Fuel for the combustion equipment of the engine of a vertical take off aircraft is supplied via pumps 50,24 through a metering valve 22, which is under the control of a digital electronic control unit (DECU) 10. A pressure drop regulator 30 and spill valve 32 ensure there is a constant pressure drop across the metering valve 22. To provide a rapid reduction in fuel output when the aircraft pilot slams shut his control lever 16 when he touches down after a vertical landing, and therefore prevent the aircraft from bouncing, the DECU 10 detects the sudden slamming closed of the lever 16, and opens a solenoid valve 60. Fuel now bleeds along line 58, and the input 54 of the regulator 30 sees an increased pressure drop determined by a potentiometer formed by two flow restricting orifices 56,62. The regulator 30 therefore opens the spill valve 32 giving the desired rapid reduction in fuel flow through the metering valve 22.