Error - model and theories

It is the nature of man to err. -  Cicero.
 James Reason : “Error will be taken as a generic term to encompass all those occasions in which a planned sequence of mental or physical activities fails to achieve its intended outcome, and when these failures cannot be attributed to the intervention of some chance agency”.

“Because civil aircraft are designed to fly safely for unlimited time provided defects are detected and repaired, safety becomes a matter of detection and repair rather than one of aircraft structure failure. 
In an ideal system, all defects which could affect flight safety will have been predicted in advance, located positively before they become dangerous, and eliminated by effective repair. In one sense, then, we have changed the safety system from one of physical defects in aircraft to one of errors in complex human-centred systems”

Various models and theories for error are :

Design- versus operator-induced errors  : 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.

Variable versus constant errors : variable errors in  are random in nature, whereas the constant errors in  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.

Reversible versus irreversible errors; The reversible error can be recovered from, whereas the irreversible errors typically cannot be. For example, if a pilot miscalculates the fuel he should carry, he may have to divert to a closer airfield, but if he accidentally dumps his fuel, he may not have many options open to him.

Slips, lapses and mistakes; 

       Slips can be thought of as actions not carried out as intended or planned, e.g. ‘transposing digits when copying out numbers, or misordering steps in a procedure.

       Lapses are missed actions and omissions, i.e. when somebody has failed to do something due to lapses of memory and/or attention or because they have forgotten something, e.g. forgetting to replace an engine cowling.

        Mistakes are a specific type of error brought about by a faulty plan/intention, i.e. somebody did something believing it to be correct when it was, in fact, wrong, e.g. an error of judgement such as mis-selection of bolts when fitting an aircraft windscreen.

        Violations sometimes appear to be human errors, but they differ from slips, lapses

and mistakes because they are deliberate ‘illegal’ actions,

Skill-, rule- and knowledge-based errors;

       Skill-based behaviors are those that rely on stored routines or motor programmes that have been learned with practice and may be executed without conscious thought.

       Rule-based behaviors are those for which a routine or procedure has been learned. The components of a rule-based behavior may comprise a set of discrete skills.

       Knowledge-based behaviors are those for which no procedure has been established. These require the [aircraft maintenance engineer] to evaluate information, and then use his knowledge and experience to formulate a plan for dealing with the situation.”