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"Human factors" refers to the study of human capabilities and limitations in the workplace. Human factors researchers study system performance. That is, they study the interaction of maintenance personnel, the equipment they use, the written and verbal procedures and rules they follow, and the environmental conditions of any system. The aim of human factors is to optimise the relationship between maintenance personnel and systems with a view to improving safety, efficiency and well-being”.
In 1940, it was calculated that approximately 70% of all aircraft accidents were attributable to man’s performance, that is to say human error¹. When the International Air Transport Association (IATA) reviewed the situation 35 years later, they found that there had been no reduction in the human error component of accident statistics.
Types of Error in Maintenance Tasks
As aircraft maintenance engineers are human, errors in the industry are inevitable. Any maintenance task performed on an aircraft is an opportunity for human error to be introduced. Errors in aircraft maintenance engineering tend to take two specific forms:
i) an error that results in a specific aircraft problem that was not there before the maintenance task was initiated;
ii) an error that results in an unwanted or unsafe condition remaining undetected while performing a maintenance task designed to detect aircraft problems, i.e. something is missed.
Examples of errors highlighted in (i) above are incorrect installation of line-replaceable units, failure to remove a protective cap from a hydraulic line before reassembly or damaging an air duct used as a foothold while gaining access to perform a task.
Examples of errors in (ii) are a structural crack unnoticed during a visual inspection task or a faulty avionics box that remains on the aircraft because incorrect diagnosis of the problem led to removal of the wrong box. The actual error type responsible can be any of those highlighted in the previous section of this document.
Errors During Regular and Less Frequent Maintenance Tasks
A large proportion of maintenance tasks are fairly routine, such as regular, periodic checks on aircraft. Thus, engineers will use a certain set of procedures relatively frequently and, as noted in the previous section, slips and lapses can occur when carrying out procedures in the busy hangar or line environment. Chapter 6, Section 2 “Repetitive Tasks” noted that engineers will often become so accustomed to doing a regular, often repeated task, that they will dispense with written guidance altogether. It would be unrealistic and unnecessarily time consuming to expect them to constantly refer to familiar guidance material. However, errors may occur if they do not keep up-to-date with any changes that occur to these frequently used procedures. These routine tasks are also prone to complacency, environmental capture and rule-based errors.
When undertaking less frequently performed tasks, there is the possibility of errors of judgement. If the engineer does not familiarise or re-familiarise himself properly with what needs to be done, he may mistakenly select the wrong procedure or parts.
Violation in Aircraft Maintenance
It is an unfortunate fact of life that violations occur in aviation maintenance. Most stem from a genuine desire to do a good job. Seldom are they acts of vandalism or sabotage. However. They represent a significant threat to safety as systems are designed assuming people will follow the procedures. There are four types of violations:
• Routine violations;
• Situational violations;
• Optimising violations;
• Exceptional violations
Routine violations are things, which have become ‘the normal way of doing something’ within the person’s work group (e.g. a maintenance team). They can become routine for a number of reasons: engineers may believe that procedures may be over prescriptive and violate them to simplify a task (cutting corners), to save time and effort.
Situational violations occur due to the particular factors that exist at the time, such as time pressure, high workload, unworkable procedures, inadequate tooling, poor working conditions. These occur often when, in order to get the job done, engineers consider that a procedure cannot be followed.
Optimising violations involve breaking the rules for ‘kicks’. These are often quite unrelated to the actual task. The person just uses the opportunity to satisfy a personal need.
Exceptional violations are typified by particular tasks or operating circumstances that make violations inevitable, no matter how well intentioned the engineer might be.
Examples of routine violations are not performing an engine run after a borescope inspection (“it never leaks”), or not changing the ‘O’ seals on the engine gearbox drive pad after a borescope inspection (“they are never damaged”).
An example of a situational violation is an incident, which occurred where the door of a B747 came open in-flight. An engineer with a tight deadline discovered that he needed a special jig to drill off a new door torque tube. The jig was not available, so the engineer decided to drill the holes by hand on a pillar drill. If he had complied with the maintenance manual he could not have done the job and the aircraft would have missed the service.
An example of an optimising violation would be an engineer who has to go across the airfield and drives there faster than permitted
Time pressure and high workload increase the likelihood of all types of violations occurring. People weigh up the perceived risks against the perceived benefits, Unfortunately the actual risks can be much higher.
Reason analysed* the reports of 122 maintenance incidents occurring within a major airline over a 3 year period. He identified the main causes as being:
- Omissions (56%)
- Incorrect installation (30%)
- Wrong parts (8%)
- Other (6%)
It is likely that Reason’s findings are representative for the aircraft maintenance industry as a whole. Omissions can occur for a variety of reason, such as forgetting, deviation from a procedure (accidental or deliberate), or due to distraction. The B7372** double engine oil loss incident, in which the HP rotor drive covers were not refitted is an example of omission. Incorrect installation is unsurprising, as there is usually only one way in which something can be taken apart but many possible ways in which it can be reassembled.
When an error occurs in the maintenance system of an airline, the engineer who last worked on the aircraft is usually considered to be ‘at fault’. The engineer may be reprimanded, given remedial training or simply told not to make the same error again. However, blame does not necessarily act as a positive force in aircraft maintenance: it can discourage engineers from ‘coming clean’ about their errors. They may cover up a mistake or not report an incident. It may also be unfair to blame the engineer if the error results from a failure or weakness inherent in the system, which the engineer has accidentally discovered (for example, a latent failure such as a poor procedure drawn up by an aircraft manufacturer - possibly an exceptional violation).
Ultimately, maintenance organisations have to compromise between implementing measures to prevent, reduce or detect errors, and making a profit. Some measures cost little (such as renewing light bulbs in the hangar); others cost a lot (such as employing extra staff to spread workload). Incidents tend to result in short term error mitigation measures but if an organisation has no incidents for a long time (or has them but does not know about them or appreciate their significance), there is a danger of complacency setting in and cost reduction strategies eroding the defences against error. Reason* refers to this as “the unrocked boat”
