Apparatus for determining the life usage of components subject to variable operating conditions particularly helicopter gears subject to variable torque. A monitor 10 provides an analogue signal of the level of operating parameter which is passed through conditioning means 26 including pre-amplifier 29, filter stage 31, multiplexer 38 and analogue to digital converter 40. A memory 42 stores life usage data for the component and a microprocessor 43 repeatedly samples the monitored level of the parameter and determines the life usage in successive time intervals. Electromechanical counter 53 provides a cumulative output of the life usage of the components. The microprocessor 43 also analyses the parameter levels into ranges or bands and the total time each component is operating in each range is stored and printed out.