System and method for operating a multi-engine aircraft in an auxiliary power unit mode

A method for controlling an aircraft system, the method comprising, at a processing unit:

• receiving, from an aircraft avionics system, a status of a present position of an actuator, the actuator operatively associated with at least a first engine of the aircraft and moveable between a first position in which at least the first engine drives an aircraft accessory and drivingly engages a gearbox for driving a propulsor of the aircraft, and a second position in which at least the first engine is disengaged from the gearbox and only drives the aircraft accessory, the present position being the first position;
• receiving, from at least the first engine and the second engine, a status of a present governing state and a present speed of at least the first engine and a second engine;
• determining a speed threshold for engaging/disengaging at least the first engine from the gearbox, wherein a value of the speed threshold is set such that substantially zero torque is applied on the gearbox by at least the first engine and a predetermined speed difference is achieved between the present speed of the first engine and the present speed of the second engine when at least the first engine operates at the speed threshold;
• receiving, from the aircraft avionics system, a request for movement of the actuator from the first position to the second position;
• in response to the request for movement of the actuator, determining whether the aircraft is on the ground;
• responsive to determining that the aircraft is on the ground,
• comparing the present speed of at least the first engine to the speed threshold;
• when the present speed of at least the first engine differs from the speed threshold, outputting a first control signal to at least the first engine for causing the present speed of at least the first engine to be adjusted towards the speed threshold and for setting a lowest available torque output for at least the first engine prior to disengaging at least the first engine from the gearbox;
• and
• when the present speed of at least the first engine corresponds to the speed threshold, outputting via the aircraft avionics system a second control signal to an actuator controller in electrical communication with the actuator to indicate that movement of the actuator is permitted and to cause the actuator controller to move the actuator from the first position to the second position, causing at least the first engine to be disengaged from the gearbox and to only drive the aircraft accessory;
• and
• responsive to determining that the aircraft is airborne, outputting via the aircraft avionics system a third control signal to the actuator controller to indicate that movement of the actuator between the first position and the second position is not permitted.

A system for controlling an aircraft, the system comprising:

• means for receiving, from an aircraft avionics system, a status of a present position of an actuator, the actuator operatively associated with at least a first engine of the aircraft and moveable between a first position in which at least the first engine drives an aircraft accessory and drivingly engages a gearbox for driving a propulsor of the aircraft, and a second position in which at least the first engine is disengaged from the gearbox and only drives the aircraft accessory, the present position being the first position;
• means for receiving, from at least the first engine and the second engine, a status of a present governing state and a present speed of at least the first engine and a second engine;
• means for determining a speed threshold for engaging/disengaging at least the first engine from the gearbox, wherein a value of the speed threshold is set such that substantially zero torque is applied on the gearbox by at least the first engine and a predetermined speed difference is achieved between the present speed of the first engine and the present speed of the second engine when at least the first engine operates at the speed threshold;
• means for receiving, from the aircraft avionics system, a request for movement of the actuator from the first position to the second position;
• in response to the request for movement of the actuator, means for determining whether the aircraft is on the ground;
• responsive to determining that the aircraft is on the ground,
• means for comparing the present speed of at least the first engine to the speed threshold;
• means for outputting, when the present speed of at least the first engine differs from the speed threshold, a first control signal to at least the first engine for causing the present speed of at least the first engine to be adjusted towards the speed threshold and for setting a lowest available torque output for at least the first engine prior to disengaging at least the first engine from the gearbox;
• and
• means for outputting via the aircraft avionics system, when the present speed of at least the first engine corresponds to the predetermined speed threshold, a second control signal to an actuator controller in electrical communication with the actuator to indicate that movement of the actuator is permitted and to cause the actuator controller to move the actuator from the first position to the second position, causing at least the first engine to be disengaged from the gearbox and to only drive the aircraft accessory;
• and
• responsive to determining that the aircraft is airborne, outputting via the aircraft avionics system a third control signal to the actuator controller to indicate that movement of the actuator between the first position and the second position is not permitted.