A simple rotary bobweight overspeed governor mechanism is conveniently incorporated in a main fuel control system for a gas turbine aeroengine in order to prevent overspeeds of one and more of its compressor/turbine rotating spools. In order to prevent so-called "dormant" failures of the overspeed governor mechanism going undetected, the invention provides automatic checking of governor function by means of a microswitch linked to a main digital electronic controller of the fuel control system. The position of a spindle in the governor mechanism is proportional to the speed of the spool from which it is driven and each time the spindle is at the position which should correspond to a predetermined proportion of the maximum allowable spool speed, the microswitch is actuated and signals the electronic controller. The latter already receivers a spool speed signal from a tachometer on the engine and if the microswitch signal oocurs at the predetermined proportion of maximum spool speed as measured by the tachometer, the electronic controller outputs a validation signal, otherwise it outputs a warning signal.