Patent Description:
When operating aircraft with multiple engines, there may be certain portions of a mission that do not require both engines to be operating at full power. In cruising conditions, operating a single engine at a relatively high power, instead of multiple engines at lower power, may allow for better fuel efficiency. For example, one or more engine(s) are operated at high power, and one or more remaining engine(s) are operated in what is sometimes referred to as a "standby" mode. However, engine governing can be challenging at certain power/ratings.

<CIT> discloses a system and method for exiting an asymmetric engine operating regime.

<CIT> discloses an air system switching system to allow aero-engines to operate in standby mode.

<CIT> discloses a turboshaft gas turbine engine.

According to an aspect of the present invention, there is provided a method for operating an aircraft having two or more engines in accordance with claim <NUM>.

In an embodiment of the foregoing, the trim values are determined during a production phase of the engine.

In a further embodiment of any of the foregoing, the at least one parameter comprises fuel consumption.

In a further embodiment of any of the foregoing, the at least one parameter comprises at least one of surge margin and flameout margin.

In a further embodiment of any of the foregoing, the given operating mode is a standby mode providing substantially no motive power to an aircraft.

In a further embodiment of any of the foregoing, the gas turbine engine is part of an aircraft having two or more engines operating in an asymmetric operating regime, wherein a first of the two or more engines is in an active mode to provide motive power to the aircraft and a second of the two or more engines is in a standby mode to provide substantially no motive power to the aircraft, and wherein the first engine is governed using a first governing logic and the second engine is governed using a second governing logic.

According to another aspect of the present invention, there is provided a system for operating an aircraft having two or more engines in accordance with claim <NUM>.

There are described herein methods and systems for governing an engine in an aircraft having two or more engines. Under certain conditions, it can be desirable to operate an aircraft in a so-called "asymmetric operating regime" (AOR) which is described in greater detail hereinbelow. When operated in the AOR, multiple engines of the aircraft, which may be a multi-engine helicopter or other rotorcraft, are operated at different output power levels.

<FIG> depicts an exemplary multi-engine aircraft <NUM>, which in this case is a helicopter. The aircraft <NUM> includes at least two gas turbine engines <NUM>, <NUM>. These two engines <NUM>, <NUM> may be interconnected, in the case of the depicted helicopter application, by a common gearbox to form a multi-engine system <NUM>, as shown in <FIG>, which drives a main rotor <NUM>.

Turning now to <FIG>, illustrated is an exemplary multi-engine system <NUM> that may be used as a power plant for an aircraft, including but not limited to a rotorcraft such as the helicopter <NUM>. The multi-engine system <NUM> may include two or more gas turbine engines <NUM>, <NUM>. In the case of a helicopter application, these gas turbine engines <NUM>, <NUM> will be turboshaft engines. Control of the multi-engine system <NUM> is effected by one or more controller(s) <NUM>, which may be Full Authority Digital Engine Control(s) (FADEC(s)), electronic engine controller(s) (EEC(s)), or the like, that are programmed to manage, as described herein below, the operation of the engines <NUM>, <NUM> to reduce an overall fuel burn, particularly during sustained cruise operating regimes, wherein the aircraft is operated at a sustained (steady-state) cruising speed and altitude. The cruise operating regime is typically associated with the operation of prior art engines at equivalent part-power, such that each engine contributes approximately equally to the output power of the system <NUM>. Other phases of a typical helicopter mission include transient phases like take-off, climb, stationary flight (hovering), approach and landing. Cruise may occur at higher altitudes and higher speeds, or at lower altitudes and speeds, such as during a search phase of a search-and-rescue mission.

More particularly, the multi-engine system <NUM> of this embodiment includes first and second turboshaft engines <NUM>, <NUM> each having a respective transmission <NUM> interconnected by a common output gearbox <NUM> to drive a common load <NUM>. In one embodiment, the common load <NUM> may comprise a rotary wing of a rotary-wing aircraft. For example, the common load <NUM> may be a main rotor <NUM> of the aircraft <NUM>. Depending on the type of the common load <NUM> and on the operating speed thereof, each of turboshaft engines <NUM>, <NUM> may be drivingly coupled to the common load <NUM> via the output gearbox <NUM>, which may be of the speed-reduction type.

For example, the gearbox <NUM> may have a plurality of transmission shafts <NUM> to receive mechanical energy from respective output shafts <NUM> of respective turboshaft engines <NUM>, <NUM>. The gearbox <NUM> may be configured to direct at least some of the combined mechanical energy from the plurality of the turboshaft engines <NUM>, <NUM> toward a common output shaft <NUM> for driving the common load <NUM> at a suitable operating (e.g., rotational) speed. It is understood that the multi-engine system <NUM> may also be configured, for example, to drive accessories and/or other elements of an associated aircraft. As will be described, the gearbox <NUM> may be configured to permit the common load <NUM> to be driven by either of the turboshaft engines <NUM>, <NUM> or, by a combination of both engines <NUM>, <NUM> together.

In the present disclosure, while the aircraft conditions (cruise speed and altitude) are substantially stable, the engines <NUM>, <NUM> of the system <NUM> may be operated asymmetrically, with one engine operated in a high-power "active" mode and the other engine operated in a lower-power (which could be no power, in some cases) "standby" mode. Doing so may provide fuel saving opportunities to the aircraft, however there may be other suitable reasons why the engines are desired to be operated asymmetrically. This operation management may therefore be referred to as an "asymmetric mode" or the aforementioned AOR, wherein one of the two engines is operated in a lower-power (which could be no power, in some cases) "standby mode" while the other engine is operated in a high-power "active" mode. Such an asymmetric operation may be engaged for a cruise phase of flight (continuous, steady-state flight which is typically at a given commanded constant aircraft cruising speed and altitude). The multi-engine system <NUM> may be used in an aircraft, such as the helicopter <NUM>, but also has applications in suitable marine and/or industrial applications or other ground operations.

Referring still to <FIG>, according to the present disclosure, the multi-engine system <NUM> is driving in this example the helicopter <NUM> which may be operated in the AOR, in which a first of the turboshaft engines (say, <NUM>) may be operated at high power in an active mode and the second of the turboshaft engines (<NUM> in this example) may be operated in a lower-power (which could be no power, in some cases) standby mode. In one example, the first turboshaft engine <NUM> may be controlled by the controller(s) <NUM> to run at full (or near-full) power conditions in the active mode, to supply substantially all or all of a required power and/or speed demand of the common load <NUM>. The second turboshaft engine <NUM> may be controlled by the controller(s) <NUM> to operate at lower-power or no-output-power conditions to supply substantially none or none of a required power and/or speed demand of the common load <NUM>. Optionally, a clutch may be provided to declutch the low-power engine.

Controller(s) <NUM> may control the engine's governing on power according to an appropriate schedule or control regime, as will be described in more detail below. The controller(s) <NUM> may comprise a first controller for controlling the first engine <NUM> and a second controller for controlling the second engine <NUM>. The first controller and the second controller may be in communication with each other in order to implement the operations described herein. In some embodiments, a single controller <NUM> may be used for controlling the first engine <NUM> and the second engine <NUM>.

In another example, the AOR of the engines may be achieved through the one or more controller's <NUM> differential control of fuel flow to the engines, as described in <CIT>. Low fuel flow may also include zero fuel flow in some examples.

Although various differential control between the engines of the engine system <NUM> are possible, in one particular embodiment the controller(s) <NUM> may correspondingly control fuel flow rate to each engine <NUM>, <NUM> accordingly. In the case of the standby engine, a fuel flow (and/or a fuel flow rate) provided to the standby engine may be controlled to be between <NUM>% and <NUM>% less than the fuel flow (and/or the fuel flow rate) provided to the active engine. In the AOR, the standby engine may be maintained between <NUM>% and <NUM>% less than the fuel flow to the active engine. In some embodiments, the fuel flow rate difference between the active and standby engines may be controlled to be in a range of <NUM>% and <NUM>% of each other, with fuel flow to the standby engine being <NUM>% to <NUM>% less than the active engine. In some embodiments, the fuel flow rate difference may be controlled to be in a range of <NUM>% and <NUM>%, with fuel flow to the standby engine being <NUM>% to <NUM>% less than the active engine.

In another embodiment, the controller <NUM> may operate one engine (say <NUM>) of the multiengine system <NUM> in a standby mode at a power substantially lower than a rated cruise power level of the engine, and in some embodiments at substantially zero output power and in other embodiments less than <NUM>% output power relative to a reference power (provided at a reference fuel flow). Alternatively still, in some embodiments, the controller(s) <NUM> may control the standby engine to operate at a power in a range of <NUM>% to <NUM>% of a rated full-power of the standby engine (i.e. the power output of the second engine to the common gearbox remains between <NUM>% to <NUM>% of a rated full-power of the second engine when the second engine is operating in the standby mode).

In another example, the engine system <NUM> of <FIG> may be operated in an AOR by control of the relative speed of the engines using controller(s) <NUM>, that is, the standby engine is controlled to a target low speed and the active engine is controlled to a target high speed. Such a low speed operation of the standby engine may include, for example, a rotational speed that is less than a typical ground idle speed of the engine (i.e. a "sub-idle" engine speed). Still other control regimes may be available for operating the engines in the AOR, such as control based on a target pressure ratio, or other suitable control parameters.

Although the examples described herein illustrate two engines, AOR is applicable to more than two engines, whereby at least one of the multiple engines is operated in a low-power standby mode while the remaining engines are operated in the active mode to supply all or substantially all of a required power and/or speed demand of a common load.

In use, the first turboshaft engine (say <NUM>) may operate in the active mode while the other turboshaft engine (say <NUM>) may operate in the standby mode, as described above. During operation in the AOR, if the helicopter <NUM> needs a power increase (expected or otherwise), the second turboshaft engine <NUM> may be required to provide more power relative to the low power conditions of the standby mode, and possibly return immediately to a high- or full-power condition. This may occur, for example, in an emergency condition of the multi-engine system <NUM> powering the helicopter <NUM>, wherein the "active" engine loses power the power recovery from the lower power to the high power may take some time. Even in the absence of an emergency, it will be desirable to repower the standby engine to exit the AOR.

With reference to <FIG>, the turboshaft engines <NUM>, <NUM> can be embodied as gas turbine engines. Although the foregoing discussion relates to engine <NUM>, it should be understood that engine <NUM> can be substantively similar to engine <NUM>. In this example, the engine <NUM> is a turboshaft engine generally comprising in serial flow communication a low pressure (LP) compressor section <NUM> and a high pressure (HP) compressor section <NUM> for pressurizing air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, a high pressure turbine section <NUM> for extracting energy from the combustion gases and driving the high pressure compressor section <NUM>, and a lower pressure turbine section <NUM> for further extracting energy from the combustion gases and driving at least the low pressure compressor section <NUM>.

The low pressure compressor section <NUM> may independently rotate from the high pressure compressor section <NUM>. The low pressure compressor section <NUM> may include one or more compression stages and the high pressure compressor section <NUM> may include one or more compression stages. A compressor stage may include a compressor rotor, or a combination of the compressor rotor and a compressor stator assembly. In a multistage compressor configuration, the compressor stator assemblies may direct the air from one compressor rotor to the next.

The engine <NUM> has multiple, i.e. two or more, spools which may perform the compression to pressurize the air received through an air inlet <NUM>, and which extract energy from the combustion gases before they exit via an exhaust outlet <NUM>. In the illustrated embodiment, the engine <NUM> includes a low pressure spool <NUM> and a high pressure spool <NUM> mounted for rotation about an engine axis <NUM>. The low pressure and high pressure spools <NUM>, <NUM> are independently rotatable relative to each other about the axis <NUM>. The term "spool" is herein intended to broadly refer to drivingly connected turbine and compressor rotors.

The low pressure spool <NUM> includes a low pressure shaft <NUM> interconnecting the low pressure turbine section <NUM> with the low pressure compressor section <NUM> to drive rotors of the low pressure compressor section <NUM>. In other words, the low pressure compressor section <NUM> may include at least one low pressure compressor rotor directly drivingly engaged to the low pressure shaft <NUM> and the low pressure turbine section <NUM> may include at least one low pressure turbine rotor directly drivingly engaged to the low pressure shaft <NUM> so as to rotate the low pressure compressor section <NUM> at a speed proportional to the low pressure turbine section <NUM> speed. The high pressure spool <NUM> includes a high pressure shaft <NUM> interconnecting the high pressure turbine section <NUM> with the high pressure compressor section <NUM> to drive rotors of the high pressure compressor section <NUM>. In other words, the high pressure compressor section <NUM> may include at least one high pressure compressor rotor directly drivingly engaged to the high pressure shaft <NUM> and the high pressure turbine section <NUM> may include at least one high pressure turbine rotor directly drivingly engaged to the high pressure shaft <NUM> so as to rotate the high pressure compressor section <NUM> at a same speed as the high pressure turbine section <NUM>. In some embodiments, the high pressure shaft <NUM> may be hollow and the low pressure shaft <NUM> extends therethrough. The two shafts <NUM>, <NUM> are free to rotate independently from one another.

The engine <NUM> may include a transmission <NUM> driven by the low pressure shaft <NUM> and driving a rotatable output shaft <NUM>. The transmission <NUM> may vary a ratio between rotational speeds of the low pressure shaft <NUM> and the output shaft <NUM>.

The engine <NUM> comprises one or more variable geometry mechanism (VGM), such as inlet guide vanes (IGVs) <NUM> moveable for directing air into the compressor section <NUM> (e.g. compressor inlet guide vanes). For example, the IGVs <NUM> may comprise low-pressure compressor inlet guide vanes, mid-pressure compressor inlet guide vanes, and/or high-pressure compressor inlet guide vanes. It should however be understood that the VGM may in some embodiments consist of outlet guide vanes for directing air out of the compressor section <NUM>, variable stator vanes for directing incoming air into rotor blades of the engine <NUM>, variable nozzles, handling bleed valves, and the like.

As described hereinabove, control of the operation of the engine <NUM> can be effected by one or more control system, for example the controller <NUM>. The controller <NUM> can modulate a fuel flow (Wf) provided to the engine <NUM>, the position and/or orientation of the VGMs within the engine <NUM>, a bleed level of the engine <NUM>, and the like. In some embodiments, the controller <NUM> is configured for controlling operation of multiple engines, for instance the engines <NUM> and <NUM>. For example, the controller <NUM> can be provided with one or FADECs or similar devices. Each FADEC can be assigned to control the operation of one or more of the engines <NUM>, <NUM>. Additionally, in some embodiments the controller <NUM> can be configured for controlling operation of other elements of the aircraft <NUM>, for instance the main rotor <NUM>.

In some embodiments, the controller <NUM> is configured for governing the engine <NUM> operating in standby mode using a governing logic that differs from a governing logic used at higher power ratings. For example, some governing logics based on fuel flow may be challenging in standby mode, due to the increased inaccuracy of the engine commanded fuel flow at low fuel flows. In addition, governing logics based on compressor or output shaft speed can result in variations from one engine to the other at low speeds. Therefore, there is described herein a governing logic suitable for governing an engine operating in a standby mode.

With reference to <FIG>, the controller <NUM> comprises an operating mode selector <NUM>, which determines, in response to pilot input <NUM>, an operating mode of the engines <NUM>, <NUM>. For example, the pilot input <NUM> may cause the engines <NUM>, <NUM> to be operated in the AOR or in any other operating regime where different governing logics are suitable for the engines <NUM>, <NUM>. The operating mode selector <NUM> would then cause the two engines <NUM>, <NUM> to be governed using respective logics for each respective operating mode. More specifically, engine <NUM> operating in an active mode is governed by the engine governing logic <NUM> in accordance with a first governing logic, engine <NUM> operating in a standby mode is governed by the engine governing logic <NUM> in accordance with a second governing logic. The first governing logic is selected to be suitable for the active mode. The second governing logic is selected to be suitable for the standby mode.

In some embodiments, the first governing logic and the second governing logic are preselected, or predetermined, and set in a respective one of the engine governing logic <NUM>, <NUM>. The operating mode selector <NUM> may selectively connect the engine governing logic <NUM>, <NUM> to engines <NUM>, <NUM> as a function of the operating mode of each respective engine <NUM>, <NUM>.

In some embodiments, the engine governing logic <NUM>, <NUM> are configured to select a suitable governing logic from two or more available governing logics upon receipt of a signal from the operating mode selector <NUM> indicative of the operating mode of a respective engine <NUM>, <NUM>. Engine governing logic <NUM> is associated with engine <NUM> and will select a suitable governing logic for engine <NUM> as a function of a signal received from the operating mode selector <NUM> indicative of the operating mode of engine <NUM>. Engine governing logic <NUM> is associated with engine <NUM> and will select a suitable governing logic for engine <NUM> as a function of a signal received from the operating mode selector <NUM> indicative of the operating mode of engine <NUM>.

It will be understood that the embodiment of <FIG> is merely one example of a configuration for the controller <NUM>. For example, the operating mode selector <NUM> may also respond to other inputs in order to trigger the different governing logics for the engines <NUM>, <NUM>, such as engine operating parameters, aircraft operating parameters, emergency signals, and the like. In addition, the engine governing logics <NUM>, <NUM> may be provided in separate implementations of the controller <NUM>, whereby each engine <NUM>, <NUM> has its own controller <NUM> capable of governing in accordance with more than one governing logic, as a function of the operating mode of the respective engine <NUM>, <NUM>.

Turning to <FIG>, there is illustrated an example embodiment of the engine governing logic <NUM> for the first engine <NUM> operating in the active mode. As described herein, the active mode causes the engine <NUM> to provide motive power to the aircraft. A control loop <NUM> adjusts fuel flow (Wf) based on an error <NUM> between an actual target parameter <NUM> and a nominal target parameter <NUM>. The actual target parameter <NUM> may be measured, synthesized, or simulated in real-time. The target parameter may be a gas generator speed (Ng), a low-pressure rotor speed (Np), a high pressure rotor speed (Nh), or any other engine parameter on which primary engine governing may be based. The control loop <NUM> also adjusts the position of the VGMs based on a VGM schedule <NUM>. The governing logic <NUM> as applied to the active engine <NUM> may vary from the example illustrated in <FIG>. For example, open-loop governing based on fuel flow or other closed-loop governing may also be used.

<FIG> illustrates an example embodiment of the engine governing logic <NUM> for the second engine <NUM> operating in the standby mode. As described herein, the standby mode causes the engine <NUM> to provide substantially no motive power to the aircraft. A control loop <NUM> adjusts fuel flow (Wf) based on an error <NUM> between an actual engine compressor speed <NUM> and a trimmed target engine compressor speed <NUM>. The actual target engine compressor speed <NUM> may be measured, synthesized, or simulated in real-time. The control loop <NUM> also adjusts the position of the VGMs based on a trimmed VGM schedule <NUM> for corresponding compressor speeds. As used herein, the expressions "trimmed", "trimming, and "trim" refer to an adjustment or bias applied to a nominal or original value. In some embodiments, the trim is applied directly to the nominal target parameter or schedule and only the trimmed target parameter or trimmed schedule is used in the engine governing logic <NUM>. In some embodiments, the nominal target parameter or schedule is provided to the engine governing logic <NUM> and the trim is applied within the governing logic <NUM>. In both embodiments, the control loop <NUM> bases its adjustments to fuel flow and VGM positions on the trimmed values instead of on the nominal values.

The trimmed target parameter is determined to optimize one or more parameter of the engine <NUM> when operating in the standby mode. In some embodiments, the optimized parameter is fuel consumption, such that fuel consumption targets specific to the engine <NUM> operating in standby mode may be used in order to find the trim values for the target compressor speed of the engine <NUM>. The trim values are associated with a specific engine and differ from one engine to another, i. e the trimmed values are associated with an engine serial number. In some embodiments, the trim values are associated with an engine model and the same trim values may be applied for all engines of the given model. In some embodiments, the trim values are associated with engines of a given engine model having one or more common characteristic, such as a deterioration index or an efficiency level. Other criteria may also be applied to associate trim values with one or more engine.

In some embodiments, the optimized parameter is a compressor stall margin, a compressor surge margin, a compressor flameout margin, or any other suitable engine parameter. In some embodiments, the trim values are set to optimize a plurality of parameters, such as fuel flow and pressure ratio, or stall margin and inlet temperature. Optimization of three or more parameters may also be performed.

In some embodiments, a multi-variable approach is used to optimize the one or more parameter. Indeed, trimming only one engine parameter, such as compressor speed, may allow optimizing of a given parameter such as fuel flow, but cause another parameter, such as pressure ratio, to no longer meet requirements. Trimming is therefore performed on a plurality of engine parameters, such as compressor speed and VGMs, concurrently in order to meet required criteria and optimize at least one parameter.

An example is illustrated in <FIG> showing a target parameter (y-axis) vs compressor speed (x-axis). Curve <NUM> illustrates a set of nominal values for compressor speed as a function of the target parameter, which may be, for example, fuel flow. In this example, the nominal compressor speed is S<NUM> for a target parameter of TP<NUM>. Applying a trim <NUM> to the nominal compressor speed <NUM> shifts the compressor speed to curve <NUM>, which corresponds to trimmed compressor speed values. For a same target parameter of TP<NUM>, the trimmed compressor speed is S<NUM>.

<FIG> illustrates a similar example of trimming, applied to a VGM schedule. Curve <NUM> illustrates a nominal VGM schedule as a function of engine compressor speed. The nominal position for the VGM is set to POSi for a compressor speed of S<NUM>. Applying a trim <NUM> to the nominal VGM schedule <NUM> shifts the VGM position to curve <NUM>, which corresponds to a trimmed VGM schedule. For a same compressor speed S<NUM>, the trimmed VGM position is POS<NUM>.

<FIG> illustrates the application of trims on Ng and on VGM by combining the effects of trims <NUM>, <NUM>. Curve <NUM> represents a nominal target compressor speed with a nominal VGM schedule. Applying trim <NUM> shifts the value for the compressor speed over to curve <NUM>, which represents a trimmed compressor speed with a trimmed VGM schedule. Applying a trim only to compressor speed would shift point <NUM> along curve <NUM> to point <NUM>. Applying a trim only to VGM position would shift point <NUM> to point <NUM> on curve <NUM>. Applying trim <NUM> on the compressor speed and VGM scheduling causes target parameter TP<NUM> to be reached while allowing other engine requirements (or criteria) to be met. This multi-variable approach allows the governing logic <NUM> to be tailored to the engine <NUM> as it operates in standby mode such that one or more engine parameter may be optimized using two or more control variables.

In some embodiments, the trims are determined during production of the engine <NUM>, while running the engine in a test cell. Trim values may be uploaded to the controller <NUM>, and/or to the engine governing logic <NUM>. Trim values may also be determined through simulation and/or modeling of the engine, either offline or through the controller <NUM>. Trim values may be proportionally scaled as a function of one or more parameter, such as altitude, outside air temperature, etc., to determine a final trimmed target compressor speed and/or trimmed VGM schedule. Scaling may be performed by the governing logic <NUM> as a function of a specific operating point in the envelope at a given point in time.

In some embodiments, the controller <NUM> is implemented as a system <NUM> for operating an aircraft having two or more engines, in one or more computing device <NUM>, as illustrated in <FIG>. For simplicity only one computing device <NUM> is shown but the system <NUM> may include more computing devices <NUM> operable to exchange data. For example, each engine governing logic <NUM>, <NUM> may be implemented in a separate computing device <NUM>. The computing devices <NUM> may be the same or different types of devices. Note that the system <NUM> can be implemented as part of a full-authority digital engine controls (FADEC) or other similar device, including electronic engine control (EEC), engine control unit (ECU), electronic propeller control, propeller control unit, and the like. Other embodiments may also apply.

The processing unit <NUM> may comprise any suitable devices configured to implement a method such that instructions <NUM>, when executed by the computing device <NUM> or other programmable apparatus, may cause the functions/acts/steps to be executed.

With reference to <FIG>, there is illustrated an example method <NUM> as performed by the system <NUM> for operating an aircraft having two or more engines. At step <NUM>, the two or more engines are operated in the asymmetric operating regime. As such, a first of the engines is in an active mode to provide motive power to the aircraft and a second of the engines is in a standby mode to provide substantially no motive power to the aircraft.

At step <NUM>, the first engine is governed using a first governing logic. The first governing logic may be an open-loop or closed-loop governing logic. The first governing logic may govern on the basis of fuel flow, Ng, Nh, Np, or any other engine parameter.

At step <NUM>, the second engine is governed using a second governing logic. The second governing logic is a closed-loop governing logic based on compressor speed and VGM settings. The nominal target compressor speed and nominal VGM settings are adjusted using trim values dependent on at least one parameter of the second engine in the standby mode. The at least one parameter comprises fuel consumption and may further comprise one or more of stall margin, flameout margin, and any other parameter that differs when the engine is operating in the standby mode vs a regular or active power mode. The VGM settings may be for variable inlet guide vanes, bleed-off valves, or any other mechanism of the engine having a variable position and for which varying the position causes a change in engine operation.

In some embodiments, the trim values are determined during a production phase of the engine and uploaded to the computing device <NUM> for use with the second governing logic. In some embodiments, the trim values are determined by the computing device <NUM>. In some embodiments, the computing device <NUM> and/or the governing logic retrieves the trim values from a storage medium as needed. In some embodiments, the target compressor speed and VGM settings provided to the second governing logic have been adjusted with the trim values.

It will be understood that the present disclosure also teaches a method for operating a gas turbine engine operating in a given operating mode, whereby the operating mode causes the engine to operate in power range having thermodynamic/aerodynamic particularities. The thermodynamic/aerodynamic particularities are addressed by using trimmed values for target compressor speed and VGM settings that are dependent on at least one parameter of the engine when in the given operating mode. The engine governing logic applied while in the operating mode determines an error between the adjusted target compressor speed and an actual compressor speed of the engine, and adjusts fuel flow to the engine based on the error when the engine is in the given operating mode, as illustrated in the example governing logic of <FIG>.

The methods and systems described herein may be implemented in a high level procedural or object oriented programming or scripting language, or a combination thereof, to communicate with or assist in the operation of a computer system, for example the computing device <NUM>. Alternatively, the methods and systems may be implemented in assembly or machine language. The language may be a compiled or interpreted language. Program code for implementing the methods and systems may be stored on a storage media or a device, for example a ROM, a magnetic disk, an optical disc, a flash drive, or any other suitable storage media or device. The program code may be readable by a general or special-purpose programmable computer for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. Embodiments of the methods and systems may also be considered to be implemented by way of a non-transitory computer-readable storage medium having a computer program stored thereon. The computer program may comprise computer-readable instructions which cause a computer, or more specifically the processing unit <NUM> of the computing device <NUM>, to operate in a specific and predefined manner to perform the functions described herein, for example those described in the method <NUM>.

Claim 1:
A method for operating an aircraft (<NUM>) having two or more engines (<NUM>, <NUM>), the method comprising:
operating the two or more engines (<NUM>, <NUM>) of the aircraft (<NUM>) in an asymmetric operating regime, wherein a first of the engines (<NUM>, <NUM>) is in an active mode to provide motive power to the aircraft (<NUM>) and a second of the engines (<NUM>, <NUM>) is in a standby mode to provide substantially no motive power to the aircraft (<NUM>); and
governing the first engine (<NUM>, <NUM>) in the active mode using a first governing logic (<NUM>),
characterised in that the method further comprises:
governing the second engine (<NUM>, <NUM>) in the standby mode using a second governing logic (<NUM>) different from the first governing logic (<NUM>), the second governing logic (<NUM>) based on a trimmed target compressor speed (<NUM>) and trimmed variable geometry mechanism (VGM) settings (<NUM>), the trimmed target compressor speed (<NUM>) including a nominal target compressor speed adjusted using a first engine-specific trim value, the trimmed VGM settings (<NUM>) including nominal VGM settings adjusted using a second engine-specific trim value, and the first engine-specific trim value and the second engine-specific trim value being specific to the second engine (<NUM>, <NUM>) and dependent on at least one parameter of the second engine (<NUM>, <NUM>) in the standby mode, the at least one parameter comprising fuel consumption.