How Autopilot Works

Autopilot Control Systems

An autopilot is an example of a control system. Control systems apply an action based on a measurement and almost always have an impact on the value they are measuring. A classic example of a control system is the negative feedback loop that controls the thermostat in your home. Such a loop works like this:

It's summertime, and a homeowner sets his thermostat to a desired room temperature -- say 78°F.
The thermostat measures the air temperature and compares it to the preset value.
Over time, the hot air outside the house will elevate the temperature inside the house. When the temperature inside exceeds 78°F, the thermostat sends a signal to the air conditioning unit.
The air conditioning unit clicks on and cools the room.
When the temperature in the room returns to 78°F, another signal is sent to the air conditioner, which shuts off.

It's called a negative feedback loop because the result of a certain action (the air conditioning unit clicking on) inhibits further performance of that action. All negative feedback loops require a receptor, a control center and an effector. In the example above, the receptor is the thermometer that measures air temperature. The control center is the processor inside the thermostat. And the effector is the air conditioning unit.

Automated flight control systems work the same way. Let's consider the example of a pilot who has activated a single-axis autopilot -- the so-called wing leveler we mentioned earlier.

The pilot sets a control mode to maintain the wings in a level position.
However, even in the smoothest air, a wing will eventually dip.
Gyroscopes (or other position sensors) on the wing detect this deflection and send a signal to the autopilot computer.
The autopilot computer processes the input data and determines that the wings are no longer level.
The autopilot computer sends a signal to the servos that control the aircraft's ailerons. The signal is a very specific command telling the servo to make a precise adjustment.
Each servo has a small electric motor fitted with a slip clutch that, through a bridle cable, grips the aileron cable. When the cable moves, the control surfaces move accordingly.
As the ailerons are adjusted based on the input data, the wings move back toward level.
The autopilot computer removes the command when the position sensor on the wing detects that the wings are once again level.
The servos cease to apply pressure on the aileron cables.

This loop, shown above in the block diagram, works continuously, many times a second, much more quickly and smoothly than a human pilot could. Two- and three-axis autopilots obey the same principles, employing multiple processors that control multiple surfaces. Some airplanes even have autothrust computers to control engine thrust. Autopilot and autothrust systems can work together to perform very complex maneuvers.