Speed control in Aircraft Engine:
Because the APU is designed to run at a constant rpm, some means must be provided to control this speed. Such a control device is known as a speed or rpm governor.
The speed governor is linked mechanically to the APU drive. As speed increases above 95%, the bob weights start to move outwards and begin to by-pass the fuel back to the inlet of the pump and as speed increases up to 100% rpm, it causes sufficient fuel to be by-passed by the governor, to maintain this rpm. Increase or decrease in the speed setting is achieved by adjustment of the governor spring. Note that at speeds below 95% rpm the by-pass valve controls the acceleration up to a maximum speed of 95% rpm.