FUEL METERING SYSTEM

Abstract

The present disclosure provides a fuel metering system for a gas turbine engine, the fuel metering system comprising: a fuel supply line; a fuel metering valve configured to pass an amount of fuel received from the fuel supply line to the gas turbine engine; an engine control unit configured to control the position of the fuel metering valve according to a demanded fuel flow to the gas turbine engine; a flow sensor configured to provide a measurement of a flow of fuel in the fuel metering system; wherein the engine control unit is further configured to determine a fuel flow to the gas turbine engine based upon the measurement from the flow sensor; and wherein the engine control unit is further configured to identify a loss of fuel flow control by comparing the demanded fuel flow to the determined fuel flow to the gas turbine engine.

Claims

1. A fuel metering system for a gas turbine engine, the fuel metering system comprising: a fuel supply line; a fuel metering valve configured to pass an amount of fuel received from the fuel supply line to the gas turbine engine; an engine control unit configured to control the position of the fuel metering valve according to a demanded fuel flow to the gas turbine engine; a flow sensor configured to provide a measurement of a flow of fuel in the fuel metering system; wherein the engine control unit is further configured to determine a fuel flow to the gas turbine engine based upon the measurement from the flow sensor; and wherein the engine control unit is further configured to identify a loss of fuel flow control by comparing the demanded fuel flow to the determined fuel flow to the gas turbine engine.

2. The fuel metering system according to claim 1, wherein the engine control unit is configured to identify a loss of fuel flow control when the fuel flow measured by the flow sensor exceeds the demanded fuel flow by a threshold amount.

3. The fuel metering system according to claim 1, further comprising: a spill line configured to receive excess fuel from the fuel supply line; and a spill valve provided in the spill line and configured to control the flow of fuel through the spill line.

4. The fuel metering system according to claim 3, wherein the engine control unit is further configured to limit closure of the spill valve in response to identifying a loss of fuel flow control.

5. The fuel metering system according to claim 3, further comprising a servo-valve associated with the spill valve, and wherein the engine control unit is configured to control the position of the spill valve via the servo-valve.

6. The fuel metering system according to claim 1, wherein the engine control unit is further configured to close the fuel metering valve in response to identifying a loss of fuel flow control.

7. The fuel metering system according to claim 3, further comprising a pump supplying fuel to the fuel supply line.

8. The fuel metering system according to claim 7, wherein the spill line is configured to receive the excess fuel from the fuel supply line and deliver it back to the pump.

9. The fuel metering system according to claim 3, wherein the system is configured such that fuel not passed by the fuel metering valve to the gas turbine engine is received by the spill line as the excess fuel.

10. The fuel metering system according to claim 1, wherein the flow sensor is configured to measure a flow of fuel downstream of the fuel metering valve.

11. The fuel metering system according to claim 1, wherein the flow sensor comprises a passive flow sensing valve configured to directly measure the downstream flow of fuel from the fuel metering valve.

12. The fuel metering system according to claim 1, wherein the flow sensor comprises a position sensor configured to determine the position of a valve downstream of the fuel metering valve.

13. The fuel metering system according to claim 1, wherein the flow sensor is configured to measure a flow of fuel upstream of the fuel metering valve.

14. The fuel metering system according to claim 13, further comprising: a spill line configured to receive excess fuel from the fuel supply line; and a spill valve provided in the spill line and configured to control the flow of fuel through the spill line, wherein the flow sensor is configured to measure the fuel flow in the spill line.

15. A method of operating a fuel metering system for a gas turbine engine, the method comprising the steps of: controlling supply of fuel to a gas turbine engine according to a demanded fuel flow; taking a measurement of a flow of fuel in the fuel metering system; determining a flow of fuel to the gas turbine engine based upon the measurement; and comparing the demanded fuel flow to the determined fuel flow.

16. The method according to claim 15, further comprising the step of: limiting a maximum fuel flow to the gas turbine engine in response to identifying that the determined fuel flow exceeds the demanded fuel flow by more than a threshold amount.

17. The method according to claim 16, wherein the step of limiting comprises limiting the closure of a spill valve in a spill line receiving excess fuel from the fuel supply line.

18. The method according to claim 16, wherein the step of limiting comprises closing a fuel metering valve controlling the supply of fuel to the gas turbine engine.

19. The method according to claim 15, wherein the fuel metering system is a system comprising: a fuel supply line; a fuel metering valve configured to pass an amount of fuel received from the fuel supply line to the gas turbine engine; an engine control unit configured to control the position of the fuel metering valve according to a demanded fuel flow to the gas turbine engine; a flow sensor configured to provide a measurement of a flow of fuel in the fuel metering system; wherein the engine control unit is further configured to determine a fuel flow to the gas turbine engine based upon the measurement from the flow sensor; and wherein the engine control unit is further configured to identify a loss of fuel flow control by comparing the demanded fuel flow to the determined fuel flow to the gas turbine engine.

20. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades; a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; and a fuel metering system according to claim 1.

21. The gas turbine engine according to claim 20, wherein: the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft; the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

Description

DESCRIPTION OF THE DRAWINGS

[0054] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0055] FIG. 1 is a sectional side view of a gas turbine engine;

[0056] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0057] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0058] FIG. 4 is a flow diagram of a fuel pumping unit and fuel metering unit;

[0059] FIG. 5 is a flow diagram of engine control loops;

[0060] FIG. 6 is a flow diagram of a fuel metering system;

[0061] FIG. 7 is a flow diagram of a fuel metering system;

[0062] FIG. 8 is a flow diagram of a fuel metering system;

[0063] FIG. 9 is a diagram of a fuel system incorporating a splitter and drain unit; and

[0064] FIG. 10 is a diagram of a modification of the fuel system of FIG. 9.

DETAILED DESCRIPTION

[0065] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0066] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0067] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0068] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0069] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the present disclosure. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0070] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0071] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0072] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0073] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0074] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0075] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0076] FIG. 4 shows a schematic diagram of a conventional gas turbine engine fuel control system. The system is broadly divided into a fuel pumping unit (FPU) 60 and a fuel metering unit (FMU) 70.

[0077] As shown in FIG. 4, fuel from aircraft tanks 50 is pumped via engine tank pumps to the FPU 60. The FPU 60 typically comprises a low pressure (LP) pump 61 followed by a high pressure (HP) pump 62. The FPU 60 is typically mounted on the accessory gearbox (AGB) with the LP and HP pumps 61, 62 driven off the gearbox at a fixed ratio of engine shaft speed. The LP pump 61 pressurises the fuel before it passes to the downstream HP pump 62 via a fuel oil heat exchanger (FOHE) 63 and a main engine filter 64. The LP stage 61 is capable of handling the low inlet pressures and multiphase flow that will be presented to the fuel pump with failed airframe tank pumps; it is designed to generate sufficient pressure to ensure that, even with high inter-stage losses which will occur if the filter 64 and FOHE 63 are running on bypass, the HP gear stages 62 are not starved of flow. In other words the LP pump 61 helps prevent cavitation at the HP gear pump 62 inlet.

[0078] The HP pump 62 is usually a fixed displacement pump, and so it delivers a fuel flow proportional to its speed of rotation. The pump 62 is sized to deliver a flow that exceeds the burner flow requirement of the engine at all operating conditions. Any surplus flow is spilled back to HP pump 62 inlet via the spill valve (SV) 71 in the FMU 70. Total HP pump 62 delivery flow passes from the FPU outlet 65 to the FMU inlet 72.

[0079] On entering the FMU 70 the total flow from the HP pump 62 is split. The required burner flow passes through a fuel metering valve (FMV) 73, and any excess or surplus flow passing through the SV 71 back to HP pump 62 inlet, or to provide servo flow to actuation systems for engine variable geometry (such as Variable Stator Vane Actuators).

[0080] Normal fuel metering is achieved via the valves of the FMU 70i.e. the FMV 73 and SV 71. FMV 73 is moved via servo flow from an associated servo-valve 74 to vary the opening of a flow metering port. The pressure drop across the metering port can be controlled to a nominally constant value via a pressure drop regulator (PDR) 75 and the SV 71. The PDR 75 senses any change in the FMV 73 pressure drop and adjusts a servo orifice to vary the servo flow to the SV 71 so that the SV 71 moves to restore the FMV 73 pressure drop by adjusting the amount of excess flow from the HP pump 62 which is spilt back to pump inlet.

[0081] Thus, in normal operation, metered flow to the engine 10 is essentially a function of FMV 73 position. The FMV 73 position can be sensed by an appropriate position sensor and fed back to the electronic engine control (EEC) to facilitate so-called inner loop (i.e. fuel system levelsee discussion below) control of FMV 73 position.

[0082] Downstream of the FMV 73, flow passes through a pressure raising shut off valve (PRSOV) 76 before exiting the FMU 70 and passing to the engine burners via a delivery line and burner manifold. Typically, the PRSOV 76 comprises a piston moveable within a sleeve to control the opening of a flow port that connects the FMV 73 outlet line to the downstream burner delivery line. One side of the piston is exposed to downstream metering valve pressure and the other side has a spring load acting on it and also a reference pressure set by an associated shut off valve servo-valve (SOVSVnot shown).

[0083] In normal operation, a low pressure supply is ported to the spring side of the PRSOV 76. The valve will only open against this pressure and the spring load, to allow flow to the burners, once a certain level of pressure is achieved on the upstream side, downstream of the FMV. In shut off mode, the SOVSV is energised to port HP fuel to the spring side of the PRSOV 76 so that the valve closes to provide a drip tight seal, closing off fuel flow to the engine 10.

[0084] The conventional fuel control described above has limited capability for detecting or accommodating an un-commanded high engine thrust (UHT). This is explained by considering the typical fuel control loops associated with the FMU 70, which are shown schematically in FIG. 5.

[0085] FMV 73 position is controlled by an EEC 80. The EEC controls a slow outer control loop 81 and a fast inner control loop 82.

[0086] The outer engine loop 81 sets a demand on an engine control parameter (for example, fan speed (N1) or engine pressure ratio (EPR), responding to changes set by the pilot's throttle. The demanded value for the engine parameter is compared to an actual measured value and the resultant error signal is converted into a fuel flow demand by the EEC 80.

[0087] The fuel flow demand is the input to the minor or inner control loop 82. This is converted into a FMV 73 demanded position which is compared to the actual FMV 73 position, as measured by an appropriate position sensor. FMV 73 position error is used to generate a current to drive the FMV servo-valve 74 which in turn drives the FMV 73 to its demanded position as previously described. As the FMV 73 moves to its demanded position, flow to the engine 10 changes and the engine 10 responds, the engine parameter under outer loop control 81 changing to meet the outer loop 81 demand.

[0088] The inner loop 82 response (FMV 73 control) is much faster than the outer loop 81 response. This is because the latter relies on the engine 10 to respond before the engine control parameter can change.

[0089] As a result, in the event of a failure internal to the FMU 70, causing an un-commanded increase in fuel flow to the burners, this system relies on a change in the engine control parameter to detect the potential UHT threat. That means UHT threat detection is slow.

[0090] Further, the only subsequent action which can be taken is to shut down the engine via the SOVSV and PRSOV. That means options for UHT accommodation are limited. Engine shut down is a limited option and is only really applicable on the ground. In-flight (and in some cases on the ground as well), slow detection of a UHT event on one engine can lead to the aircraft experiencing gross asymmetric thrust. With limited aircraft rudder authority/response this can be hazardous, particularly at low altitude conditions such as approach to a runway.

[0091] Once detected, shut down of an engine 10 experiencing UHT can itself lead to the aircraft experiencing gross asymmetric thrust, particularly if the aircraft rudder has been adjusted in an attempt to compensate for the UHTsudden shut down of the engine can result in asymmetric thrust caused by a combination of having one functional engine and an incorrectly positioned rudder.

[0092] FIG. 6 presents a UHT protection system. In this system, normal fuel metering is achieved via the valves of the FMU 70.

[0093] As in FIG. 4, FMV 73 of FIG. 5 is moved via servo flow from an associated servo-valve 74 to vary the opening of a flow metering port. The pressure drop across the metering port is controlled to a nominally constant value via the PDR 75 and the SV 71. The PDR 75 senses any change in the FMV 73 pressure drop and adjusts a servo orifice to vary the servo flow to the SV 71. This causes the SV 71 to move to restore the FMV 73 pressure drop by adjusting the amount of excess flow from the HP pump 62 that is spilt back to pump inlet. Thus, in normal operation, metered flow to the engine 10 is essentially a function of FMV 73 position. That position is sensed by an appropriate position sensor and fed back to the EEC 80 to facilitate inner loop 82 (i.e. fuel system level) control of FMV 73 position.

[0094] In the event of a FMV 73 fault (e.g. upward runaway of the FMV 73 or a jammed FMV 73), an overthrust threat is detected via the engine control loop (outer loop 81) which senses a change in an engine parameter. The EEC 80 then acts to energise a thrust control malfunction (TCM) servo-valve 77 which moves to vent the SV 71 servo pressure back to HP pump 62 inlet pressure (LP). This opens the SV 71 to increase spill flow and simultaneously reduces flow through the FMV 73 to the burners. Thus the level of overfuelling/overthrust is limited.

[0095] Similarly, in the event of an overthrust threat caused by a jammed SV 71 (too far closed), this can be sensed by the outer control loop and accommodated by the EEC 80 closing the FMV 73.

[0096] In both of the above scenarios, the main drawback is the slow speed of response. The slow outer control loop 81 has to wait for a change in a key engine parameter to first detect and then accommodate a potential UHT threat. The measured engine parameter provides the feedback signal to the EEC 80 in TCM control mode. Consequently, an appreciable increase in thrust can occur before the burner flow is pulled back.

[0097] There are other ways of achieving control to avoid UHT. Many of these suffer from the same issue, their response to an upward runaway of the FMV or a jammed FMV/SV or an incorrect/lost FMV position signal being slow because of their reliance on the slow outer engine control loop 81.

[0098] Thus, the systems described above are limited in their ability to meet UHT detection and accommodation requirements on modern engines. Detection via the outer loop 81 is slow (the engine 10 can reach an unacceptable level of overthrust before action is taken) and accommodation by shutting down the engine 10 can exacerbate the UHT problem.

[0099] Therefore, it is preferable to have a system capable of rapidly detecting a UHT threat and, before it manifests to any significant extent at engine level, that is subsequently able to maintain control the fuel flow to the engine, thereby controlling engine thrust. This can be achieved by providing a direct or inferred measurement of burner flow which can be used by the fast inner loop 82, as set out below. Preferably, any measurement device used to provide that measurement should be sufficiently responsive so as to not compromise the dynamic capability of the loop to control burner flow. In other words, the dynamic capability of the flow measurement device is preferably faster than that of the engine to provide a benefit in detection speed.

[0100] FIG. 7 shows an arrangement where a flow sensing device 78 is located in the main flow line to the burners. In general, the device 78 can be downstream of the FMV 73 in the FMU 70 (as shown) or upstream of the FMV 73 in the FMU 70 or downstream of the FMU 70 in a separate body.

[0101] In FIG. 7, the flow measuring device 78 is shown as a flow sensing valve (FSV) 78. The FSV 78 in FIG. 7 incorporates a position sensor 79. However, the FSV 78 can take several different forms as long as it provides a measure of flow. Possible options for the FSV 78 include:

[0102] a flowmeter e.g. a turbine flow meter, provided it is sufficiently responsive;

[0103] an orifice plate arrangement, using an orifice of known size with a pressure drop sensor across it, from which flow can be computed from the area of the orifice and the pressure drop across it;

[0104] a single stage flow sensing valve; and

[0105] a two stage flow sensing valve.

[0106] A single stage flow sensing valve can comprise a piston in a sleeve with a spring at one end of the piston. Inlet fuel flow can be fed into the non-spring end of the valve and pass by a metering edge on the piston through a flow profile cut into the sleeve. Thus, the non-spring end of the piston is exposed to high inlet pressure while the spring chamber is flooded, with lower pressure from downstream of the profile. As flow increases, the piston moves to open the exposed area of the profile. A position sensor is used to detect piston position, the measured position being a measure of flow passing through the flow profile.

[0107] A two stage flow sensing valve can be considered as a two stage version of the single stage valve. A first stage piston can sense the pressure drop across a flow profile cut into a second stage sleeve and balances it against a first stage spring load. The first stage piston pilots a second stage piston which moves to vary the flow area of the flow profile. High upstream pressure acts at the flowing end of the second stage piston whilst the other end is exposed to a secondary spring load and servo pressure. An orifice potentiometer, formed by a fixed first orifice and a second orifice variable with position of the pilot stage piston, can take fuel pressure from upstream of the FSV, and returns flow to downstream of the FSV. The intermediate servo pressure between the two restrictions can act at the spring end of the second stage piston. When flow through the flow profile changes, the pressure drop across it changes. This is sensed by the first stage piston which moves to vary the restriction of the variable orifice. This disturbs the servo flow balance across the potentiometer arrangement such that there is a net flow into/out of the second stage servo chamber. This displaces the second stage piston to vary the flow profile area until the profile pressure drop is restored. In other words, the first stage senses and maintains a nominally constant pressure drop across the flow profile by piloting the second stage piston. As flow increases, the second stage piston moves to open the flow area of the profile so that piston position, measured by an appropriate position sensor is a measure of flow.

[0108] A two stage FSV 78 might have a bandwidth of around 4 Hz. A single stage FSV 78 bandwidth can be significantly higher, for example greater than 10 Hz. In general a flow sensor 78 for use in the embodiments of the present disclosure preferably has a bandwidth of more than 0.5 Hz, more preferably more than 0.75 Hz, still more preferably more than 1 Hz, still more preferably more than 2 Hz, and still more preferably greater than or equal to 4 Hz.

[0109] In any case, whatever form of flow sensing device 78 is used, it plays no role in setting the metered flow to the engine in normal conditions. It is simply a passive device, monitoring the metered flow which is typically set by the FMV 73. That is, although the device may include moving parts (as described in options above) it is passive in the sense of only reacting to the flow and pressure conditions around it, and not being directly controlled by a signal from the EEC 80.

[0110] A FMV servo-valve 74 can provide servo flow to either end of the FMV 73. The FMV 73 can comprise a piston moveable in a sleeve with a metering edge to vary the opening of a metering profile cut into its sleeve. Varying the input current to the FMV servo-valve 74 varies the servo flow to the FMV 73 so that FMV 73 moves to change the area of the metering profile exposed to fuel flow. The pressure drop across the metering port can be controlled to a nominally constant value by the PDR 75 and SV 71, as has been discussed in connection with previous figures. That is, the PDR 75 can sense the FMV 73 pressure drop and adjusts a servo orifice to vary the servo flow to the SV 71, so that the SV 71 moves to adjust the amount of excess flow from the HP pump 62 which is spilt back to the pump inlet. Thus, in normal operation, metered flow to the engine 10 is essentially a function of FMV 73 position, the latter being sensed by an appropriate position sensor and fed back to the EEC 80 to facilitate inner loop 82 control of FMV 73 position.

[0111] By directly measuring the metered flow, the FSV 78 provides an accurate and rapid means of detecting any fault where the actual metered flow significantly exceeds the demanded flow, for example. A threshold amount between the actual flow and the demanded flow may be set, to trigger a control response. In other words, the flow measurement feedback signal can be used by the EEC 80 to identify a potential UHT threat. If the threshold difference is passed, this can be detected by the fast EEC inner loop control 82, without relying on the slow outer engine control loop 81 to detect a change in an engine parameter. The threshold difference between the actual flow and the demanded flow amount may be a negative amount or a positive amount and may vary with engine condition including during engine transients. The threshold may be set at some absolute difference or percentage amount difference compared to the determined flow.

[0112] A high measured flow, in comparison to the demanded flow, can be indicative of a loss of fuel flow control. That may be caused by an upward runaway/jamming of the FMV 73 (i.e. a loss of position control that tends to open the FMV 73 and increase burner flow) or downward failure/jamming of the SV 71 (i.e. closure of the SV 71 spill ports, resulting in an increase in burner flow). FMV 73 position faults may be caused by a failure of the FMU 70 mechanical valves or by an EEC 80 drive circuit fault.

[0113] Upon detection of a high burner flow, in comparison to the demanded flow, the EEC 80 can switch into TCM fuel control mode. The EEC 80 may attempt to close the FMV 73 as the fault may be due to the SV 71 being in too closed a position (low spill flow, high burner flow). Alternatively, if flow does not decrease as a result of attempting to close the FMV 73 (in this case, the cause of the high burner flow is a loss of control of the FMV 73), the TCM servo-valve can be energised to vent SV 71 servo pressure back to a low pressure source (typically HP pump 62 inlet pressure) so that the SV 71 opens to increase spill flow, thereby reducing/limiting the burner flow.

[0114] In either case, the EEC 80 can maintain control of the metered flow either by modulating the FMV (in the case of as spill valve failure) or by modulating the spill valve (in the case of an FMV failure). As such, the level of overfuelling/overthrust is limited and closed loop control is achieved using the fast inner control loop 82 via the EEC 80, the FSV 78 providing a flow feedback signal. This means there is no reliance on the slower outer/engine control loop 81 to provide a feedback signal for flow control. That is, burner flow continues to be controlled by the inner loop with the outer loop not having TCM related control components in its algorithms. In other words, the outer loop will continue to try to control according to normal operation, so will still be required for engine control, but UHT can be controlled by the inner loop.

[0115] In summary, closed inner loop control can be achieved for any fault where the burner flow is significantly higher than demand. The fault may be caused by, amongst others: FMV 73 upward failure, position known or unknown; FMV 73 being jammed; incorrect FMV 73 position measurement; SV 71 downward failure; or SV being jammed too far closed.

[0116] In a non-UHT scenario, the scheme also offers the potential to partially accommodate any jamming of the FMV 73 or SV 71. Should the FMV 73 jam, closed loop flow control can be achieved by modulating the SV 71 via the TCM servo-valve 77, using the FSV 78 for inner loop flow feedback. Here, the maximum achievable burner flow may be limited by a pump pressure relief valve (PPRVnot shown). The PPRV will open to spill pump delivery flow back to pump inlet should the increase in flow through the FMV 73 (as the SV 71 closes) raise the system pressure to a level above the PPRV cracking point. Similarly, should the SV 71 jam, closed loop control can be achieved by modulating the FMV 73 via its servo-valve 74, again using the FSV 78 for flow feedback. In this case, the maximum achievable flow may be limited at the maximum FMV 73 opening for a given position of the failed SV 71.

[0117] FIG. 8 shows an alternative arrangement in which the FSV 78 is located in the spill return line to monitor spill flow. In this arrangement, the EEC 80 can compute burner flow from pump speed and spill flow measurement, approximating pump and FMU leakage from burner flow demand, pump speed and combustion chamber pressure measurements. Alternatively, the system can be run at a condition where the burner flow is zero (e.g. the normal scenario during engine spool-up prior to commencing delivery of burner flow to the engine for light-up), to compute combined leakage of the pump 62, FMU 70 and any other component between pump outlet and inlet pressure, for use in later pump health monitoring. In any case, a burner flow indicative of the health of the pump 62 is given by:

Burner Flow=Net pump flowSpill FlowFMU (& other component) leakage

[0118] Comparing demanded flow to calculated burner flow, the EEC 80 can detect a high flow fault and limit it through fast inner loop control of the FMV 73 or SV 71 control as described above.

[0119] Compared to the FIG. 7 arrangement, the FIG. 8 arrangement may have some drawbacks in certain scenarios. It may be less accurate because it does not directly measure the burner flow, relying instead on a calculated approximation. Similarly, the inner loop 82 response may be slower, as burner flow has to be calculated from a number of other signals. However, the benefit of this arrangement is that the FSV 78 can be used for direct measurement of the spill flow. This can provide a direct measurement of pump health; as the pump wears, spill flow at any given condition will decrease.

[0120] FIGS. 9 and 10 relate to another alternative arrangement that could be employed if there are difficulties associated with installation of the FMU 70 on the engine 10.

[0121] This might occur, for example, if the additional UHT protection functionality within the FMU cannot be accommodated in the space available. An additional TCM servo-valve 77 and FSV 73 downstream of the metering valve could cause such issues in some engines. In any case, the relative positions of the burner manifold supplying fuel the engine 10 burners, the FMU 70 and manifold drains tank are important and can impose installation constraints.

[0122] As mentioned above, the position of the FSV 73 can vary. As such, it can be downstream of the PRSOV 76, and the functionality could be integrated into any flow throttle by adding a position sensor. An example of such an arrangement is a use in a flow splitter valve, used for distributing fuel between manifolds.

[0123] A schematic view of such a system is shown in FIG. 9, in which it can be seen that between FMU 70 and the splitter and drain valve unit 100, there may also be a flowmeter and HP filter 90. This flowmeter is of the highly accurate type for measuring the total fuel supplied to the engine for data gathering purposes. However, technologies suitable for the engine environment, and that can provide sufficient accuracy for this flowmeter, typically cannot also provide sufficiently high bandwidth for fuel metering. For example, a two stage FSV 78 might have a bandwidth of around 4 Hz. A single stage FSV 78 bandwidth can be significantly higher, for example greater than 10 Hz. A conventional engine flow meter bandwidth is around 0.5 Hz.

[0124] Further, as shown in FIG. 10, a position sensor 179 can be added to the splitter/drain valve 100 so that it can also be used as a flow sensing device in an analogous manner to the FSV 78 in FIGS. 7 and 8. The piston 101 senses upstream pressure and upper manifold pressure (i.e. downstream valve pressure, the same as lower manifold), so piston 101 position can be used to infer a measure of burner flow, operating as a fast acting flow sensing device downstream of the FMV 73 that can be used for control in an analogous manner to the FSV 78 of FIG. 7. This also further simplifies size reduction of the FMU 70 as the FSV 78 of the other arrangements can be omitted because the functionality of the splitter/drain valve 100 is extended to provide a flow sensing function instead. However, the arrangements of FIG. 7 or FIG. 8 may be preferable if the splitter & drain unit 100 is located in a hot region, e.g. close to the burner ring, which may not be suitable conditions for the position sensor 79 and associated wiring.

[0125] In summary, the various arrangements discussed above facilitate rapid detection and accommodation of a higher than demanded fuel flow, thereby minimising the level of any engine overthrust. By measuring the metered flow, detection of a fault and UHT threat is fast. Unlike other systems, such detection does not rely on the slow outer engine control loop to detect a change in an engine parameter. Subsequent EEC control of flow can also be via a fast inner loop using measured flow feedback rather than relying on an engine parameter fed back via the slow outer loop. These arrangements are reliable and accurate because the metered flow measurement is direct (i.e. based on a measured flow) and is not inferred from other parameter measurements.

[0126] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.