The trimming in Airbus aircraft is pretty conventional (A320, A330 and A340). In A320, the THS (trimmable horizontal stabilizer) is connected to the trim wheel in the cockpit via cables. When the pilot moves the trim wheel the movement is transferred to the THS actuator through cables which is powered by two hydraulically actuated motors.
In normal operations however, the control of the trim wheel is not required in Airbus aircraft. It is only used to set the proper stabilizer trim before take off. In flight, the pitch trimming is achieved automatically. When the pilot puts in a pitch command using the side stick, the Elevator and aileron control computer (ELAC 2) sends a signal to an electrical motor which in turn powers the hydraulic motors, deflecting the stabilizer to trim the aircraft as required. In the cockpit, the trim wheels move, giving feed back to the pilots.
The electrical trim can always be overriden by the pilot using enough force on the trim wheel. This is done through an override mechanism. When the wheel is let go, it reverts back to electrical trim control. If you do so with the autopilot on, it will disengage the autopilot. The THS is mechanical for the reason to give the aircraft some sort of back up if the fly by wire flight control system were to fail for some reason. In A320 and A330, the rudder and the THS are thus fully mechanical flight controls.
The A320 trim wheel and THS control.
In A380 and A350, the trim wheel is replaced with a pitch trim rocker switch. The reliability in fly by wire controls mean, that these aircraft are built with no mechanical controls whatsoever. All the flight controls are computer controlled. In A380 and in A380, there are more electrical back ups so that electrical failures do not hamper the controllability of the aircraft. The trim back up is also thus achieved through backup electrical systems. And this can work even if all the flight control computers fail in flight.
Author – Anas Maaz ( Airline Pilot )
