The autopilot is an important component of an aircraft. It reduces the mental and physical fatigue of flying an aircraft during less eventful flight phases, such as the cruise, and keeps the pilot sharp for more testing flight regimes, such as the approach and the landing. With the autopilot active, the pilot can focus on other important flying tasks, such as navigation, communication, and weather analysis and avoidance.

The autopilot also gives a much smoother ride to the passengers as it reacts faster to disturbances than a human pilot. It corrects the trajectory of an aircraft with fewer oscillations. As such, autopilot is a great tool to have in an aircraft.


Photo: Jeppesen

The first autopilots

Autopilot has a long history since being invented by the Sperry corporation in 1912, just nine years after Wright Brothers’ first flight in 1903. The invention was led by Lawrence Sperry, son of Elmer Sperry. Early autopilots were simple, and used gyros to sense the aircraft’s movement and compare it to the pilot’s inputs.

The gyro gimbals were strapped to the aircraft and allowed to move with it. A spinning gyro maintains its position due to rigidity, and it retains its fixed position even if the gimbals move. This difference between the gimbal and the gyro is used to deduce the behavior of the aircraft. For this, the gyro is provided with electrical power.

If there is a movement in the gimbals (due to aircraft movement), a potential difference is generated in the circuit, generating a signal. This generated signal is amplified by an amplifier that sends the information to the autopilot servo motors, physically moving the control surface(s) to maintain the dictated state.

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For example, a vertical gyro is used to sense the changes in pitch. With the pilot commanding the autopilot to maintain level flight, if an atmospheric disturbance were to pitch the aircraft nose up, the gyro gimbals move up.

This generates a signal, which then commands the pitch servomotor to deflect the elevator down to pitch the aircraft nose down, so that level flight is maintained. A vertical gyro is also used in the roll axis, and a directional gyro is used in the yaw axis. The roll and yaw autopilot control works similarly to the pitch control. The roll control keeps the wings level, and the yaw control keeps the aircraft from yawing.


Image: Oxford ATPL

These autopilot control systems are known as inner loop control systems, and only provide augmented stability. These autopilots were not by any means “on and leave” systems, as the pilots had to constantly nurse the system with control switches and/or knobs to keep the aircraft from drifting.

For instance, if the pilot wants to roll the aircraft, they must manually turn a roll control knob until the aircraft starts to bank. Then, as the aircraft approaches the desired heading, the knob must be centered to maintain level flight.

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Honeywell C-1 autopilot

Types of autopilots

There are three main types of autopilots, with these being:

1) Single-axis autopilots

These autopilots are single-channel, and can only control the roll axis of the aircraft. They are also known as wing leveler systems.

2) Two-axis autopilots

The two-axis autopilots can control the aircraft both in pitch and roll axes. One autopilot channel controls the elevators in pitch, and the second controls the ailerons/roll spoilers in the roll.

3) Three-axis autopilots

Three axes autopilots can control the aircraft in roll, pitch and yaw. These autopilots can control the elevators, the ailerons/roll spoilers, and the rudder. With such an autopilot, an aircraft can be used to perform automatic landings and automatic rollout after touchdown.

Modern autopilots

When compared to early-generation autopilots, modern autopilots are highly advanced. One of these autopilots’ key features is the greater integration level. Modern autopilots are a part of the aircraft’s Auto Flight Control System (AFCS), or simply the Autoflight system.

At the heart of the AFCS is the Flight Management Guidance Computer (FMGC) or the Flight Management Guidance System (FMGS). Some manufacturers also call it the Autopilot Flight Director System (AFDS). The FMGC is fed by multiple systems, such as inertial reference units, GPS, navigational beacons, flight control computers, air data systems, and Full Authority Digital Engine Control (FADEC).

The FMGC has two main components, namely a management component and a guidance component. The management system controls the following aspects:

  • Flight planning.
  • Flight path prediction and performance optimization.
  • Aircraft navigation, navigation radio tuning, and control.
  • Lateral and vertical flight path control when following the FMGC calculated path.

Meanwhile, the guidance part of the FMGC controls:

  • The autopilot.
  • Flight Director.
  • Engine control through the thrust levers (autothrust or autothrottle).

The pilot interacts with the AFCS in two ways. One is through the Flight Control Unit (FCU) or the Mode Control Panel (MCP). With the FCU, the pilot can use buttons and knobs to control the aircraft’s speed, heading, altitude, and vertical speed in climb and descent through the autopilot.

It is, in a way, similar to the controls of an early-generation autopilot. For example, if the pilot wants to fly on a heading of 035 degrees, they simply turn the heading knob until 035 degrees is selected. The autopilot then turns to 035 and levels out at the heading.

The difference nowadays is that autopilot is now smart enough to know when to level out. As such, the pilot does not have to anticipate it and turn the knob back to zero. The FMGC, with the data it collects from various sources, tells the autopilot when to turn and when to level out.

The speed control is also very similar. If the pilot commands a speed by using the control panel, the autothrust or autothrottles react as necessary by decreasing the power or adding power. This information is fed to the FMGC by the FADEC, which tells the autopilot how it needs to maneuver to maintain the pilot-dictated speed.

The control of the autopilot through the FCU/MCP is highly advanced. However, this is nothing compared to what the autopilot can achieve through the flight management system. When the pilot hands over the aircraft to the flight management system, it orders the autopilot steering commands through the guidance computers.

During the pre-flight preparations, the pilot programs the flight management system through the Command Display Unit (CDU) or the Multifunctional Control Display Unit (MCDU). Pilots also call it simply ‘the box.’

The pilot inputs the flight level to be flown, the cost index, the weights, fuel, and, finally, the flight plan into the FMGC through the box during pre-flight. Once the MCDU is set, the flight management system is ready to navigate and control nearly all aspects of the flight.

With the autopilot engaged and the control given to the flight management system, everything is automatic; it knows when to increase the speed, when to reduce the speed, when to level off, when to begin the descent, and more.

The important thing to remember is that the MCDU is a computer, and the GIGO (or Garbage In, Garbage Out) applies here. As such, pilots must always cross-check the data inserted into the MCDU to avoid errors in the FMGC.

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Airbus MCDU

The FMGC also provides the pilot with flight directors, which appear on the Primary Flight Display (PFD). The flight directors, or FDs, are simply command bars that show the pilot how to maneuver the aircraft, and thus can be said to be a very basic form of autopilot.

The FD bars can show commands either from the FCU/MCP or the flight management system. The vertical FD bar commands roll changes, and the horizontal bar commands pitch changes. The pilots can follow these bars when in manual flight for accurate flight path tracking.


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