Abstract: 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.

Abstract: A fuel control system for a three spool gas turbine aeroengine has a fuel contorl unit incorporating an intermediate pressure spool overspeed limiter. A fuel control signal is produced which drives a fuel metering unit to meter the flow of fuel to the engine in accordance with control and sensor inputs as acted upon by predetermined control laws incorporated in the fuel control unit. The engine has variable inlet guide vanes (VIGV's) in one or more of its compressor stages and a safety problem arises in that if the VIGV's fail at an angle for low engine speed while the engine is accelerating to a high speed, the rotational speed of the intermediate pressure spool can increase very quickly. This can lead to overshooting of maximum acceptable levels of spool speed before known overspeed limiters have time to regain control.

Abstract: A fuel control system for gas turbine aeroengine includes an overspeed limiter unit operative to limit fuel flow when engine speed exceeds a predetermined safe value. It is conventional for such overspeed limiters to be activated only in the event of an overspeed, so as to obviate the possibility that the limiter unit may fail unnoticed while the unit is dormant, the invention utilises the limit unit as an active part of the fuel control system on a continuous basis. Specifically, the overspeed limiter unit is used to control the pressure drop across a fuel metering unit as a function of engine speed, except when it is actually performing its overspeed limiting task. Specific means of accomplishing this are disclosed. In this way, any failure in the limiter unit will be immediately apparent due to its effect on the functioning of the system, and appropriate action can be taken.

Abstract: A fuel control system for a gas turbine engine comprises a main fuel controller and an overspeed limiter arranged to prevent overspeed of the engine in the case of a fault in the main controller. To ensure that the limiter is tested regularly, the logic capacity of the main controller (which is preferably a digital electronic unit) is used to vary the parameters of the limiter each time the engine is started up. This variation is such as to cause the limiter to operate and produce an observable reduction in the engine speed which is detected by the main controller and uses as an indication of correct limiter operation.

Abstract: A fuel control system for a gas turbine engine comprises a main fuel controller which varies the fuel flow to the engine in accordance with the engine pressure ratio and an idling fuel control arrangement. Control of idling fuel flow on a pressure ratio basis is somewhat wasteful of fuel. Therefore in the present invention the idling fuel control arrangement is arranged to control to a constant value of engine HP rotor speed. In the embodiment a variable bleed orifice is allowed to reduce the control pressure which operates the main fuel valve.

Abstract: A fuel control system for a gas turbine engine comprises a main steady-state fuel control unit which adjusts the fuel flow to the engine to produce an approximation to a desired thrust and a trimmer device which trims the fuel flow demanded by the main unit to produce a closer approximation to the desired thrust. During transients such as accelerations a trimmer suitable for steady operation will apply zero trim until the desired thrust is almost attained, when an amount of trim may have to be applied rapidly. This can lead to overshoots.To mitigate the problem the trimmer is provided with a model of the main control unit which derives an approximation to the trim which will be required when the steady state is reached. This is applied to the main unit throughout the transients, and when the desired thrust is achieved only second-order changes need to be made.

Abstract: A gas turbine engine fuel system of the type using the rotational speed of the high pressure spool as the main control parameter has a trimmer device which reduces the effect of variation of engine geometry in cold starts etc. The trimmer uses the throttle lever angle and engine inlet pressure to compute the desired value of a thrust-related parameter such as weighted mean exhaust pressure, this desired value being compared with the actual value to produce a trim input to the main fuel control unit.