A device used to control an aircraft in flight automatically.

Many aircraft are equipped with autopilots that will fly an aircraft automatically while the pilot accomplishes other tasks. These systems vary greatly in sophistication, from simple wing levelers to completely integrated flight control systems.

The simplest autopilot is a single-axis system. Most single-axis autopilots are designed to control the motion of the aircraft around the aircraft’s longitudinal axis, passing from the front of the aircraft to the rear. When movement around the longitudinal axis becomes unstable, then the aircraft will roll, or tip, from side to side. In its simplest form, the single-axis autopilot may be referred to by pilots as a wing leveler. Upon activation, a wing leveler will stabilize the aircraft by leveling the wings. By adding features such as turn, heading, and navigational control, pilots can use a single-axis system throughout most of the flight.

Another common type of single-axis system is known as the yaw damper. This autopilot maintains control of the aircraft around the vertical axis, running through the aircraft from top to bottom. When movement around the vertical axis becomes unstable, the aircraft is considered to be slipping or skidding sideways. This motion is known as yaw. Yaw dampers are designed to prevent slipping and skidding.

A form of autopilot commonly used on medium-sized aircraft is the dual-axis system. A dual-axis autopilot will maintain control of the aircraft around both the lateral and the longitudinal axes. The lateral axis of an aircraft is an imaginary line passing from wingtip to wingtip. Movement around the lateral axis causes the front of the airplane to move up or down.

For example, a dual-axis autopilot will be able to keep both the wings and the nose of the aircraft level. Pilots may use the dual-axis system to hold a particular direction, follow commands from a navigation system, maintain an altitude, and climb or descend at a specified rate.

The three-axis autopilot is a combination of a dual-axis system and a yaw damper. Airliners and large business aircraft are normally equipped with a three-axis autopilot. Three-axis systems are connected with navigation and flight-management systems. In addition, they may include features such as throttle control and ground steering.

Many autopilots can connect to, or be integrated with, a navigation system. In a single-axis autopilot, this may merely be a connection to the directional gyro. In a complex three-axis system, all of the navigation devices may be connected to the autopilot. In this case, the autopilot could be considered an integrated flight control system.

Most integrated flight control systems include a special attitude indicator known as a flight director indicator. In addition to the symbolic airplane and horizon reference line found in most attitude indicators, a flight director indicator includes a special set of needles called flight director, or command, bars. The flight director bars will move up, down, right, and left to indicate where the autopilot intends to fly. Often, these bars are operated by a special computer running in parallel with the autopilot computer. In case of an autopilot failure, the flight director computer will still be able to manipulate the flight director bars. Pilots can manually fly a precise flight path by keeping the bars centered. By allowing the flight director computer to make the complex calculations involved in flying a precise flight path, pilots are still able to reduce their workload.

In order to control the aircraft, an autopilot must be able to sense attitude. To do this, autopilots rely on gyroscopic instruments, or accelerometer-based sensors. Often, the attitude gyro is used to transmit information regarding pitch and roll attitude to the autopilot computer. A turn and bank indicator or a turn and slip indicator can be used to supply yaw information. The autopilot computer will compare the actual flight attitude of the aircraft with the desired flight attitude and, if necessary, move the appropriate control surface.

The device that operates the control surfaces of the aircraft is called a servo. A servo converts electrical energy into mechanical energy. Servos may be electric, hydraulic, or pneumatic. Electric and hydraulic servos are quite common. Electric servos are widely used on aircraft with mechanical or fly-by-wire controls, and hydraulic servos are widely used on aircraft with hydraulic controls.

Electric servos contain a small, electric motor. In this type of system, the computer sends a voltage to the servo, causing the motor to rotate. The motor is connected to the aircraft controls, and as the motor turns, the controls are moved.

Hydraulic servos contain a small, electrically controlled, hydraulic actuator. In this type of system, the computer sends a voltage to the actuator. Valves within the actuator channel hydraulic fluid in and out of small cylinders containing pistons. The pistons are connected to the control surface, and, as they move, the surface moves.

Pneumatic servos contain electrically operated valves. These valves channel air into bellows that are connected to the aircraft controls. The inflation and deflation of the bellows causes the controls to move.

• Brown, Carl A. A History of Aviation. 2d ed. Daytona Beach, Florida: Embry-Riddle Aeronautical University, 1980. A well-illustrated book that covers the history of flight from ancient times to the space age.
• Eismin, Thomas K. Aircraft Electricity and Electronics. 5th ed. Westerville, Ohio: Glencoe, 1994. A beginner’s text starting with the fundamentals of electricity and ending with electric instruments and autoflight systems.
• Helfrick, Albert. Principles of Avionics. Leesburg, Va.: Avionics Communications, 2000. A very complete avionics text that includes history.
• Jeppesen Sanderson. Instrument Rating Manual. 7th ed. Englewood, Colo.: Jeppesen Sanderson, 1993. A textbook designed to assist pilots to prepare to add an instrument rating to their pilot license.

Airplanes

Avionics

Flight control systems

Instrumentation

Pilots and copilots

Roll and pitch