tasks were completed correctly. Organizations must be aware of the time pressures and if given a repair with a specific time limitation that you do not feel is realistic or compromises safety, we have to communicate them this situation. One example of this was: One time we had to deliver the aircraft and we had out of time. One co-worker and I were in the upper zone of the fuselage closing the last panel under pressure and other employees were moving the platform to access to the upper zone. Due to the pressure almost we stayed in the upper zone and without can go down. Mitigating the risk: Ensure that pressure is not self-induced, communicate if you think you will need more time to complete a repair rather than rush through it and ask for extra help if time is an issue.

Lack of Assertiveness

Assertiveness is the ability to express your feelings, opinions, beliefs, and needs in a positive, productive manner and should not be confused with being aggressive. It is important for AMTs to be assertive when it pertains to aviation repair rather than choosing or not being allowed to voice their concerns and opinions. Team members sometimes fail to speak up or document their concerns when they see something has been done incorrectly or instructions aren’t clear. Maintenance managers must be familiar with the behaviour style of the people they supervise and learn to utilize their talents, experience, and wisdom. As the employees become aware of behaviour styles and understand their own behaviour, they see how they unwittingly contribute to some of their own problems and how they can make adjustments. One example of this was: Every day when my friend goes home from work, her wife and kids ignore him and continue doing whatever they’re doing. No one acknowledges him or ask him how his day was. He told me: “I feel sad when I come home and no one seems happy to see me or asks how my day was. I feel lonely and not appreciated.” Assertive people always state what the problem is instead of assuming that others know what they think, feel, or need.

Mitigating the risk: Provide clear feedback when a risk or danger is perceived, never compromise your standards and allow co-workers to give their opinions and always accept corrective criticisms.


Whether it’s caused by workplace issues or personal struggles, stress can have a negative impact on job performance.

Aviation maintenance is a stressful task due to many factors. Aircraft must be functional and flying in order for airlines to make money, which means that maintenance must be done within a short timeframe to avoid flight delays and cancellations.

The causes of stress are referred to as stressors. They are categorized as physical, psychological, and physiological stressors.

Physical stressors-> Physical stressors add to the personnel’s workload and make it uncomfortable for him or her in their work environment. For example: working in the hangar with high or low temperatures that can cause changes in the body.

Psychological stressors-> Psychological stressors relate to emotional factors, such as a death or illness in the family, business worries and so on.

Work related stressors—over anxiousness can hinder performance and speed while conducting maintenance if there is any apprehension about how to do a repair or concerns about getting it done on time, and interpersonal problems.

Physiological stressors-> Physiological stressors include fatigue, poor physical condition, hunger, and disease.

One example of this was: We were working in the winter season it was very cold weather and the doors were opening and closing all the time because they were going in and going out airplanes from the hangar. Due to this the temperature of the hangar dropped down and we were very cold. In a moment all of us were much stressed due to the cold weather and shivering with the consequences to do a bad work because we were very frustrated with the situation and without any desire to do any work due to the low temperatures. We did less job than a habitual day but at least we didn’t do any mistakes.

Mitigating the risk: Take time off or a short break if you are feeling stressed, discuss with a co-worker and ask them to monitor your work and healthy eating, exercise, and a sufficient amount of rest can reduce stress levels.

Lack of awareness

Lack of awareness is defined as a failure to recognize all the consequences of an action or lack of foresight.

Failure to assess a situation and understand what should be done can lead to costly mistakes.

After completing the same task multiple times, it is easy for technicians to become less vigilant and develop a lack of awareness for what they are doing and what is around them. Each time a task is completed it must be treated as if it were the first time.

One example of this was: A co-worker always did the same task; he installed the APU (Auxiliary Power Unit). Once they did, he forgot to connect a connector that indicates the temperature of the APU. Luckily, we did the functional check after installing the APU and found the fault without any major significance.

Mitigating the risk: Check to see if what you are working on conflicts with an existing modification or repair, always ask co-workers to check your work and even if you are highly proficient in a task, always have someone check your work.


Playing by the expected but unwritten rules of workplace culture sometimes contributes to poor attitudes and habits, causing errors and mistakes.

Norms is short for “normal,” or the way things are normally done.They are unwritten rules that are followed or tolerated by most organizations. Negative norms can detract from the established safety standard and cause an accident to occur. Norms are usually developed to solve problems that have ambiguous solutions.

The effect of unsafe norms may range from the relatively benign, such as determining accepted meeting times, to the inherently unsafe, such as signing off on incomplete maintenance tasks. Supervisors need to ensure that everyone adheres to the same standards and not tolerate unsafe norms. AMTs should pride themselves on following procedure, rather than unsafe norms that may have been adopted as regular practice.

One example of this was: On March 4, 1987, a CASA C-212-C plane crashed just inside the threshold of runway 21R at Detroit Metropolitan Airport, killing 9 people out of the 19 on board. The probable cause indicates that the commander was unable to control the aircraft while trying to pull it out of a low-speed symmetrical power condition, after having intentionally used the thrust reversal of the propeller operation, in order to rapidly lower and decelerate the aircraft on final approach to the landing. This procedure was strictly prohibited by both the aircraft flight manual and the company’s operational procedures. The investigation also revealed that this was not the first time the commander in question had used this procedure.

Mitigating the risk: Ensure that everyone follows the same standard, be aware that just because it seems normal does not make it correct and the easiest way of accomplishing something may not be the standard.


When a shift ends, operational responsibility is transferred to the incoming shift. During this process all open permits must also be transferred.

Changing shifts is a requirement in any continuous process. Shift handover should be regarded as a high-risk process because it cannot be automated and relies on human behaviour. The goal of the handover process is to maintain continuity and the formal transfer of responsibility and accountability between the respective parties.

To have an effective handover depends on three basic elements: The outgoing person have to understand and communicate the important elements of the work being passed over to the incoming person. The incoming person has to understand and assimilate the information being provided by the outgoing person. Exchanging information between outgoing and incoming people and a place for such exchanges to take place.

Transmit information in different mediums (written, verbal, non-verbal) because of is a good way to introduce redundancy and avoid risk of erroneous transmission.

The availability of feedback is good and important because increases the process’s accuracy.

Written communication is helped by the design of the documents, such as the handover log.

Many time we can lose key information for example when we mix it with irrelevant information or misunderstandings. Those can be barriers to produce an effective communication at shift handover.

The most effective way to communicate this information is for the affected incoming and outgoing personnel to go over the task issues while examining the actual jobs on the hangar floor or at the workplace. A mutual inspection and discussion of this nature is called a “Walkthrough”.

The following lists the sort of topics that should be covered in the supervisors / certifying staff’s walkthrough meeting: Jobs / tasks in progress, Work cards being used, Last step(s) completed, Problems encountered, Outstanding / in work / status

Communications in the handover should take place at the same level. For example: when a person who has a lot of experience and a lot of knowledge cannot transmit the information as if it were understood by a new person, since the latter would not understand many things and would result in loss of information.

Face to face communication is not possible therefore total reliance has to be placed on written communication.

One example of this was: I finished my work and I gave my shift handover to one co-worker of my team and I didn’t record anything due to it was very late and I had to go out. At the end, this co-worker was moved to another team that day and the others member of the team didn’t know anything and finally they had to call me because they didn’t know for where I left the task.


Error is an unavoidable part of being human. However, maintenance errors can have more serious consequences, and are not always caught and corrected easily.

The most common mistake made in maintenance is when someone is checking the work of another, there is a 10 percent chance that they will miss the problem.

We can take appropriate precautions to limit its effects.

In maintenance we can have two types of errors in according with Professor Reason’s Model of Error: unintended actions and intentional actions.

Unintended actions are:

Slips-> Occur when we perform a routine action that was out of place in the situation, usually because we are distracted and habit takes over. It’s due to: attention failures, omissions, misordering, etc.

Lapssesà A lapse occurs when we forget to complete an action we had been intending to perform. It’s due to: memory failure, losing place, omitting items etc.

Intended errors are:

Mistakes-> Mistakes are a type of error where the problem has occurred during thinking rather than doing. In according with the professor Reason’s model, we have two types of these mistakes: ruled based and knowledge based.

Ruled based errors are the ones that appears and you have a rules defined to avoid the mistakes but the person or in our case the ATM doesn’t follow these rules and the mistake occur.

Knowledge based errors are the ones that the people don’t know very well about the task, dangers and due to lack of awareness or knowledge the mistake can appear. All the people must to know all the dangers and be prepared to the task that he/she is going to perform.

And the last one is Violations:

Violations are intentional deviations from procedures or good practice.

One example of them in my experience was: I was installing a panel on the wing when a co-worker called me to help him to carry a panel to the sheet metal worker repairs. After that was the break and we went to lunch and rest a little. When I came back to my job, I forget that I remained some screws of the panel to install and the supervisor told me.