A fuel system is described that comprises a pressure raising and shut-off valve (PRSOV), and a control valve arrangement operable to control a fuel pressure applied to a part of the PRSOV urging the PRSOV towards a closed position, wherein the control valve arrangement is supplied with fuel at a pressure controlled by a variable fluid pressure potentiometer network including a variable flow restrictor which is variable in response to changes in the position occupied by the PRSOV.
1. A fuel system comprising a pressure raising and shut-off valve (PRSOV), and a control valve arrangement operable to control a fuel pressure applied to a part of the PRSOV urging the PRSOV towards a closed position, wherein the control valve arrangement comprises a two stage control valve arrangement including a first stage and a second stage, the first stage being operable to control the position occupied by the second stage, and wherein the first stage is supplied with fuel at a pressure controlled by a variable fluid pressure potentiometer network including a variable flow restrictor which is variable in response to changes in the position occupied by the PRSOV.
2. A system according to claim 1 , wherein the variable flow restrictor comprises a port with which a chamber formed in the PRSOV is registerable.
3. A system according to claim 1 , wherein the variable flow restrictor is closable to permit the application of fuel under high pressure to the control valve arrangement, when desired.
4. A system according to claim 1 , wherein the control valve arrangement comprises a two-stage electro-hydraulic servo-valve (EHSV).
5. A system according to claim 1 , further comprising a pressure drop control valve (PDCV), the control valve arrangement being operable to control a fuel pressure applied to the PDCV.
6. A system according to claim 5 wherein the PDCV allows fuel to flow to a low pressure line in the event of the pressure drop across a metering valve (MV) exceeding a predetermined level.
7. A system according to claim 5 , wherein the PDCV serves to control the operation of a variable output pump.
8. A system according to claim 7 , wherein the variable output pump comprises a variable displacement vane pump.
9. A system according to claim 1 , wherein fuel at the fuel pressure controlled by the variable fluid pressure potentiometer network is additionally supplied to at least one further device.
10. A system according to claim 9 , wherein the said at least one further device comprises a control valve.
11. A system according to claim 10 , wherein the control valve is a proportional control servo-valve.
12. A system according to claim 11 , wherein the proportional control servo-valve is used in controlling the operation of a fuel metering valve.