Switching method applied by a first processing unit (1, 2) of a flight control system of an aircraft comprising at least one engine, said first processing unit (1, 2) being configured for controlling at least one first actuator so as to control the engine of the aircraft, and being configured to be connected to at least one first sensor and to be connected via at least one link (3, 4) to a second processing unit (2, 1), configured for controlling at least one second actuator and being connected to at least one second sensor, said first processing unit (1, 2) being further configured so as to be connected to emergency communication means (13, 14) giving the possibility of ensuring exchanges of data between the first processing unit (1, 2) and the second processing unit (2, 1) in the case of failures of said at least one link (3, 4) connecting them, said emergency communication means comprising a network of sensors or actuators and/or an onboard secure network for avionics, said method comprising steps of: - sending to the second processing unit acquisition data relating to the at least one first sensor and actuator data relating to the at least one first actuator, - receiving from the second processing unit acquisition data relating to the at least one second sensor and actuator data relating to the at least one second actuator, - receiving a health data item relating to the health condition of the second processing unit (2, 1), called the second health data item, - determining a health data item relating to the health of said first processing unit (1, 2), called the first health data item, - switching of said first processing unit (1, 2) from a first state to a second state, depending on said received second health data item and on said determined first health data item, said steps of sending and receiving acquisition data and of receiving a health data item being applied via said at least one link or said emergency communication means, and said states being from among an active state in which the first processing unit (1, 2) ensures control of the engine of the aircraft, a passive state in which the first processing unit (1, 2) does not ensure the control of the engine of the aircraft and a slave state in which the first processing unit (1, 2) gives over to the second processing unit (2, 1) the control of said first actuators for controlling the engine of the aircraft, wherein the switching step comprises: - a step of determining, from the first health data item, a state data item relating to the state of said first processing unit (1, 2) and from a health status data item of the first processing unit relating to the capability of the first processing unit of ensuring control of the engine, and - a step of switching said first processing unit (1, 2) into the slave state: - when the state data item indicates that the first processing unit is in a passive state, and - when the health status data item indicates a status in which: ▪ the first processing unit is capable of communicating with the second processing unit, ▪ the first processing unit (1, 2) is incapable of ensuring the control of the engine, ▪ and the first processing unit (1, 2) is capable of controlling the first actuators. - sending to the second processing unit acquisition data relating to the at least one first sensor and actuator data relating to the at least one first actuator, - receiving from the second processing unit acquisition data relating to the at least one second sensor and actuator data relating to the at least one second actuator, - receiving a health data item relating to the health condition of the second processing unit (2, 1), called the second health data item, - determining a health data item relating to the health of said first processing unit (1, 2), called the first health data item, - switching of said first processing unit (1, 2) from a first state to a second state, depending on said received second health data item and on said determined first health data item, - a step of determining, from the first health data item, a state data item relating to the state of said first processing unit (1, 2) and from a health status data item of the first processing unit relating to the capability of the first processing unit of ensuring control of the engine, and - a step of switching said first processing unit (1, 2) into the slave state: - when the state data item indicates that the first processing unit is in a passive state, and - when the health status data item indicates a status in which: ▪ the first processing unit is capable of communicating with the second processing unit, ▪ the first processing unit (1, 2) is incapable of ensuring the control of the engine, ▪ and the first processing unit (1, 2) is capable of controlling the first actuators. ▪ the first processing unit is capable of communicating with the second processing unit, ▪ the first processing unit (1, 2) is incapable of ensuring the control of the engine, ▪ and the first processing unit (1, 2) is capable of controlling the first actuators. ▪ and the first processing unit (1, 2) is capable of controlling the first actuators.