Patent Description:
In some modes of operation, fuel consumption of an aircraft with multiple engines can be reduced by only running one engine, such as during aircraft descent. However, operating an aircraft with a single engine producing thrust can create a yawing moment on the aircraft that may necessitate compensation actions. Further, the engine that is non-operational may need time to support starting prior to a mode transition of the aircraft or in the event that the fuel-burning engine shuts off unexpectedly.

<CIT> relates to a method and an apparatus for asymmetric hybrid aircraft idle. <CIT> relates to a method and apparatus for operating a hybrid gas turbine electric propulsion system during descent. <CIT> relates to a power-assist system in which power is transferred between an auxiliary power unit (APU) and a high spool of a primary engine. <CIT> relates to generating electrical power at high thrust conditions.

According to a first aspect of the invention, a system of a hybrid aircraft is as claimed in claim <NUM>. Further embodiments are as claimed in the dependent claims thereof.

According to another aspect of the invention, a method is as claimed in claim <NUM>. Further embodiments are as claimed in the dependent claims thereof.

A technical effect of the apparatus, systems and methods is achieved by performing hybrid electric single engine descent power extraction control.

<FIG> schematically illustrates a hybrid aircraft <NUM> that includes a pair of hybrid electric propulsion systems 100A, 100B (also referred to as hybrid gas turbine engines 100A, 100B or hybrid propulsion systems 100A, 100B). Each of the hybrid electric propulsion systems 100A, 100B includes a gas turbine engine <NUM> (e.g., a first gas turbine engine 20A and a second gas turbine engine 20B) with a low speed spool <NUM> configured to drive rotation of a fan <NUM>. Each gas turbine engine 20A, 20B also includes a high speed spool <NUM> that operates at higher speeds and pressures than the low speed spool <NUM>. A low spool electric machine 12A can be configured to augment rotational power of the corresponding gas turbine engine 20A, 20B, for instance, by driving rotation of the low speed spool <NUM> and fan <NUM> in a motor mode. The low spool electric machine 12A can be configured to extract power from the low speed spool <NUM> and output electrical power in a generator mode. In some embodiments, one or more of the gas turbine engines 20A, 20B can include a high spool electric machine 12B configured to drive the high speed spool <NUM> in a motor mode. The high spool electric machine 12B can be configured to extract rotational power from the high speed spool <NUM> of the corresponding gas turbine engine 20A, 20B and produce electric power. At least one power source <NUM> of the hybrid aircraft <NUM> can provide at least a portion of electrical power to the electric machines 12A, 12B of the gas turbine engines 20A, 20B and/or other components of the hybrid aircraft <NUM>. The power source <NUM> can be a stored energy source or a generator driven by an engine. For example, the power source <NUM> can include one or more of a battery, a supercapacitor, an ultracapacitor, a fuel cell, a flywheel, and the like. Where the hybrid aircraft <NUM> includes an additional thermal engine (not depicted), such as an auxiliary power unit or a supplemental power unit, the power source <NUM> can be a generator driven by the thermal engine.

Further, electric machines 12A, 12B of one of the hybrid electric propulsion systems 100A, 100B can provide power to the other hybrid electric propulsion systems 100A, 100B and/or power to the power source <NUM>. For example, if the hybrid electric propulsion system 100A is combusting fuel, the hybrid electric propulsion system 100B may operate without burning fuel and can drive the low speed spool <NUM> and fan <NUM> based on either or both of the electric machines 12A, 12B of the hybrid electric propulsion system 100B receiving electric power from either or both of the electric machines 12A, 12B of the hybrid electric propulsion system 100A and/or the power source <NUM>. Further, if the hybrid electric propulsion system 100B is combusting fuel, the low speed spool <NUM> of the hybrid electric propulsion system 100A can be driven based on either or both of the electric machines 12A, 12B of the hybrid electric propulsion system 100A receiving electric power from either or both of the electric machines 12A, 12B of the hybrid electric propulsion system 100B and/or the power source <NUM>.

While the example of <FIG> illustrates a simplified example of the gas turbine engines 20A, 20B, it will be understood that any number of spools, and inclusion or omission of other elements and subsystems are contemplated. Further, although not claimed, rotor systems described herein can be used in a variety of applications and need not be limited to gas turbine engines for aircraft applications. For example, rotor systems can be included in power generation systems, which may be ground-based as a fixed position or mobile system, and other such applications, although those are not claimed.

Further, each of the electric machines 12A, 12B can be separated and implemented as a separate electric motor and a generator rather than switching each of the electric machines 12A, 12B between a motor mode and a generator mode.

<FIG> illustrates a hybrid electric propulsion system <NUM> (also referred to as hybrid gas turbine engine <NUM> or hybrid propulsion system <NUM>) as a further example of the hybrid electric propulsion system 100A, 100B of <FIG>. In the example of <FIG>, the hybrid electric propulsion system <NUM> includes gas turbine engine <NUM> operably coupled to an electrical power system <NUM> as part of a hybrid electric aircraft, such as hybrid aircraft <NUM> of <FIG>. One or more mechanical power transmissions <NUM> (e.g., 150A, 150B) can be operably coupled between the gas turbine engine <NUM> (e.g., first gas turbine engine 20A, second gas turbine engine 20B) and the electrical power system <NUM>. The gas turbine engine <NUM> includes one or more spools, such as low speed spool <NUM> and high speed spool <NUM>, each with at least one compressor section and at least one turbine section operably coupled to a shaft (e.g., low pressure compressor <NUM> and low pressure turbine <NUM> coupled to inner shaft <NUM> and high pressure compressor <NUM> and high pressure turbine <NUM> coupled to outer shaft <NUM>). The electrical power system <NUM> can include a low spool electric machine 12A configured to augment rotational power of the low speed spool <NUM> and a high spool electric machine 12B configured to augment rotational power of the high speed spool <NUM>. The low spool electric machine 12A can control thrust by driving rotation of the fan <NUM>, and the high spool electric machine 12B can act as a motor in driving rotation of the high speed spool <NUM>.

Although two electric machines 12A, 12B are depicted in <FIG>, it will be understood that there may be only a single electric machine (e.g., only high spool electric machine 12B) or additional motors (not depicted). Further, the electric machines 12A, 12B can be electric motors/generators or alternate power sources may be used, such as hydraulic motors, pneumatic motors, and other such types of motors known in the art. In some embodiments, the electrical power system <NUM> can include a low spool generator 213A configured to convert rotational power of the low speed spool <NUM> to electric power and/or a high spool generator 213B configured to convert rotational power of the high speed spool <NUM> to electric power. For example, where the low spool electric machine 12A is implemented as an electric motor, the low spool generator 213A can be a separate component, and/or where the high spool electric machine 12B is implemented as an electric motor, the high spool generator 213B can be a separate component. The combination of low spool electric machine 12A and low spool generator 213A can be collectively referred to as an electric machine or a motorgenerator, and similarly, the combination of high spool electric machine 12B and high spool generator 213B can be collectively referred to as an electric machine or a motorgenerator. Further, although two electric generators 213A, 213B (generally referred to as generators <NUM>) are depicted in <FIG>, it will be understood that there may be only a single electric generator (e.g., only electric generator 213B) or additional electric generators (not depicted). In some embodiments, one or more of the electric machines 12A, 12B can be configured as a motor or a generator depending upon an operational mode or system configuration, and thus one or more of the electric generators 213A, 213B may be omitted.

In the example of <FIG>, the mechanical power transmission 150A includes a gearbox operably coupled between the inner shaft <NUM> and a combination of the low spool electric machine 12A and low spool generator 213A. The mechanical power transmission 150B can include a gearbox operably coupled between the outer shaft <NUM> and a combination of the high spool electric machine 12B and high spool generator 213B. In embodiments where the electric machines 12A, 12B are configurable between a motor and generator operating mode, the mechanical power transmission 150A, 150B can include a clutch or other interfacing element(s).

The electrical power system <NUM> can also include electric machine drive electronics 214A, 214B operable to condition current to/from the electric machines 12A, 12B. The electrical power system <NUM> can also include rectifier electronics 215A, 215B operable to condition current from the electric generators 213A, 213B (e.g., AC-to-DC converters). The electric machine drive electronics 214A, 214B and rectifier electronics 215A, 215B can interface with an energy storage management system <NUM> that further interfaces with an energy storage system <NUM>. The energy storage management system <NUM> can be a bi-directional DC-DC converter that regulates voltages between energy storage system <NUM> and electronics 214A, 214B, 215A, 215B. The energy storage system <NUM> can include one or more energy storage devices, such as a battery, a supercapacitor, an ultracapacitor, and the like. The energy storage management system <NUM> can facilitate various power transfers within the hybrid electric propulsion system <NUM>. The energy storage management system <NUM> may also transfer power to/from one or more electric machines on the engine, or to external loads <NUM> and receive power from one or more external power sources <NUM> (e.g., power source <NUM> of <FIG>, aircraft power, auxiliary power unit power, supplemental power unit, crossengine power, and the like).

A power conditioning unit <NUM> and/or other components can be powered by the energy storage system <NUM>. The power conditioning unit <NUM> can distribute electric power to support actuation and other functions of the gas turbine engine <NUM>. For example, the power conditioning unit <NUM> can power an integrated fuel control unit <NUM> to control fuel flow to the gas turbine engine <NUM>. The power conditioning unit <NUM> can also power a plurality of actuators (not depicted), such as bleed actuators, vane actuators, and the like.

One or more accessories <NUM> can also be driven by or otherwise interface with the gas turbine engine <NUM>. Examples of accessories <NUM> can include oil pumps, fuel pumps, and other such components. As one example, the accessories <NUM> include an oil pump driven through gearing, such as mechanical power transmission 150B, in response to rotation of the high speed spool <NUM> and/or the high spool electric machine 12B. Alternatively, accessories <NUM> can be electrically driven through power provided by the energy storage management system <NUM> or other such sources of electrical power.

Engagement and operation of the low spool electric machine 12A, low spool generator 213A, high spool electric machine 12B, and high spool generator 213B can change depending upon an operating state of the gas turbine engine <NUM> and any commands received. Collectively, any effectors that can change a state of the gas turbine engine <NUM> and/or the electrical power system <NUM> may be referred to as hybrid electric system control effectors <NUM>. Examples of the hybrid electric system control effectors <NUM> can include the electric machines 12A, 12B, electric generators 213A, 213B, integrated fuel control unit <NUM>, and/or other elements (not depicted).

<FIG> is a schematic diagram of control signal paths <NUM> of the hybrid electric propulsion system <NUM> of <FIG> and is described with continued reference to <FIG> and <FIG>. A controller <NUM> can interface with the electric machine drive electronics 214A, 214B, rectifier electronics 215A, 215B, energy storage management system <NUM>, integrated fuel control unit <NUM>, accessories <NUM>, and/or other components (not depicted) of the hybrid electric propulsion system <NUM>. In embodiments, the controller <NUM> can control and monitor for fault conditions of the gas turbine engine <NUM> and/or the electrical power system <NUM>. For example, the controller <NUM> can be integrally formed or otherwise in communication with a full authority digital engine control (FADEC) of the gas turbine engine <NUM>. Alternatively, the controller <NUM> can be an aircraft level control or be distributed between one or more systems of the hybrid aircraft <NUM> of <FIG>. In embodiments, the controller <NUM> can include a processing system <NUM>, a memory system <NUM>, and an input/output interface <NUM>. The controller <NUM> can also include various operational controls, such as a hybrid engine control <NUM> that controls the hybrid electric system control effectors <NUM> further described herein, for instance, based on a thrust command <NUM>. The thrust command <NUM> can be a throttle lever angle or a command derived based on a throttle lever angle control of the hybrid aircraft <NUM> of <FIG>.

The processing system <NUM> can include any type or combination of central processing unit (CPU), including one or more of: a microprocessor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. The memory system <NUM> can store data and instructions that are executed by the processing system <NUM>. In embodiments, the memory system <NUM> may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and algorithms in a non-transitory form. The input/output interface <NUM> is configured to collect sensor data from the one or more system sensors and interface with various components and subsystems, such as components of the electric machine drive electronics 214A, 214B, rectifier electronics 215A, 215B, energy storage management system <NUM>, integrated fuel control unit <NUM>, accessories <NUM>, and/or other components (not depicted) of the hybrid electric propulsion system <NUM>. The controller <NUM> provides a means for controlling the hybrid electric system control effectors <NUM> using a hybrid engine control <NUM> that can be dynamically updated during operation of the hybrid electric propulsion system <NUM>. The means for controlling the hybrid electric system control effectors <NUM> can be otherwise subdivided, distributed, or combined with other control elements.

The controller <NUM> with hybrid engine control <NUM> can apply control laws and access/update models to determine how to control and transfer power between the low speed spool <NUM> and high speed spool <NUM>, as well as power transfers between multiple gas turbine engines <NUM>. For example, sensed and/or derived parameters related to speed, flow rate, pressure ratios, temperature, thrust, and the like can be used to establish operational schedules and transition limits to maintain efficient operation of the gas turbine engine <NUM>, as well as for the hybrid aircraft <NUM> collectively. For instance, an operating mode of the gas turbine engine <NUM>, such as idle, takeoff, climb, cruise, and descent can have different power settings, thrust requirements, flow requirements, and temperature effects. With respect to the hybrid aircraft <NUM> of <FIG>, each of the gas turbine engines 20A, 20B can have different settings and splits between electric and fuel-burn based operations in one or more of the operating modes. The hybrid engine control <NUM> can control electric current provided to the low spool electric machine 12A and high spool electric machine 12B in a motor mode and loading effects of the low spool electric machine 12A and high spool electric machine 12B in a generator mode. The hybrid engine control <NUM> can also determine a power split between delivering fuel to the combustor <NUM> and using the low spool electric machine 12A and/or high spool electric machine 12B to power rotation within the gas turbine engine <NUM>.

In embodiments, the controller <NUM> can blend the power distribution between the hybrid electric system control effectors <NUM> and fuel burn in the combustor <NUM>. From a pilot's perspective, the setting of a throttle lever angle produces thrust command <NUM> without the pilot having to distinguish between whether motor-based thrust or fuel burn based thrust is needed, although the pilot may control whether fuel is on or off. With respect to the hybrid aircraft <NUM>, the hybrid electric propulsion systems 100A, 100B can be independently controlled such that one of the hybrid electric propulsion systems 100A, 100B is operating in a fuel burning mode while the other of the hybrid electric propulsion systems 100A, 100B is operated using the low spool electric machine 12A and/or the high spool electric machine 12B. Such a mixed operating mode may be used, for instance, during descent of the hybrid aircraft <NUM>, where thrust is desired from both gas turbine engines 20A, 20B, but only one of the gas turbine engines 20A, 20B actively burns fuel.

In embodiments, the controller <NUM> can perform thrust balancing between a first gas turbine engine 20A of the first hybrid electric propulsion system 100A and a second gas turbine engine 20B of the second hybrid electric propulsion system 100B prior to activation of single engine descent mode and after activation of single engine descent mode. During descent of the hybrid aircraft <NUM> of <FIG>, both gas turbine engines 20A, 20B may combust fuel to maintain a desired airspeed. As thrust demand is reduced for the hybrid aircraft <NUM>, one of the gas turbine engines 20A, 20B may have a sufficient capacity to produce thrust without the other gas turbine engine 20A, 20B producing thrust. However, operating only one of the gas turbine engines 20A, 20B through combusting fuel can result in an imbalance and could result in performance issues if the fuel combusting engine experiences a fault while the depowered engine takes time to get up to speed for relighting. Embodiments can operate the fuel burning engine with an operating margin to extract power and provide electric power to either or both of the low spool electric machine 12A and the high spool electric machine 12B of the gas turbine engine <NUM> not combusting fuel such that the resulting thrust from being electrically driven substantially balances the thrust produced by the gas turbine engine <NUM> that is fuel driven. Supplemental electrical power can be provided from the energy storage system <NUM> and/or one or more external power sources <NUM> as needed.

The controller <NUM> can select and balance power extraction from the high speed spool <NUM>, low speed spool <NUM>, energy storage system <NUM>, and/or external power sources <NUM> to power either or both of the low spool electric machine 12A and the high spool electric machine 12B of the gas turbine engine <NUM> being electrically driven. To provide electrical power from the gas turbine engine <NUM> being fuel-driven, fuel flow to the gas turbine engine <NUM> may increase to accommodate the loading of the low spool electric machine 12A and/or the high spool electric machine 12B operating in a generator mode. As one example, the high spool electric machine 12B of the gas turbine engine <NUM> being fuel-driven may be initially used as the primary source of electrical power as the gas turbine engine <NUM> is driven above idle during descent. If the speed of the fan <NUM> of the gas turbine engine <NUM> being fuel-driven results in excess thrust, the low spool electric machine 12A of the gas turbine engine <NUM> being fuel-driven can be used to extract power and add loading. The controller <NUM> can be configured as a dynamic multi-variable control that determines power distribution based on thrust and/or speed of the fan <NUM> (N1) to maintain minimum fuel flow and airflow requirements and provide electrical power for the gas turbine engine <NUM> being electrically driven. The electrical power can be distributed between the low spool electric machine 12A and/or the high spool electric machine 12B of the gas turbine engine <NUM> being electrically driven. For example, the high spool electric machine 12B of the gas turbine engine <NUM> being electrically driven may be the primary motor to maintain relight conditions for rapid restarting. The low spool electric machine 12A of the gas turbine engine <NUM> being electrically driven can be directly driven to make more rapid thrust adjustments. Control loops can balance electrical production by the gas turbine engine <NUM> being fuel driven with electrical demand by the gas turbine engine <NUM> being electrically driven and electrical power available from the energy storage system <NUM> and/or external power sources <NUM>.

Referring now to <FIG> with continued reference to <FIG>, <FIG> is a flow chart illustrating a method <NUM> for providing hybrid electric single engine descent power extraction control, in accordance with an embodiment. The method <NUM> may be performed, for example, by the hybrid aircraft <NUM> through the hybrid electric propulsion systems 100A, 100B of <FIG>. For purposes of explanation, the method <NUM> is described primarily with respect to the hybrid electric propulsion system <NUM> of <FIG>; however, it will be understood that the method <NUM> can be performed on other configurations (not depicted).

Method <NUM> pertains to the controller <NUM> executing embedded code for power extraction, transfer, and thrust control using hybrid engine control <NUM> along with other control functions, where the controller <NUM> can be an aircraft-level control or distributed between aircraft and engine system levels of control. At block <NUM>, the controller <NUM> can determine an operating mode of the hybrid aircraft <NUM> including a first gas turbine engine 20A of hybrid electric propulsion system 100A and a second gas turbine engine 20B of hybrid electric propulsion systems 100B, where the first gas turbine engine 20A includes a first low spool electric machine 12A and a first high spool electric machine 12B, and the second gas turbine engine 20B includes a second low spool electric machine 12A and a second high spool electric machine 12B. The controller <NUM> can receive a thrust command <NUM> for each gas turbine engine 20A, 20B, where each gas turbine engine 20A, 20B includes a low speed spool <NUM>, a high speed spool <NUM>, and a combustor <NUM>. The thrust command <NUM> can be different between the first and second gas turbine engines 20A, 20B, or the thrust command <NUM> can be the same for both the first and second gas turbine engines 20A, 20B.

At block <NUM>, the controller <NUM> can control power extraction from either or both of the first low spool electric machine 12A and the first high spool electric machine 12B of the first gas turbine engine 20A while a single engine descent mode is active. The controller <NUM> can be configured to apply a higher weighting on extracting power from the first high spool electric machine 12B than from the first low spool electric machine 12A to balance power extraction with thrust or speed of the first gas turbine engine 20A. Further, the controller <NUM> can be configured to apply a higher weighting on extracting power from the first low spool electric machine 12A to lower a thrust output of the first gas turbine engine 20A.

While single engine descent mode is active, fuel combustion can be commanded as a complete shut off of fuel flow to prevent fuel burn depending upon an operating state of the second gas turbine engine 20B. The first gas turbine engine 20A of hybrid electric propulsion system 100A can combust fuel, and fuel combustion can be inhibited in the second gas turbine engine 20B of hybrid electric propulsion system 100B for at least a portion of the time while the single engine descent mode is active. For example, the controller <NUM> can output a command of no fuel, fuel flow off, and/or otherwise effectively disable or reduce fuel flow as targeted. The operating state can depend on a combination of commands, conditions, and modes, such as an e-taxi mode, a starting mode, a ground idle mode, a takeoff mode, a climb mode, a cruise mode, an in-flight idle mode, a descent mode, a landing mode, and other such modes. The controller <NUM> can determine an allocation of the thrust command <NUM> between commanding fuel flow to the combustor <NUM> and electric current to the low spool electric machine 12A and/or high spool electric machine 12B based on the operating state of the first and second gas turbine engines 20A, 20B and a throttle lever angle, where the throttle lever angle can be received from a pilot control, an auto-pilot control, or other such source on the hybrid aircraft <NUM>. In a motor mode, the low spool electric machine 12A and/or the high spool electric machine 12B can be powered by one or more of a generator, an energy storage system <NUM>, and a power source <NUM> external to the gas turbine engine <NUM>. The single engine descent mode can be active based on determining that the first gas turbine engine 20A has a thrust generation capacity to maintain a targeted airspeed during descent of the hybrid aircraft <NUM> in combination with providing electric power to either or both of the second low spool electric machine 12A and the second high spool electric machine12B of the second gas turbine engine 20B.

The power source <NUM> can include an energy storage system. The controller <NUM> can be operable to provide at least a portion of the power extracted from either or both of the first low spool electric machine 12A and the first high spool electric machine 12B to the energy storage system <NUM>. The controller <NUM> can be operable to power either or both of the second low spool electric machine 12A and the second high spool electric machine 12B at least in part by one or more of the energy storage system <NUM> and/or another power source external to the second gas turbine engine 20B (e.g., external power source <NUM>).

At block <NUM>, the controller <NUM> can provide electric power to either or both of the second low spool electric machine 12A and the second high spool electric machine 12B while the single engine descent mode is active to balance thrust between the first gas turbine engine 20A and the second gas turbine engine 20B.

The controller <NUM> can control a high spool electric machine 12B to accelerate the high speed spool <NUM> and augment rotational power of the high speed spool <NUM>, while the low spool electric machine 12A can control thrust produced by the low speed spool <NUM>. The controller <NUM> can be configured to apply a higher power input weighting to the second high spool electric machine 12B than to the second low spool electric machine 12A to maintain relight readiness of the second gas turbine engine 20B.

Power source selection can depend on the available power and allocation of power between systems of the hybrid aircraft <NUM>. For instance, using electric power from one of the gas turbine engines <NUM> burning fuel can allow that engine to operate at a higher thermal efficiency by using a higher power setting. A greater amount of battery power or other stored energy from the energy storage system <NUM> may be available after a recharge event. Some embodiments can support recharging during operation of the hybrid aircraft <NUM>, such as during cruise.

A designation of the first gas turbine engine 20A and the second gas turbine engine 20B can be changed between flights of the hybrid aircraft <NUM> to alternate which engine is burning fuel when single engine descent mode is active while the other operates on electric power. The designation needed not change for each flight and may be based on various selection criteria, such as deterioration, in order to optimize fleet management.

Embodiments of the invention can provide a number of advantages and benefits. For instance, compared to conventional descent, fuel burn can be reduced. Using the energy storage system <NUM> with recharging during cruise can support the use of stored energy collected nearer to cruise efficiency to power descent. Driving rotation of the fan <NUM> of both gas turbine engines 20A, 20B can reduce a yawing moment and improve aerodynamics of the hybrid aircraft <NUM> during descent as compared to fully shutting down one of the gas turbine engines 20A, 20B. This can also improve engine thermal efficiency of the gas turbine engine <NUM> that is fuel-driven by continuing to burn fuel with higher power operation and improve engine restarting by keeping components of the electrically-driven gas turbine engine <NUM> rotating. Further, one or more accessories <NUM> of the first gas turbine engine 20A and one or more accessories <NUM> of the second gas turbine engine 20B can be powered while the single engine descent mode is active.

Also, it is clear to one of ordinary skill in the art that, the asymmetric hybrid aircraft idle described herein can be combined with aircraft and propulsion system control features, such as fuel flow control, power management, emergency operation, and the like.

Claim 1:
A system for a hybrid aircraft (<NUM>), the system comprising:
a first gas turbine engine (20A) comprising a first low spool electric machine (12A) and a first high spool electric machine (12B);
a second gas turbine engine (20B) comprising a second low spool electric machine (12A) and a second high spool electric machine (12B); and
a controller (<NUM>) operable to:
determine an operating mode of the hybrid aircraft (<NUM>); and
control power extraction from the first low spool electric machine (12A) and/or the first high spool electric machine (12B) while a single engine descent mode is active,
wherein the first gas turbine engine (20A) combusts fuel and fuel combustion is inhibited in the second gas turbine engine (20B) while the single engine descent mode is active,
the controller (<NUM>) is operable to provide electric power to the second low spool electric machine (12A) and/or the second high spool electric machine (12B) while the single engine descent mode is active to balance thrust between the first gas turbine engine (20A) and the second gas turbine engine (20B); characterised in that
the single engine descent mode is active based on determining that the first gas turbine engine (20A) has a thrust generation capacity to maintain a targeted airspeed during descent of the hybrid aircraft (<NUM>) in combination with providing electric power to the second low spool electric machine (12A) and/or the second high spool electric machine (12B).