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Smaller and less complex aircraft such as the Piper Cherokee PA28-161 N21170 pictured on the right use a simple cable, pulley and lever system to move the primary flight control surfaces.

The amount of force required to move the surfaces varies with the speed and angle of attack of the aircraft. Dynamic forces acting on the primary flight control surfaces during all phases of the aircraft operation provide "feel" feedback to the pilot as to what the aircraft is doing. This dynamic change in control force pressures allows the pilot to develop a "feel" of the way aircraft is reacting to the control forces.

As aircraft became larger they have correspondingly larger primary flight control surfaces needing higher and higher pilot control forces to move them. At some point the control forces have become so high that the pilots body has lost the ability to control the primary flight control surfaces. The simple direct cable control of flight surfaces has become cable control of hydraulic and mechanical control actuators, which provide the power to move the primary flight control surfaces. With this type of system there is still the mechanical connection between the pilot and the flight control surface.

Some commercial aircraft flight controls systems have done away with the cables altogether replacing them with a complex fly-by-wire system using servoactuators and rotary geared actuators for control and movement of the primary flight control surfaces.

There are two independent hydraulic systems “A” and “B” providing hydraulic pressure for the 737 systems. Each independent hydraulic system has two sources of hydraulic power, an engine driven hydraulic pump and an electric hydraulic pump. Hydraulic system “A” or “B” can alone power any primary flight control surface of the 737

The ailerons and elevator each have two independently powered power control units for primary flight control surface movement. One power control unit is powered from the “A” hydraulic system and the other power control unit is powered from the “B” hydraulic system. The rudder also has two independently powered control units plus a third standby power control unit.

From Delta Airlines 737 Technical Training Manual
Elevator Feel System:
The elevator feel system supplies feel forces to the control system. The feel forces developed will vary depending upon airspeed and stabilizer trim. The feel system contains two basic units:
1. The feel and centering mechanism.
2. The feel computer.
The feel and centering unit consists of a cam, spring-loaded follower, two hydraulic actuators and related linkage.

From Boeing document "Elevator Feel Actuator" dated Oct 16, 1984
On larger aircraft, when a pilot operates a device to actuate various control surfaces on the aircraft he is actually initiating action on two devices. His control column moment initiates the actuation of powered units that furnish the power needed to move the control surfaces, and it also actuates feel devices that resist movement of the control column to give the pilot, the feel of the movement.

One well-known means for providing the feel device calls for the combination of hydraulic actuators and a spring arrangement. The two units work together.

For normal takeoff and landing, the feel force must be low enough to allow one handed control operation.

The minimum feel force that can be produced is determined by the minimum hydraulic pressure required in the hydraulic actuators, to overcome friction in the actuators, and the force provided by the spring mechanism. The spring force acts as the only source of control feel in the event of hydraulic system failure, and it must be sufficiently large to provide acceptable feel forces under those failure conditions.