How an Aircraft Autopilot Flies the Plane
An autopilot reads sensors and moves the controls to hold a course, altitude or approach path. Here is how it works, its modes, autoland and what pilots must still watch.
In this article
An aircraft autopilot is a system that guides the aeroplane without the pilot moving the controls by hand. It reads sensors that say where the aircraft is and how it is pointing, compares that with what the crew has selected, and moves the flight controls to close the gap. Pilots remain responsible for the flight and can take over at any time.
The first autopilot
According to Wikipedia, the Sperry Corporation built the first gyroscopic autopilot in 1912. It linked gyroscopic heading and attitude indicators to hydraulic controls on the elevators and rudder, so the aircraft held a straight and level compass course. In 1914 Lawrence Sperry demonstrated it at a Paris aviation safety contest, flying with his hands visibly off the controls. In 1947 a US Air Force C-53 flew across the Atlantic, including takeoff and landing, under autopilot control, and in 1949 Bill Lear won the Collier Trophy for his F-5 autopilot and automatic approach system.
The control loop
Every autopilot works as a loop. Wikipedia uses the yaw damper as the simple example: a sensor, such as a gyroscope, detects the aircraft starting to swing sideways. A computer works out how much rudder is needed to oppose the motion, and an actuator moves the rudder. Then the sensor checks again, many times a second.
On large aircraft, position and attitude usually come from an inertial guidance system. Gyroscope errors build up over time, so the system's readings are combined with radio aids such as DME and GPS. Wikipedia says a Kalman filter, a mathematical method for blending sensors that disagree, handles this.
One, two or three axes
- Single-axis: controls roll only, sometimes called a wing leveller.
- Two-axis: controls roll and pitch.
- Three-axis: adds yaw control.
An autothrottle can also adjust engine power to hold a chosen speed.
Modes pilots can choose
Wikipedia describes two broad ways of using the system. In control wheel steering the pilot flies through the yoke or stick and the autopilot holds the resulting heading and attitude. In command mode the autopilot has full control and follows selected headings and altitudes, radio navigation signals or the route in the flight management system (FMS).
SKYbrary, the aviation safety knowledge base, says modern autopilots integrated with the FMS can control the aircraft from just after takeoff to landing. In climb, cruise and descent the aircraft follows the cleared altitude, speed and route. On approach the system tracks the glide path and the localiser, which are the vertical and sideways guidance beams of the runway's instrument landing system (ILS).
Autoland
An autoland flies the ILS approach, the flare and sometimes the rollout automatically. Wikipedia lists the ICAO approach categories:
- CAT I: decision height of 200 ft; the autopilot is not required.
- CAT II: decision height between 200 and 100 ft; a fail-passive autopilot is required.
- CAT IIIa: decision height as low as 50 ft.
- CAT IIIb: adds automatic rollout; a fail-operational autopilot is needed to land with no decision height.
A fail-passive system leaves the aircraft controllable if it fails, so the pilot can take over. A fail-operational system can finish the approach and landing after a failure.
What the autopilot does not remove
SKYbrary stresses that safe use depends on understanding what the system can and cannot do. It lists events in which crews did not notice a mode change or a disengagement, including an autothrottle that dropped out on a Boeing 737 at Bristol in 2024 and went unnoticed during takeoff, and an unnoticed go-around mode selection on a Boeing 767 near Houston in 2019. It also notes that on a Boeing 737 near Kazan in 2013 the crew did not recognise that the autopilot would disconnect automatically in an all-engine go-around.
The takeaway is that automation changes the pilot's job from moving the controls to monitoring and managing the system. This article is general information drawn from the cited sources, not operating guidance.
Sources
Share
Keep reading
Related articles
How Cabin Pressurisation Keeps Airliner Air Breathable
At cruise height the outside air is too thin to breathe. Airliners pump in air and control how fast it leaves. Here is how, and why cabins sit near 6,000 to 8,000 feet.
How ADS-B Lets Aircraft Broadcast Their Own Position
ADS-B aircraft work out their position from satellite navigation and broadcast it about once a second. See what is sent, who hears it and what limits it.
How Ailerons, Elevators and Rudder Steer an Aeroplane
Three movable surfaces control roll, pitch and yaw. Learn what each one does, why turning needs more than ailerons, and how trim tabs ease the pilot's load.