Error due to Individual Practices and Habits

 Where procedures allow some leeway, AMT often develop their own strategies or preferred way of carrying out a task. Often, a ‘good’ rule or principle is one that has been used successfully in the past. 
These good rules become ‘rules of thumb’ that an engineer might adopt for day-to-day use. 
Problems occur when the rule or principle is wrongly applied. For example, aircraft pipe couplings are normally right-hand threads but applying thi s ‘normally good rule’ to an oxygen pipe (having a different thread) could result in damage to the pipe. Also, there can be dangers in applying rules based on previous experience if, for example, the design
 philosophy differs, as in the case of Airbus and Boeing. This may have been a factor
in an A320 locked spoiler incident, where subtle differences between the operation
of the spoilers on the A320 and those of the B767 (with which the engineers were
more familiar) meant that actions which would have been appropriate on the B767
were inappropriate in the case of the A320.
In addition, engineers may pick up some ‘bad rules’, leading to bad habits during their
working life, as a driver does after passing his driving test. 
An example of applying a bad rule is the British Rail technician in the Clapham train accident who had acquired the practice of bending back old wires rather than cutting them off and insulating them.
Errors Associated With Visual Inspection

Reason’s  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. Reason illustrates this with a simple example of a bolt and
 several nuts, asking the questions (a) how many ways can this be disassembled? (the answer being 1) and (b) how many ways can it be reassembled?
(the answer being about 40,000, excluding errors of omission!).

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