The job of an aircraft maintenance engineer can be physically demanding. In addition, his work may have to be carried out in widely varying physical environments, including cramped spaces, extremes of temperature, etc. Fitness and health can have a significant effect upon job performance. Day-to-day fitness and health may be diminished by illness or injury. In fact, it is a legal requirement for aircraft maintenance engineers to make sure they are fit for work: Part 66 imposes that “certifying staff must not exercise the privileges of their certification authorisation if they know or suspect that their physical or mental condition renders them unfit.”