Apparatus for determining the deterioration 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 deterioration data for the component and a microprocessor (43) repeatedly samples the monitored level of the parameter and determines the deterioration in successive time intervals. Electromechanical counter (53) provides a cumulative output of the deterioration 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.