How autopilot controls the aircraft

The autopilot controls the aircraft using servos.

The autopilot system is capable of keeping aircraft stabilized in all three axes laterally, longitudinally, and vertically. Early autopilot systems did this function during cruise only, but most autopilot now controls all flight from take-off to landing.

The main purpose of the autopilot system is to relieve the flight-crew of controlling the aircraft’s attitude for the entire duration of the flight.

Autopilot has two modes of operation i.e manual and automatic.

In manual mode, the pilot selects each maneuver and makes small inputs into autopilot. Then autopilot moves the flight control surfaces to perform the maneuver.

In automatic mode, the pilot selects the attitude and direction for a particular flight. Then autopilot moves the flight control surfaces to attain and maintain these parameters.

Basics of autopilot

To understand the working of autopilot first you should know what the pilot actually does before autopilot takes the control of aircraft.

You might have heard the term cockpit preparation.

During cockpit preparation, pilots use computers to program an entire flight profile ahead of time allowing them to supervise its execution. These computers are called Flight Management Computers (FMC) and the whole system to manage flights is called Flight Management System (FMS).

The basis of autopilot operation is error correction. When an aircraft fails to meet the inputs given by FMC or pilots, an error signal is generated, and this error has to be corrected to achieve desired outputs. The autopilot system automatically corrects the error and restores the flight attitude desired by the pilot.

Now, we can say autopilot closely works with flight management computers to take programmed inputs. However, pilots can change or override the autopilot command during flight. For that, a separate flight control panel is given.

Autopilot system can be categorised into –

• One-Axis: Control the aileron.
• Two-Axis: Control the ailerons and elevators.
• Three-Axis: Control the ailerons, elevators, and rudder.

Single-axis autopilot usually found in light aircraft. Two-axis and three-axis autopilot systems can be found on aircraft of all sizes.

There are many autopilot systems available in the industry but they feature a wide range of capabilities and complexity. High-performance transport categories of aircraft have autopilots with higher capabilities than light aircraft.

Automatic Flight Control System (AFCS)

Some high performance and large transport category aircraft have very elaborated autopilot systems. These systems are called automatic flight control systems (AFCS).

The nomenclature of the autopilot system might be a little different for different aircraft models, but the system and working principle are almost the same.

AFCS capabilities vary from aircraft to aircraft autopilot systems. Most AFCS are now more advanced and they are integrated with navigational aids, flight directors, and autothrottle systems. AFCS combines these all commands into a single flight control interface for better management and control of aircraft.

If we see the level of integration, auto landings are also possible using autopilot. For that autothrottle system is integrated into the flight director and autopilot systems with glide scope modes.

How does autopilot work

When the autopilot (A/P) is selected ON, and if the aircraft moves from its given path, this is sensed by the sensing unit (usually gyro). A signal is sent to the computer (A/P computer or controller) where it is amplified and sent to the autopilot servo in the flight control system. A/P servos are an electrically powered motor or hydraulically powered jack. This moves the control surfaces to return the aircraft to the datum. Then feedback signals from the servo (output signal) and sensing unit (input signal) are sent back to the A/P computer.

Basic components of autopilot

There are 4 basic components of autopilot –

1. Sensing Unit – Input
2. A/P Computer – Processing
3. A/P servo – Outputs
4. Command unit – Pilot interface

1. Sensing Unit

The sensing element is usually called pick-off. It is a device to generate an electrical signal in response to an aircraft movement. A transducer is connected to a gyro. This transducer can be a potentiometer or variable resistor type. Most modern aircraft use an LVDT or RVDT. In each case, the basic principle of generating a signal is – moving the primary coil relative to the secondary and the induction into the secondary is proportional to the amount of overlap between the two coils.

2. A/P Computer

The A/P computer is a controller that collects and sums the signals. An amplifier is also a part of the autopilot computer which is used to strengthen the signal for processing. It takes the input from gyro and other systems like navigation, and combines them, then sends it to the A/P servo to input the flying control system.

3. A/P Servo

A/P servos are output elements that cause the actuation of the flight control surfaces. Autopilot servos are designed on the basis of the method of actuation of the flight controls. Cable-actuated flight control systems typically utilize an electric motor autopilot servos. Hydraulic actuated flight control systems use electro-hydraulic autopilot servos.

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

4. Command Unit

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

Autopilot Integration

Autopilots are integrated with other systems to make systems (AFCS) more capable. The capabilities of autopilot depend on how many other systems are integrated. Most advanced autopilot systems are typically integrated with navigational aids, flight directors, autothrottles, yaw dampers, and fly-by-wire systems.

Navigational Aids

Autopilot follows the route programmed by the pilot into the FMS.

Flight Director

A flight director is an instrument that shows the aircraft’s attitude required to maintain the desired flight. A command bar of the flight director on the aircraft attitude indicator displays the pilot to attain and maintain pre-selected flight conditions. It helps pilots to make decisions to achieve the desired flight profile by changing the direction and attitude of the aircraft.

The computed command indications by the flight director relieve the pilot from the many calculations required for the desired flight.

Actually, the flight director system is an autopilot system without the A/P servos because all the sensing and computations are made the same as autopilot except, here the pilot controls the aircraft by moving the control surfaces by following commands displayed by the flight director.

Flight director systems can be part of an autopilot system. Aircraft that do not have full autopilot systems may lack flight directors. Many autopilot systems allow for the option of engaging or disengaging a flight director display. This means that autopilot can work with or without flight directors.

Yaw Damper

The yaw damper keeps the airplane stable around the airplane yaw (vertical) axis. During the flight, the yaw damper commands rudder movement in proportion to and opposite to the airplane yaw moment.

Now one thing we need to clear here, aircraft oscillate around their vertical axis while flying a fixed heading. This is natural and common for many aircraft.

This unwanted airplane yaw motion is caused by the dutch roll or air turbulence.

But anyways these oscillations are not good for aircraft stability. So they have to be corrected. Continuous rudder input is needed to counteract this effect.

Then the yaw damper comes into play. By using a yaw damper, oscillatory motion can be corrected.

A yaw damper receives error signals from the turn coordinator rate gyro. Oscillating yaw motion is counteracted by the rudder movement, which is made automatically by the rudder servo in response to the polarity and magnitude of the error signal.

For the 3-axis autopilot system, the yaw damper is part of autopilot, but for the 2-axis, it can be a completely independent unit.

Autothrottle

An autothrottle system will automatically set the thrust levers in the cockpit to the correct power setting and adjust the fuel flow into the engines accordingly. This system is simply an extension of the autopilot to allow the pilot to maintain the aircraft’s speed or engine power settings automatically.

Fly-by-wire

Fly by wire technology manages the flight controls by electrical power instead of traditional cables, rods, and pulleys design.

Some high performance and large transport category aircraft utilize large control surfaces which require high torque (power) to move these control surfaces. And this power can’t be achieved through electrical motors, so hydraulic actuators are used to move these control surfaces.