Why: Life is consumed by usage of life units. Some life units occur as a sum of the different cases, for example on a gas turbine aircraft engine take-offs consume more life than landings or enroute conditions which requires a synthetic value for how life is consumed on a mission. For a land based, heavy -duty gas turbine used in the generation of electrical power the number of starts is not equivalent to hours of operation as other wear mechanisms are involved, however, 1 trip cycle = 8 normal shutdown cycles and thus decreases the time between required maintenance actions.
When: Development of a life consuming profile may be more important than the literal measurement of an elapsed time to adequately measure consumption of life that in the end will result in a failure.
Where: Life units have different measures and must be considered to obtain the proper "common denominator" for calculations.
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com