Man - the “Liveware” - can perform a wide range of activities. Despite the fact that modern aircraft are now designed to embody the latest self-test and diagnostic routines that modern computing power can provide, one aspect of aviation maintenance has not changed: maintenance tasks are still being done by human beings. However, man has limitations. Since Liveware is at the centre of the model, all other aspects -Software, Hardware and Environment - must be designed or adapted to assist his performance and respect his limitations. If these two aspects are ignored, the human - in this case the maintenance engineer - will not perform to the best of his abilities, may make errors, and may jeopardise safety.