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 regime. In cruising conditions, operating a single engine at a relatively high regime, instead of both engines at lower regimes, may allow for better fuel efficiency.

Improvements are needed for managing the various engine operating regimes.

<CIT> discloses a prior art method for operating an aircraft according to the preamble of claim <NUM>.

<CIT> discloses a prior art multi-engine aircraft power plant with heat recuperation.

According to an aspect of the present invention, there is provided a method as set forth in claim <NUM>.

In a preferred embodiment, the plurality of available exit protocols comprise different engine acceleration rates.

In a preferred embodiment according to any of the previous embodiments, the at least one parameter distinguishes between at least two levels of exit protocols, a first one of the two levels associated with a non-emergency exit and a second one of the two levels associated with an emergency exit.

In a preferred embodiment according to any of the previous embodiments, the at least one parameter comprises a first parameter for distinguishing between the at least two levels of exit protocols, and a second parameter for distinguishing between a system-commanded request to exit the asymmetric operating regime and a pilot-commanded request to exit the asymmetric operating regime.

In a preferred embodiment according to any of the previous embodiments, the system-commanded request is received from a Full Authority Digital Engine Control (FADEC).

In a preferred embodiment according to any of the previous embodiments, the plurality of exit protocols comprise a transition to a power rating associated with having all engines operative at a first acceleration rate and a transition to the power rating associated with having all engines operative at a second acceleration rate greater than the first acceleration rate.

In an embodiment according to any of the previous embodiments, the plurality of exit protocols comprise shutting down a first one of the two or more engines and accelerating a second one of the two or more engines to a power rating associated with having one engine inoperative.

In a preferred embodiment according to any of the previous embodiments, the second one of the two or more engines is accelerated at a maximum permissible acceleration rate.

In a preferred embodiment according to any of the previous embodiments, the method further comprises determining that operating conditions associated with the asymmetric operating regime are no longer met, and generating the request to exit the asymmetric operating regime.

According to a further aspect of the present invention, there is provided a system for operating an aircraft as set forth in claim <NUM>.

In a preferred embodiment according to any of the previous embodiments, the plurality of exit protocols comprise shutting down a first one of the two or more engines and accelerating a second one of the two or more engines to a power rating associated with having one engine inoperative.

In a preferred embodiment according to any of the previous embodiments, the program code is further executable for determining that operating conditions associated with the asymmetric operating regime are no longer met, and generating the request to exit the asymmetric operating regime.

<FIG> illustrates a gas turbine engine <NUM>. In this example, the gas turbine <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 a fuel flow, delivered to the combustor <NUM> via fuel nozzles <NUM> from fuel system (not depicted), and ignited for generating a 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> via a high pressure shaft <NUM>, and a low pressure turbine section <NUM> for further extracting energy from the combustion gases and driving the low pressure compressor section <NUM> via a low pressure shaft <NUM>.

The turboshaft 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 optionally be provided to vary a ratio between rotational speeds of the low pressure shaft <NUM> and the output shaft <NUM>. The compressors and turbines are arranged in low and high pressures spools <NUM>, <NUM>, respectively. In use, one or more controllers <NUM>, such as one or more full authority digital controllers (FADEC) providing full authority digital control of the various relevant parts of the engine <NUM>, control operation of the engine <NUM>. The controller <NUM> may also be an engine control unit (ECU) or electronic engine control (EEC), forming part of the FADEC. Each controller <NUM> may be used to control one or more engines <NUM> of an aircraft (H). Additionally, in some embodiments the controller(s) <NUM> may be configured for controlling operation of other elements of the aircraft (H), for instance the main rotor <NUM>.

The low pressure compressor section <NUM> is configured to independently rotate from the high pressure compressor section <NUM> by virtues of their mounting on different engine spools. 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. In the embodiment shown in <FIG>, the low pressure (LP) compressor section <NUM> includes a single compressor stage 12A, which includes a single mixed flow rotor (MFR), for example such as described in <CIT>, entitled "MIXED FLOW AND CENTRIFUGAL COMPRESSOR FOR GAS TURBINE ENGINE".

The LP compressor <NUM> and the HP compressor <NUM> are configured to deliver desired respective pressure ratios in use, as will be described further below. The LP compressor <NUM> may have a bleed valve <NUM> (shown schematically) which may be configured to selectively bleed air from the LP compressor <NUM> according to a desired control regime of the engine <NUM>, for example to assist in control of compressor stability. The design of such valve <NUM> is well known and not described herein in further detail. Any suitable bleed valve arrangement may be used.

As mentioned, the HP compressor section <NUM> is configured to independently rotate from the LP compressor section <NUM> by virtue of their mounting on different engine spools. The HP compressor section <NUM> may include one or more compression stages, such as a single stage, or two or more stages 14A as shown in more detail in <FIG>. It is contemplated that the HP compressor section <NUM> may include any suitable type and/or configuration of stages. The HP compressor is configured to deliver a desired pressure ratio in use, as will be described further below. The HP compressor <NUM> may have a bleed valve <NUM> (shown schematically) which may be configured to selectively bleed air from the HP compressor section <NUM> according to a desired control regime of the engine <NUM>, for example to assist in control of compressor stability. The design of such valve <NUM> is well known and not described herein in further detail. Any suitable bleed valve arrangement may be used.

The engine <NUM> has two or more compression stages <NUM>, <NUM> to pressurize the air received through an air inlet <NUM>, and corresponding turbine stages <NUM>, <NUM> which extract energy from the combustion gases before they exit via an exhaust outlet <NUM>. In the illustrated embodiment, the turboshaft 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, and need not mean the simple shaft arrangements depicted.

The low pressure spool <NUM> may include 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>. 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 same speed as the low pressure turbine section <NUM>. In other embodiments (not depicted), the low pressure compressor section <NUM> may be connected via a suitable transmission (not depicted) to run faster or slower (as desired) than the low pressure turbine section <NUM>.

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 rotor(s) of the high pressure compressor section <NUM>. The high pressure compressor section <NUM> may include at least one high pressure compressor rotor (in this example, two rotors are provided, a MFR compressor 14A and a centrifugal compressor 14B) directly drivingly engaged to the high pressure shaft <NUM>. The high pressure turbine section <NUM> may include at least one high pressure turbine rotor (in this example there is one HP turbine 18A) directly drivingly engaged to the high pressure shaft <NUM> so as to drive 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> and the low pressure shaft <NUM> are concentric, though any suitable shaft and spool arrangement may be employed.

The turboshaft engine <NUM> may include a set of variable guide vanes (VGVs) <NUM> upstream of the LP compressor section <NUM>, and may include a set of variable guide vanes (VGVs) <NUM> upstream of the HP compressor section <NUM>. The first set of variable guide vanes 36A may be provided upstream of the low pressure compressor section <NUM>. A set of variable guide vanes 36B may be provided upstream of the high pressure compressor section <NUM>. The variable guide vanes 36A, 36B may be independently controlled by suitable one or more controllers <NUM>, as described above. The variable guide vanes 36A, 36B may direct inlet air to the corresponding stage of compressor sections <NUM>, <NUM>. The set of variable guide vanes 36A, 36B may be operated to modulate the inlet airflow to the compressors in a manner which allows for improved control of the output power of the turboshaft engines <NUM>, as described in more detail below. The VGVs may be provided with any suitable operating range. In some embodiments, VGV vanes 36B may be configured to be positioned and/or modulated between about +<NUM> degrees and about -<NUM> degrees, with <NUM> degrees being defined as aligned with the inlet airflow, as depicted schematically in <FIG>. In a more specific embodiment, the VGV vanes 36A and/or 36B may rotate in a range from +<NUM> degrees to -<NUM> degrees, or from +<NUM> degrees to -<NUM> degrees, and more particularly still from <NUM> degrees to -<NUM> degrees. The two set of VGV vanes <NUM> may be configured for a similar range of positions, or other suitable position range.

In some embodiments, the set of variable guide vanes 36A upstream of the low pressure compressor section <NUM> may be mechanically decoupled from the set of variable guide vanes 36B upstream of the high pressure compressor section <NUM>, having no mechanical link between variable guide vanes 36A, 36B to permit independent operation of the respective stages. The VGV vanes 36A, 36B may be operatively controlled by the controller(s) <NUM> described above, to be operated independently of each other. Indeed, the engines 10A, 10B are also controlled using controller(s) <NUM> described above, to carry out the methods described in this document. For the purposes of this document, the term "independently" in respects of the VGVs <NUM> means that the position of one set of the VGV vanes (e.g. 36A) may be set without effecting any change to a position of the other set of the VGV vanes (e.g. 36B), and vice versa.

Independent control of the VGVs 36A, 36B may allow the spools <NUM>, <NUM> to be operated to reduce or eliminate or reduce aerodynamic coupling between the spools <NUM>, <NUM>. This may permit the spools <NUM>, <NUM> to be operated at a wider range of speeds than may otherwise be possible. The independent control of the VGV vanes 36A, 36B may allow the spools <NUM>, <NUM> to be operated at constant speed over a wider operating range, such as from a "standby" speed to a "cruise" power speed, or a higher speed. In some embodiments, independent control of the VGVs 36A, 36B may allow the spools <NUM>, <NUM> to run at speeds close to maximum power. In some embodiments, independent control of the VGVs 36A, 36B may also allow one of the spools <NUM>, <NUM> to run at high speed while the other one run at low speed.

In use, the engine <NUM> is operated by the controller(s) <NUM> described above to introduce a fuel flow via nozzles <NUM> to the combustor <NUM>. Combustion gases turn turbine sections <NUM>, <NUM> which in turn drive the compressor sections <NUM>, <NUM>. The controller(s) <NUM> control(s) the angular position of VGVs 36A, 36B in accordance with a desired control regime, as will be described further below. The speed of the engine <NUM> is controlled, at least in part, by the delivery of a desired fuel flow rate to the engine, with a lower fuel flow rate causing the engine <NUM> to operate at a lower output speed than a higher fuel flow rate.

Such control strategies may allow for a faster "power recovery" of the engine <NUM> (when an engine is accelerated from a low output speed to a high output speed), possibly because the spools <NUM>, <NUM> can be affected relatively less by their inherent inertia through the described use of spool <NUM>,<NUM> speed control using VGVs <NUM>, as will be further described below. In some embodiments, using the vanes VGV 36A, 36B as described herein, in combination with the use of MFR-based low pressure compressor section <NUM> and/or MFR-based high pressure compressor section <NUM> may provide relatively more air and/or flow control authority and range through the core of the engine <NUM>, and/or quicker power recovery.

Where MFR compressors <NUM> and/or <NUM> of the engines 10A, 10B are provided as described herein, the control of the VGVs 36A and/or VGV 36B provides for improved stability of engine operation. This may be so even where the VGV is operated at an extreme end of its range, such as in the "closed down" position (e.g. at a position of +<NUM> degrees in one embodiment described herein). This control of the VGVs facilitates the ability of the engine to operate at a very low power setting, such as may be associated with a "standby" mode as described further below herein, wherein the compressor of an engine operating in standby mode is operating in a very low flow and/or low pressure ratio regime.

Turning now to <FIG>, illustrated is an exemplary multi-engine system <NUM> that may be used as a power plant for an aircraft (H), including but not limited to a rotorcraft such as a helicopter. The multi-engine system <NUM> may include two or more gas turbine engines 10A, 10B. In the case of a helicopter application, these gas turbine engines 10A, 10B will be turboshaft engines. Control of the multi-engine system <NUM> is effected by one or more controller(s) <NUM>, which may be 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 10A, 10B 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.

In the present description, while the aircraft conditions (cruise speed and altitude) are substantially stable, the engines 10A, 10B 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 is therefore referred to as an "asymmetric mode" or an "asymmetric operating regime", 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 is 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 a helicopter, but also has applications in suitable marine and/or industrial applications or other ground operations.

Referring still to <FIG>, according to the present description the multi-engine system <NUM> is driving in this example a helicopter (H) which may be operated in this asymmetric regime, in which a first of the turboshaft engines (say, 10A) may be operated at high power in an active mode and the second of the turboshaft engines (10B 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 10A 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 10B 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. The controller(s) <NUM> may comprise a first controller for controlling the first engine 10A and a second controller for controlling the second engine 10B. 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 10A and the second engine 10B.

In another example, an asymmetric operating regime 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 pending application <CIT>, the entire contents of which are incorporated herein by reference. 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 10A, 10B 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 asymmetric operating regime, 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>% to <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 10B) 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 at less than <NUM>% output power relative to a reference power (provided at a reference fuel flow). Alternately 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 asymmetric operating regime 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 asymmetric operating regime, such as control based on a target pressure ratio, or other suitable control parameters.

Although the examples described herein illustrate two engines, asymmetric mode 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 engine (say 10A) may operate in the active mode while the other engine (say 10B) may operate in the standby mode, as described above. During this operation in the asymmetric regime, if the helicopter (H) needs a power increase (expected or otherwise), the second engine 10B 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, wherein the "active" engine loses power and the power recovery from the lower power to the high power may take some time. Even absent an emergency, it will be desirable to repower the standby engine to exit the asymmetric operating regime.

Referring to <FIG>, there is illustrated an aircraft H, comprising two engines 10A, 10B. More than two engines 10A, 10B may be present on a same aircraft H. An AOR system <NUM> is configured for exiting the asymmetric operating regime.

In some embodiments, the AOR system <NUM> forms part or all of the controller <NUM>, which may be a FADEC, ECU, EEC, or the like. In some embodiments, the AOR system <NUM> is a separate computing device that communicates with a FADEC, an ECU, an EEC, and/or any related accessories.

In order to enter the asymmetric operating regime, both engine and aircraft parameters must meet certain operating conditions associated with the asymmetric operating regime. When one or more of these parameters no longer meet the operating conditions, the asymmetric operating regime may be exited. One or more first sensors 204A are operatively coupled to engine 10A, and one or more second sensors 204B are operatively coupled to engine 10B. The sensors 204A, 204B may be any type of sensor used to measure engine parameters, such as but not limited to speed sensors, pressure sensors, temperature sensors, and the like.

In some embodiments, sensor measurements are transmitted to a monitoring device <NUM> for monitoring the engine parameters and determining whether the engine operating conditions are met or no longer met. Note that not all engine parameters necessarily come from the sensors 204A, 204B. In some embodiments, some of the engine parameters monitored by the monitoring device <NUM> are received from one or more other source, such as but not limited to a FADEC, an ECU, an EEC, or any related accessories that control any aspect of engine performance. In some embodiments, measurements obtained from the sensors 204A, 204B are used to calculate or determine other related engine parameters.

Aircraft parameters are also monitored to determine whether certain aircraft operating conditions for the asymmetric operating regime are met or no longer met. In some embodiments, the aircraft parameters are obtained by the monitoring device <NUM> from aircraft avionics <NUM>. The aircraft avionics <NUM> may include any and all systems related to control and management of the aircraft, such as but not limited to communications, navigation, display, monitoring, flight-control systems, collision-avoidance systems, flight recorders, weather systems, and aircraft management systems. In some embodiments, the aircraft avionics <NUM> perform all monitoring of the aircraft parameters and communicate with the AOR system <NUM> and/or the monitoring device <NUM> when the aircraft operating conditions for the asymmetric operating regime are met or no longer met.

In the embodiment of <FIG>, the monitoring device <NUM> is shown to form part of the AOR system <NUM>. Alternatively, the monitoring device <NUM> is separate therefrom and communicates with the AOR system <NUM> when the engine operating parameters are met and the aircraft operating parameters are met. Alternatively or in combination therewith, monitoring of some or all of the parameters is performed externally to the AOR system <NUM> and involves a pilot monitoring some or all of the parameters.

In some embodiments, the AOR system <NUM> monitors engine and/or aircraft conditions required to enter and exit the asymmetric operating regime. Monitoring may be done continuously or by periodical queries. If at any time the conditions are not respected, the asymmetric operating regime is either exited/aborted or disabled (i.e. cannot be entered).

In some embodiments, the AOR system <NUM> receives a request to exit the asymmetric operating regime when the engine parameters no longer meet the engine operating conditions for the asymmetric operating regime, for example from the monitoring device <NUM> or from the cockpit <NUM>. In some embodiments, the AOR system <NUM> receives a request to exit the asymmetric operating regime when the aircraft parameters no longer meet the aircraft operating conditions for the asymmetric operating regime, for example from the aircraft avionics <NUM>, from the monitoring device <NUM>, or from the cockpit <NUM>. For example, if any one of airspeed, altitude, aircraft generator and/or battery status, or avionic health status for optimal asymmetric operation are not respected, a request to exit the asymmetric operating regime would be received.

A request to exit the asymmetric operating regime based on the engine and/or aircraft parameters no longer being met may be considered as a "normal" category of exit request. A normal exit request should be understood as a request where a return to a mode of operation outside of the asymmetric operating regime is not urgent. For this reason, the AOR system <NUM> may select and apply an exit protocol that will ensure that passenger comfort and engine life are optimized. An exit protocol corresponding to a slow return to an "all engines operative" (AEO) rating may be selected. For example, any engine operating at low speed during the asymmetric operating regime may be accelerated at a rate of <NUM>% per second to a desired speed. Other acceleration rates may also be used to provide a comfortable recoupling and ensure that sufficient time is allotted for engine thermal expansion.

In some embodiments, the AOR system <NUM> receives a request to exit the asymmetric operating regime when the active engine fails. For example, the active engine is subject to a loss of power or control. An exit request under these conditions may be considered as an "emergency" category of exit request. An emergency exit request should be understood as a request where a return to a mode of operation outside of the asymmetric operating regime is urgent and should be performed rapidly for safety or security reasons. If the engine experiencing loss of power or control is the active engine, the standby engine needs to be transitioned to a "one engine inoperative" (OEI) rating rapidly in order to avoid rotor droop and subsequent aircraft events. Example exit protocols comprising rapid transitions may be accelerating at <NUM>% per second, <NUM>% per second, or <NUM>% per second. Other rates may also be used.

Requests received by the AOR system <NUM> based on monitored parameters, including engine power or control, are referred to herein as "system-commanded exits" as they are commanded by a system of the aircraft. The system may be the FADEC, the EEC, the ECU, or an aircraft computer.

In some embodiments, the AOR system <NUM> receives a request to exit AOR from the cockpit <NUM>. These requests are referred to as pilot-commanded exit requests. Similarly to the system-commanded exit requests, pilot-commanded exit requests may occur in normal circumstances or in emergency circumstances. For example, a pilot may request an exit from the asymmetric operating regime in normal circumstances when the end of a cruise segment of a mission is approaching. Having both (or all) engines available for aircraft control may be required and thus the asymmetric operating regime is to be exited in favor of an AEO rating. Various mission profiles may require interleaved segments of AEO rating and of the asymmetric operating regime.

A pilot-commanded exit request may be received via a cockpit interface and sent to the AOR system <NUM>. This may be done using any interface in the cockpit, for example discrete inputs from a button press or a long hold for added protection against inadvertent selection. In some embodiments, the AOR system <NUM> may determine, upon receipt of a normal pilot-commanded exit request, whether the standby engine is capable of returning to an AEO rating. For example, the AOR system <NUM> may review faults and/or conditions which may be impacted or worsened by engine acceleration. Upon confirming that the engine can return to AEO, the engine may be transitioned using a standard exit protocol, i.e. using a low acceleration rate.

An emergency pilot-commanded exit request may be received when a need for dual engine power and control is required urgently, for example for a rapid abort or for object avoidance. Similarly to the normal pilot-commanded exit request, the AOR system <NUM> may determine whether it would be safe to return to an AEO rating and if so, transition the engines as quickly as possible. In such circumstances, passenger comfort and engine thermal expansion are sacrificed for overall aircraft occupant safety. The acceleration rate for an emergency pilot-commanded exit request may be the same as the acceleration rate for an emergency system-commanded exit request. Alternatively, different rates may apply if one exit category is viewed as more critical than the other.

An emergency pilot-commanded exit request may be received via the cockpit interface, using a different input as the normal pilot-commanded exit request, or using the same input with different parameters (i.e. a longer hold or pressing multiple times on the button). In some embodiments, emergency pilot-commanded exit requests are system-generated in response to a specific pilot-initiated aircraft maneuver. For example, if a pilot reacts to an emergency situation by having the aircraft swerve left at a sharp angle, this may cause the AOR system <NUM> to generate the pilot-commanded exit request and react accordingly. In some embodiments, emergency pilot-commanded exit requests are system-generated in response to a specific pilot command. For example, if a pilot commands a specific power requirement or a specific rate of change of a power requirement, the AOR system <NUM> may generate the pilot-commanded exit request and react accordingly.

In response to receiving the request to exit the asymmetric operating regime, the AOR system <NUM> determines which exit category the request belongs to. For example, the AOR system <NUM> may distinguish between an emergency request and a normal request. The AOR system <NUM> may also distinguish between a pilot-commanded request and a system-commanded request. One of a plurality of available exit protocols is then selected as a function of the exit category, and applied to the engines.

Referring now to <FIG>, there is illustrated a flowchart of an example method <NUM> for operating a multi-engine aircraft. In some embodiments, the method <NUM> comprises a step <NUM> of operating the engines in the asymmetric operating regime. Alternatively, the method <NUM> begins when the engines are already operating in the asymmetric operating regime and step <NUM> is omitted. At step <NUM>, a request to exit the asymmetric operating regime is received. In some embodiments, the method <NUM> comprises a step <NUM> of monitoring the engine and/or aircraft parameters and determining whether the operating conditions for the engine to remain in the asymmetric operating regime are met. Once the engine and/or aircraft operating conditions are no longer met, a system-commanded request to exit the asymmetric operating regime is generated.

Some example engine operating conditions for entering and/or remaining in the asymmetric operating regime are as follows:.

Other engine operating conditions may also be used, alone or in combination with any of the engine operating conditions listed above.

Some example aircraft operating conditions for entering and/or remaining in the asymmetric operating regime are as follows:.

Other aircraft operating conditions may also be used, alone or in combination with any of the aircraft operating conditions listed above.

At step <NUM>, the exit category of the request is determined. For example, the exit category may be a normal system-commanded exit, a normal pilot-commanded exit, an emergency system-commanded exit, or an emergency pilot-commanded exit. In some embodiments, only two exit categories are used: emergency or normal. Other exit categories are also considered.

At step <NUM>, the corresponding exit protocol for the determined exit category is selected. For example, the exit protocol may be a slow transition to AEO, a rapid transition to AEO, a very rapid transition to AEO, a maximum acceleration to OEI, etc. Various acceleration rates may be associated with each exit protocol.

At step <NUM>, the selected exit protocol is applied in order to transition the engines out of the asymmetric operating regime. This may comprise performing the change in engine speed or commanding another system of the engine or aircraft to change the engine speed.

In some embodiments, the method <NUM> is performed by the FADEC of the aircraft H. In some embodiments, a portion of the method <NUM> is performed by the FADEC. For example, the set of engine parameters are monitored and the system-commanded exit request is output by the FADEC.

With reference to <FIG>, the method <NUM> may be implemented by a computing device <NUM> as an embodiment of the AOR system <NUM>. The processing unit <NUM> may comprise any suitable devices configured to implement the functionality of the AOR system <NUM> such that instructions <NUM>, when executed by the computing device <NUM> or other programmable apparatus, may cause the functions/acts/steps performed by the AOR system <NUM> as described herein to be executed. The processing unit <NUM> may comprise, for example, any type of general-purpose microprocessor or microcontroller, a digital signal processing (DSP) processor, a central processing unit (CPU), an integrated circuit, a field programmable gate array (FPGA), a reconfigurable processor, other suitably programmed or programmable logic circuits, custom-designed analog and/or digital circuits, or any combination thereof.

The memory <NUM> may include a suitable combination of any type of computer memory that is located either internally or externally to device, for example random-access memory (RAM), read-only memory (ROM), compact disc read-only memory (CDROM), electro-optical memory, magneto-optical memory, erasable programmable read-only memory (EPROM), and electrically-erasable programmable read-only memory (EEPROM), Ferroelectric RAM (FRAM) or the like.

The methods and systems for exiting an asymmetric operating regime as 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 for exiting an asymmetric operating regime may be implemented in assembly or machine language. The language may be a compiled or interpreted language.

Embodiments of the methods and systems for exiting an asymmetric operating regime 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 having two or more engines (10A, 10B), the method comprising:
operating the two or more engines (10A, 10B) of the aircraft in an asymmetric operating regime, wherein a first of the engines (10A, 10B) is in an active mode to provide motive power to the aircraft and a second of the engines (10A, 10B) is in a standby mode to provide substantially no motive power to the aircraft,
characterised in that the method further comprises:
receiving a request to exit the asymmetric operating regime, the request having at least one parameter associated therewith, the at least one parameter distinguishing between a system-commanded request to exit the asymmetric operating regime and a pilot-commanded request to exit the asymmetric operating regime;
selecting one of a plurality of available exit protocols as a function of the at least one parameter; and
applying the exit protocol by commanding the engines (10A, 10B) accordingly.