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The control surfaces of many modern aircraft are subjected to very heavy air loads due to the airspeed and / or the size of the control surface. These loads are often greater than the pilot can comfortably overcome and he / she must therefore be assisted. It is usual to power the control surfaces hydraulically, using a basically simple hydraulic system, the control valve of which is moved by the pilot via the control system. To ensure that the surface moves only when, and as far as the pilot wants to, a feedback from the jack to the control valve must be incorporated. (Negative feedback). The basic jack and control valve are incorporated in one unit called a Powered Flying Control Unit (PFCU or PCU), and may contain the autopilot servo. The usual type of PFCU feeds off an aircraft hydraulic system, but there are other types which are self contained hydraulic systems requiring only electrical power to drive their pump motors.