Autopilot System

Aircraft · 5 min read · May 23, 2022
autopilot system

Many of us have heard the words autopilot system and immediately have thought about an aircraft flying on its own, even without a crew or just a single pilot. However, while autonomous technologies are under development, autopilot systems do not exactly work this way.

In general, autopilot systems require the pilot to input the information regarding altitude, speed, and other relevant flight parameters according to the flight plan for the specific flight stage the autopilot systems will be used. Moreover, while full attention is not really necessary, the pilot must be aware in case autopilot operation is interrupted by any cause, and flight controls need to be taken again.

If you want to learn more about how aircraft autopilot works, and the different types of autopilot systems available, we invite you to keep reading as we share all the details.

How does autopilot work?

Modern aircraft operate with autopilot systems that integrate autopilot software that processes the information the pilot inputs such as the desired aircraft attitude, speed, and direction according to the flight plan. This is usually achieved by integration with the flight management system (FMS), and what is called the autothrottle system which is the one that allows controlling of the speed.

Based on the information provided by the pilot, the autopilot system automatically corrects any deviation from the parameters set. To do so, the autopilot system moves the control surfaces accordingly. In other words, when the aircraft fails to maintain the parameters selected by the pilot, the autopilot system moves the control surfaces to correct the error. This is normally known as error correction operation, and it is the most basic principle for autopilot operation.

What are the flight control surfaces?

We have been saying that autopilot systems work by moving the flight control surfaces, so it might be interesting to remember what these surfaces are. The main control surfaces on aircraft include:

  • Aileron
  • Elevators
  • Rudder

However, the most complex aircraft like big airliners will use secondary control surfaces such as spoilers, flaps, slats, and air brakes.

An aircraft performing a maneuver with the help of an autopilot system in an evening sky.

What is the primary purpose of an autopilot system?

Autopilot systems are designed to minimize the time and effort required to fly during extended flights. Many autopilot systems are integrated with two operation modes, manual and automatic.

When operating in the manual mode, the pilot selects the type of maneuver needed for each stage of the flight while inputting the needed information to complete it in real-time. This way, the pilot does not have to manually perform any maneuver. Instead, the autopilot controls the aircraft by moving the control surfaces to complete it based on the pilot’s input.

On the other hand, when operating in the automatic mode, the autopilot controls the aircraft by moving the control surfaces to maintain the flight attitude desired as well as the direction or heading that was previously selected by the pilot.

What are the types of autopilot systems?

The types of autopilot systems normally depend on the complexity and number of axis they control. Therefore, there are single-axis, two-axis, and three-axis autopilot systems. Let’s see each of them in more detail.

Single-axis autopilot

As the name implies, this autopilot performs corrections on one axis by controlling the ailerons. Here, a sensing element called a turn coordinator is used to sense roll and make the corresponding correction. They are normally found in light aircraft and are called wing leveler systems.

Two-axis autopilot

This type controls both the ailerons and the elevators. This way, the autopilot is capable of stabilizing both the level of the wings and the pitch of the nose.

Three-axis autopilot

Most modern autopilots work on three axes. Apart from controlling the ailerons and the elevators, this type also controls the aircraft rudder. To control the rudder movement, this type of autopilot usually integrates a yaw damper, thus being able to control not only the pitch but also the yaw of the nose. With the yaw damper, oscillatory corrections can be achieved, making it possible to counter Dutch roll and air turbulence.

Is an autopilot a flight control system?

Yes, an autopilot can be considered a flight control system. In general, there are many different systems to help pilots control aircraft, but perhaps the most powerful one is the automatic one. Let’s see what this system involves.

Automatic flight control system

Also known by the acronym AFCS, it is a type of autopilot systems that do more than control surfaces to correct the aircraft’s attitude. These automatic pilot controls can intervene in more demanding stages of the flight such as take-off and landing.

An AFCS will use a three-axis autopilot integrated with other systems such as the Instrument Landing System (ILS), which is the integration that helps pilots during the approach and landing stages of the flight. This way, auto landings are possible, especially when conditions such as thick fog make it difficult for the pilots to land the aircraft themselves.

What’s more, an AFCS can be integrated with Flight Management Computers (FMC) and Flight Management Systems (FMS) that provide pilots with high-performance capabilities such as programming an entire flight profile before the flight, allowing them to only supervise the program is executed accordingly. This is achieved by integrating the autopilot software with navigation systems that make the autopilot capable of controlling the aircraft throughout the different flight stages.

Flight director

A flight director could be considered an alternative type of autopilot. We said that modern autopilots do their work based on the input provided by the pilot and their sensing element. The autopilot takes the information and makes the corresponding corrections.

However, many aircraft, even some light aircraft, use a flight director. This system provides the information required to complete the desired flight. In other words, it helps pilots to make decisions to achieve the desired flight profile by changing the direction and attitude of the aircraft while moving the corresponding surfaces based on the commands displayed by the flight director.

A control panel with illuminated buttons and switches onboard an aircraft.

What are the four basic components of an autopilot system?

The four basic components of an autopilot system include:

  • A sensing unit
  • An autopilot computer
  • An autopilot servo
  • A command unit

Sensing unit

The sensing element is a device that generates an electrical signal in response to an aircraft movement which is taken as the input for the system. It usually has a transducer connected to a gyroscope, which can be a potentiometer or a variable resistor.

Autopilot computer

The autopilot computer is a controller that receives and processes the signals. By using an amplifier, the autopilot computer can strengthen the signal for processing. It takes the input from the gyroscope and other systems like navigation, combines them, and then sends the information to the autopilot servo as input for the system that controls the flight.

Autopilot servo

Autopilot servos are output elements that cause the actuation of the surfaces mentioned above. Autopilot servos are designed on the basis of the method of actuation of the flight controls. There are two types of autopilot servos used. One is a cable-actuated system that typically uses an electric motor. The other type is a hydraulic actuated system that uses electro-hydraulic autopilot servos.

High-performance and large transport aircraft use hydraulics to move their surfaces. These are control valves that direct fluid pressure as needed to move the surfaces via the control surface actuators. When the autopilot is not engaged, the servos allow hydraulic fluid to flow unrestricted in the system for normal operation. The servo valves are usually fitted with feedback transducers to update the autopilot computer with the progress of the error correction.

Command unit

The command unit is the human interface of the autopilot. It allows the pilot to tell the autopilot what to do.

Final words

Clearly, the capabilities of these systems allow for both short and long flights to be less demanding so they can be completed safely.

The future will probably take them to the next level and provide them with capabilities that will make small aircraft and big airliners fully autonomous. We just have to wait and see what the future brings.

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Jet pilot @NASA

1 comment

  1. José Carlos says:

    Hi Admin, good afternoon! Thanks a lot for your fabulous articule! I loved it! I just work as an Aircraft Maint. Engineer and nowaday I study so much control theories. I’m from Brazil, earth of Vladimir Gontijo, one of Brazilian Engineer that projected the one of Curiosity sensors. I wait for your messages. Jacareí -Sao Paulo-Brasil

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