Autopilot in an Airplane: Everything You Need to Know

autopilot in an airplane

Ever wondered how the autopilot in an airplane keeps them on the course, which we often know little about?

Like you, I have often imagined how these navigation systems safely take airborne crafts through the plotted course.

After digging deep into this engineering feat, let me share my discoveries with you. The problem with early-day aircraft is that they require constant attention from the pilots.

You can imagine the fatigue with the overwhelming number of controls needed to keep them on the course.

Thankfully, we have an autopilot in an airplane today that takes most of the responsibilities of the pilots’ shoulders.

As simple as that sounds, the entire system is an intricate network of electronic and mechanical controls. Let us delve deeper and explore how autopilots function.

What Autopilot Systems Entail?

Autopilot has more prominence in passenger aircraft than others, and what it does will intrigue you. This system controls the aircraft’s flight path, eliminating the need for constant manual inputs.

Hence, pilots can focus on other things and remain energetic throughout the flight. The autopilot in an airplane does not replace human pilots.

Instead, they assist in keeping the plane on course. Pilots can then focus on weather, onboard systems, and trajectory with keen attention.

This system uses software and hardware to keep control, and it only works in certain situations based on what the pilot tells it to do.

The hardware components include hydraulic, electronic, and mechanical systems. We will see more of these later.

Engineers build this unique system to automatically follow a flight’s plan, stabilize speed, and maintain altitude and heading. Pilots work hand-in-hand with the autopilot in an airplane to complete the flight trajectory.

The only scenarios where pilots assume complete control are on departure and landing. However, not all passenger aircraft have autopilots. 

Before we proceed, let us look briefly at the history of this engineering marvel.

Brief History of autopilot in an airplane

With longer flight hours came the need for something to lessen the burden on the pilots. You would agree that a fully conscious pilot is a crucial safety feature of airplanes.

Sperry Corporation developed the first autopilot system in 1912. Although no longer operational, the company connected an attitude indicator and a gyroscopic heading to elevators and rudders. The latter were hydraulically operated.

The role of providing roll stability fell on the wind dihedral instead of the ailerons. This configuration allowed the aircraft to maintain a straight path and level on a compass course.

As a result, the pilot no longer had to pay constant attention, resulting in a reduced workload.

The Modern Autopilot System

Not all aircraft today have an autopilot system. Some airlines still use small aircraft that are hand-flown from take-off to landing. Such planes usually have fewer than twenty seats and service short-duration flights.

International aviation regulatory bodies mandate manufacturers to install an autopilot in an airplane with twenty or more seats.

Even at that, there are three levels of control for these aircraft, namely:

  • Wing levelers or single-axis autopilot
  • Two-axis autopilot
  • Three-axis autopilot

Single-axis autopilot systems control only the roll axis. They have a single capability and fall one function short of the two-axis variants.

The two-axis autopilots add pitch control to the roll axis. They may perform limited pitch oscillation correction in addition to wing leveling.

A more complex design will send inputs from the onboard navigation systems to them for true automatic flight guidance.

Two-axis autopilots can maintain flight control immediately after take-off until the aircraft is about to land.

The three-axis autopilot’s final design adds yaw-axis control to what the two-axis systems do. As a result, they are often absent on small aircraft.

What Modern Aircrafts Use

A flight has several sections: taxiing on the runway, takeoff, ascent, cruise, descent, approaching the runway, and landing. Excluding takeoff and landing, the autopilot in an airplane is active for the rest.

However, we have autoland, an autopilot-controlled approach to landing that keeps the plane at the runway’s center. The component responsible for that is the Instrument Landing System (ILS) Cat IIIc.

This component is crucial when visibility is poor and is particularly common in airports subject to adverse weather conditions. It can bring the aircraft to a complete stop, but pilots must disengage it to take the plane off the runway.

An aircraft’s autopilot is part of the overall Flight Management System. Now that we have that out of the way let us look at the components of this automatic flight control system.

Components of an Autopilot System

Automatic control systems read the plane’s position and attitude from the inertial guidance system. It can initiate error corrections to keep the flight on course.

Achieving control of an aircraft in flight is no easy feat. It requires integrating several systems or components, both software, and hardware.

With that in mind, here are the crucial components of the autopilot in an airplane:

  • Sensors
  • A/P Computer
  • Servos
  • Command Interface

1. Sensors

These devices provide input to the control system. They transform the aircraft’s movement into electrical signals for easy electronic processing.

Examples of sensors include gyroscopes, accelerometers, and magnetometers. Together, they give the control unit information regarding speed, altitude, etc., to detect deviations from the preset values.

2. Computer Controller

This component interprets the signals from the sensors. Sometimes, there may be signal amplifiers to aid faster recognition and processing.

Once processed, it sends signals or commands to the servos for correction. That is after comparing the signals received with the preset values the pilot established when plotting the flight’s course.

3. Servos

You could call them actuators because they control physical components to restore or keep a flight in its preset pattern. They could be cable actuated or hydraulic-actuated. Often, you will have a combination of both.

The autopilot in an airplane with a large capacity and higher performance often uses hydraulics. That is because they are better at heavy lifting.

Servos have control valves that regulate fluid pressure on the control surfaces. When not engaged, they allow unrestricted flow for regular operation from the pilot.

Servos often have feedback transducers for the control computer to keep tabs on the error correction. You can also have thrust control servos that optimize the aircraft’s airspeed.

Just to be clear on something, let me mention some control surfaces. They include the ailerons, elevators, rudder, etc., and direct the aircraft’s yaw, pitch, and roll. Those are horizontal, vertical, and longitudinal movements.

Furthermore, you could have spoilers, slats, air brakes, and flaps that modify the overall aerodynamics. They either increase or reduce the lift and drag from the airplane’s wings.

4. Command Interface

Of course, the autopilot in an airplane does not just go on by itself. Although it subtly controls the flight’s dynamics, the pilot must fully engage them manually. That is done through the command interface.

This component allows the pilot to give inputs to the automatic control system. Hence, it is crucial as it also provides feedback to the pilot.

Redundancy and Safety in Autopilot Design

One thing aircraft have in common is redundancy, especially with their engines. It is a crucial safety feature to continue flight safely if one system fails mid-flight.

Airplane manufacturers and designers often consider reliability and redundancy in the overall build. While we do not wish any essential components to fail mid-flight, the redundant ones will kick in to continue safe operations.

Regarding the autopilot in an airplane, critical software processes are often run on several computers that may sometimes use different architectures.

Usually, different engineering teams program these computers in separate programming languages to avoid making the same mistake twice.

Nevertheless, complexity and cost have kept these design iterations to a few computers. Redundancy is not only limited to software; essential hardware components also have redundant replicas.

The Control Wheel Steering – A Slight Variation of the Autopilot System

Before I wrap things up, let us look at the control wheel steering (CWS). Although not a complete autopilot in an airplane, it offers an in-between solution between fully automated and manual flight control.

The CWS is available on many airplanes today and is usually among the off and CMD functions. While the CMD (command) mode gives complete control to the autopilot, the CWS holds inputs from the pilot using the yoke or stick.

It could be input on the flight heading or attitude. The CWS will maintain stability in the pitch and roll. Nevertheless, there are limitations while using the CWS mode that the pilot cannot exceed.

Conclusion

The autopilot in an airplane can control the flight except for takeoff and landing. It directs aircraft according to preset flight parameters from the pilots. However, the pilot must disengage it before making any changes to the system.

Most flights use a partial autopilot system, pending when the pilot will engage full automated controls. You could call it an aid or safety feature.

The autopilot system uses an intricate network of sensors, software, servos, and actuators to establish control.

The sensors detect flight motion, altitude, attitude, etc., while the actuators receive commands from the computer for corrective movements.

New developments have enhanced the capabilities of autopilot in an airplane. They can perform classic flight maneuvers effortlessly. However, they remain entirely under the control of the pilot’s commands.

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