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’ are breached, that safety be compromised.