a modular control sub - assembly in a fuel control system for a turbine engine such as a jet engine , comprises a pressure drop detector constituted by a slide axially movable in a bore of a casing , and an overspeed limiter disposed coaxially in said casing at one end of the bore containing the detector slide . the slide is rotatably driven by the overspeed limiter through a fork coupling and , when the plant overspeed threshold is exceeded , the slide is moved axially by the overspeed limiter in the same direction as on detecting an increase in the pressure drop across a fuel flow feed regulator to reduce an output control pressure signal and thereby cause a control valve to open which reduces the fuel flow and hence the overspeed .