System for servo-controlling an aircraft flight control member

A servo control system for actuating an aircraft flight control member, the system comprising:

• (A) an hydraulic actuator unit for actuating the flight control member, said actuator unit comprising two hydraulic actuators;
• (B) an actuation means for actuating said hydraulic actuator unit, said actuation means comprising first and second actuation assemblies, each assembly comprising
• (1) two hydraulic circuits,
• (2) two mechanically controlled distributor valves, each distributor valve operates a hydraulic circuit, and
• (3) two drive units including motor means for actuating said distributor valves, wherein each drive unit has an active and a passive state, and one unit is in the active state and the other is in the passive state;
• (C) means for detecting a failure of at least one element of the first or second actuation assemblies;
• (D) a selector valve, having two states, for connecting the hydraulic circuits of the first and second actuation assemblies with the hydraulic circuits of each actuator wherein in a first state of the selector valve the first actuation assembly is in an active state and the second actuation assembly is in a reserve state, and in a second state of the selector valve, the first actuation assembly is in a reserve state and the second actuation assembly is in an active state;
• and
• (E) control means for
• (1) controlling the motor means to operate the drive units as a function of a detected position of the flight control member and of piloting orders, wherein the distributor valves of the first and second actuation assembly are actuated simultaneously,
• (2) controlling switching of said selector valve from one state to the other upon detection of failure by the means for detecting a failure, and
• (3) controlling, upon detection of a failure, the active drive unit in one of the actuation assemblies to change from the active to passive and controlling the passive drive unit in the same assembly to change from the passive to active state, wherein the change of drive units occurs simultaneously with the selector valve switching.