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variable versus constant errors;

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reversible versus irreversible errors;

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slips, lapses and mistakes;

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skill-, rule- and knowledge-based behaviours and associated errors;

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the ‘Swiss Cheese Model’.

2.3 Design- Versus Operator-Induced Errors
In aviation, emphasis is often placed upon the error(s) of the front line operators, who may include flight crew, air traffic controllers and aircraft maintenance engineers.
However, errors may have been made before an aircraft ever leaves the ground, by aircraft designers. This may mean that, even if an aircraft is maintained and flown as it is designed to be, a flaw in its original design may lead to operational safety being compromised. Alternatively, flawed procedures put in place by airline, maintenance organisation or air traffic control management may also lead to operational problems.
It is common to find when investigating an incident or accident that more than one error has been made and often by more than one person. The ‘error chain’ captures this concept. It may be that, only when a certain combination of errors arise and error ‘defences’ breached (see the ‘Swiss Cheese Model’) will safety be compromised.

2.4 Variable Versus Constant Errors
In his book “Human Error”, Professor Reason discusses two types of human error: variable and constant errors. It can be seen in Figure 1 that variable errors in (A) are random in nature, whereas the constant errors in (B) follow some kind of consistent, systematic (yet erroneous) pattern. The implication is that constant errors may be predicted and therefore controlled, whereas variable errors cannot be predicted and are much harder to deal with. If we know enough about the nature of the task, the environment it is performed in, the mechanisms governing performance, and the nature of the individual, we have a greater chance of predicting an error.

Figure 1 Variable versus Constant Errors
Target patterns of 10 shots fired by two riflemen. Rifleman A’s pattern exhibits no constant error, but large variable errors; rifleman B’s pattern exhibit’s a large constant error but small variable errors. The latter would, potentially, be easier to predict and to correct (e.g. by correctly aligning the rifle sight). Chapanis, 1951
However, it is rare to have enough information to permit accurate predictions; we can generally only predict along the lines of “fatigued pilots are more likely to make errors than alert pilots”, or “The SOPs for task X on aircraft type Y is known as being ambiguous and likely to result in pilot error ”. It is possible to refine these predictions with more information (e.g. The SOPs in Operator Z's QRH are known as being ambiguous), but there will always be random errors or elements which cannot be predicted.
　
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