Control systems for gas turbine aeroengines

Claim 1

A fuel control unit for a gas turbine aeroengine, the fuel control unit being adapted to form part of a control system of the aeroengine and to be in communication with a main fuel line leading to the aeroengine, the fuel control unit being controllable by the control system to take fuel from the fuel line to reduce temporarily but substantially the mass flow of fuel to the aeroengine, thereby to combat surge conditions in the aeroengine; the fuel control unit comprising a housing and free piston means provided therein to divide the housing into at least first and second variable volumes, the position of the free piston means within the housing being controlled by pressure differentials across the free piston means, the first variable volume having means for communication with a first point in the fuel line near the fuel line's point of entry into the aeroengine and the second variable volume having means for communication with a second point in the fuel line substantially upstream of the first point, the fuel pressure at the second point being substantially higher than at the first point, whereby under normal operating conditions the balance of pressure across the free piston means positions the free piston means such that the first variable volume is minimised; the fuel control unit further being provided with discharge means for discharging the contents of the second variable volume to a location having a lower pressure than the pressure in the first variable volume, and valve means adapted to obturate the discharge means under normal operating conditions but upon receipt of a signal signifying the occurence of a surge, to open the discharge means whereby the balance of pressure across the free piston means alters and the free piston means moves under the influence of the fuel pressure at the first point in the fuel line to discharge the fuel from the second variable volume and to take fuel into the first variable volume from the fuel line.