Autopilot System | How Does Autopilot Work, Autopilot Overview

Autopilot System

Autopilot System | How Does Autopilot Work, Autopilot Overview & Operation

Autopilot Systems

An aircraft Auto (Automatic) Pilot system controls the aircraft without the pilot directly maneuvering the flight controls. The autopilot maintains the aircraft’s attitude and/or direction and restores the aircraft to that condition when it is displaced from it. Automatic pilot systems are capable of keeping aircraft stabilized laterally, vertically, and longitudinally & in modern aircraft it can auto land also.

An autopilot system's main purpose is to lessen the physical and mental strain of piloting the aircraft during lengthy trips. Most of the autopilots have both manual and automatic modes of operation. In the manual mode, the pilot selects each maneuver and makes small inputs into an autopilot controller. The maneuver is carried out by the autopilot system by moving the aircraft's control surfaces. The pilot chooses the desired attitude and direction during a flying segment when in automatic mode. The control surfaces are then moved by the autopilot in order to achieve and keep these values.

An aircraft can be controlled on one, two, or three axes using autopilot systems. The ailerons are managed by the same people who control the aircraft around just one axis. They are wing leveller systems, single-axis autopilots that are often seen on small and medium-sized aircraft. Ailerons and elevators are controlled by two-axis systems, which are another type of autopilot. The ailerons, elevators, and rudder are all under the control of three-axis autopilots. There are two- and three-axis autopilot systems on all sizes of aircraft.

There are numerous autopilot systems on the market. They have many different capacities and levels of complexity. Autopilots for light aircraft often have fewer features than those on high-performance and transport category aircraft. Even on light aircraft autopilots, integration of navigational functions is typical. As autopilots get more sophisticated, they increasingly regulate not just the flight control surfaces but also other flying characteristics.

Some contemporary high-performance, transport, and small aircraft use extremely complex autopilot systems, sometimes known as automatic flight control systems (AFCS). These three-axis systems are used for much more than just aircraft maneuvering. They are in charge of flying the plane during climbs, descents, cruising, and landing approaches. Some even incorporate an auto-throttle feature that regulates engine thrust automatically, enabling auto-landings.

Flight management systems have been created for further automatic control. An full flight profile can be designed in advance using computers, allowing the pilot to see how it is carried out. Almost every aspect of a flight is coordinated by an FMS computer, including the fuel management plans, autopilot and auto throttle systems, navigation route selection, and more.

Basis for Autopilot Operation

Error correction serves as the foundation for autopilot system operation. An error is stated to have happened when an aircraft does not satisfy the chosen conditions. The aircraft is returned to the desired flight attitude by the pilot after the autopilot system automatically corrects the problem. Modern autopilot systems accomplish this in one of two primary methods. One is based on position, and the other is based on rate. A position-based autopilot adjusts the aircraft's controls to ensure that any departure from the intended attitude is avoided. To accomplish this, one must first memories the desired aircraft attitude before adjusting the control surfaces to bring the aircraft back to that position. Rate-based autopilots adjust control surfaces to counter the error-causing rate of change using data about the aircraft's rate of movement. Rate-based autopilot systems are used by most large aircraft. Either can be used by small aircraft.

Autopilot Components

The majority of autopilot systems are made up of four fundamental parts, as well as other switches and auxiliary devices. Sensing elements, processing elements, output elements, and command elements make up the four fundamental parts. A fifth component, feedback or follow-up, is present in many sophisticated autopilot systems. To inform the autopilot of the progress being made, signals are given by the output elements as corrections are being made.

Sensing Elements

The autopilot sensing components are an altitude control, a turn coordinator, and attitude and directional gyros. These gadgets pick up on the motions of the aircraft. In order to maintain the aircraft flying as intended, they produce electric signals that the autopilot uses to automatically make the necessary corrections. The sensing gyros can be found in the instruments positioned in the cockpit. They can also be mounted remotely. The autopilot computer's input signals and the servo displays in the cockpit panel are both driven by remote gyro sensors.

Various types of sensors may be used by modern digital autopilots. Solid state accelerometers and magnetometers may be used in conjunction with or in addition to MEMS gyros. Gyros may not be used at all in rate-based systems. Different input sensors may be housed in the same unit or in different units that communicate with one another via a digital data bus. Through a digital data bus connection to the avionics computers, navigational information is also included.

Computer and Amplifier

An autopilot's computing element could be analogue or digital. Its job is to decipher the data from the sensing elements, combine navigational input and commands, and transmit signals to the output elements so they can move the flight controls as needed to manage the aircraft. To make the signal stronger for processing, if necessary, and for use by output devices such servo motors, an amplifier is utilised. The computer of an analogue autopilot system consists of the amplifier and related circuitry. Information is processed in channels that match the axis of control that the signals are meant to control (i.e., pitch channel, roll channel, or yaw channel). Digital systems typically simply enhance signals provided to the output elements and use solid state microprocessor technology.

Output Elements

The servos that activate the flight control surfaces are an autopilot system's output components. For each control channel that was integrated into the standard flight control system, they are separate devices. The design of the autopilot servo varies greatly depending on how the flight controls are activated. Electro-pneumatic or electric servo motors are frequently used in cable-actuated systems. Electro-hydraulic autopilot servos are used in hydraulically operated flight control systems. The same actuators are used in digital fly-by-wire aircraft for manual and autopilot maneuvers. The actuators respond to commands from the autopilot rather than only the operator when the autopilot is activated. However, while the autopilot is not in use, the servos must permit unrestricted control surface movement.

There are two primary categories of electric motor-driven servos used by aircraft with cable-actuated control surfaces. One uses reduction gears to link a motor to the servo output shaft. The autopilot computer's commands cause the motor to start, halt, and reverse course. The other kind of electric servo uses two magnetic clutches to gear a continuously running motor to the output shaft. Energizing one clutch sends motor torque to spin the output shaft in one direction, while energizing the other clutch turns the shaft in the other direction. This is how the clutches are set up. In some autopilot systems, wire flying controls can also be driven by electro pneumatic servos. They are activated by a proper air pressure source and are controlled by electrical signals from the autopilot amplifier. A vacuum system pump or the bleed air from a turbine engine could be the source. An output linkage assembly and an electromagnetic valve assembly make up each servo.

Autopilot servos on aircraft having hydraulically actuated flight control systems are electro-hydraulic. They are control valves that, when necessary, send fluid pressure to the actuators on the control surfaces to move the surfaces. Signals from the autopilot computer provide their power. The servos permit unfettered hydraulic fluid flow in the flight control system during normal operation when the autopilot is not activated. Feedback transducers can be incorporated into the servo valves to provide updates on the autopilot's progress during error correction.

Command Elements

The human interface of the autopilot is the command unit, often known as a flight controller. It enables the pilot to direct the autopilot. The intricacy of the autopilot system affects the type of flight controller used. The pilot instructs the controller to send instruction signals to the autopilot computer, enabling it to activate the appropriate servos to carry out the instructions, by pushing the required function buttons (s). On most autopilots, there are options for level flight, climbs, descents, turning to a heading, and flying a desired heading. Many different radio navigational aids are used by many different aircrafts. These can be chosen to send orders to the autopilot computer directly.

Most autopilots have a disconnect switch on the control wheel in addition to an on/off switch on the autopilot controller (s). When the pilot wants to assume manual control of the aircraft or if there is a system fault, they can use this switch, which is activated by applying pressure with their thumbs.

Feedback or Follow-up Element

In order for the controls and the aircraft to come to rest on course when an autopilot is adjusting the flight controls to achieve a desired flight attitude, it must reduce control surface correction as the desired attitude is almost obtained. In the absence of this, the system would constantly overcorrect. The surface would deflect until the desired attitude was achieved. However, as the surface(s) restored to its pre-error location, movement would resume. Once more detecting a mistake, the attitude sensor would start the process of correction all over again.

To gradually diminish the mistake message in the autopilot and prevent continual over-correction, various electric feedback, or follow-up signals, are produced. Transducers on the surface actuators or in the autopilot servo units are commonly used for this.

The control surfaces move when a rate system gets error signals from a rate gyro that are of a specific polarity and amplitude. Follow-up signals of the opposite polarity and increasing magnitude counter the error signal as the control surfaces work to repair the error and counteract it up until the aircraft is back in its proper attitude. When the surface is relocated to the proper position, a displacement follow-up system employs control surface pickups to cancel the error message.

Autopilot Functions

A simple analogue autopilot's operation is demonstrated in the autopilot system description that follows. The majority of autopilots are much more advanced. But many of the underlying principles of operation remain the same.

The aircraft is flown by the automatic pilot system using electrical signals produced in gyro-sensing devices. Flight instruments that show direction, rate of turn, bank, or pitch are connected to these units. Electrical signals are produced in the gyros in response to changes in the magnetic heading or flight attitude. These signals are utilized to regulate the operation of servo devices and are supplied to the autopilot computer and amplifier.

Each of the three control channels has a servo that transforms electrical impulses into mechanical force that moves the control surface in response to pilot directions or corrective signals. Two signals are sent to the rudder channel, which control when and how much the rudder moves. A compass system's course signal serves as the first signal. No signal appears as long as the aircraft stays on the magnetic heading it was on when the autopilot was activated. However, any variation results in the compass system sending a signal to the rudder channel proportional to the angle the aircraft deviates from the predetermined heading.

The rate signal, which is the second signal the rudder channel receives, offers information whenever the aircraft is turning about its vertical axis. The gyro that measures turn and bank provides this data. The rate gyro generates a signal proportional to the rate of turn when the aircraft tries to veer off course, and the course gyro generates a signal related to the amount of displacement. The rudder channel of the amplifier receives the two signals, combines them, and amplifies their power. The rudder servo receives the signal after it has been amplified. To bring the aeroplane back to the chosen magnetic heading, the servo spins the rudder in the right direction.

A follow-up signal that counters the input signal develops as the rudder surface moves. The servo comes to a complete stop when the magnitude of the two signals is equal. The course signal reaches zero when the aeroplane gets on course, and the follow-up signal moves the rudder back to the streamline position.

The gyro horizon indicator's transmitter provides the aileron channel with the input signal it needs to function. The gyro-sensing unit creates a signal to account for any movement of the aircraft about its longitudinal axis. The aileron servo sends this signal after it has been phase identified, amplified, and relayed to the aileron control surfaces, which move to correct the error. A follow-up signal develops in opposition to the input signal when the aileron surfaces move. The servo comes to a complete stop when the magnitude of the two signals is equal. The aeroplane now begins to return to level flight as the ailerons are pulled away from the streamline, the input signal reducing and the follow-up signal pulling the control surfaces back toward the streamline position. The input signal returns to zero after the aircraft has reached level flight roll attitude. The follow-up signal is zero and the control surfaces are simplified at the same time.

The elevator channel detects and corrects changes in the aircraft's pitch attitude, unlike the aileron channel, whose circuits are comparable to those of the aileron channel. A remotely installed apparatus with an altitude pressure diaphragm is used to control altitude. The altitude unit emits error signals when the aircraft deviates from a preset height, much as the attitude and directional gyros. A function called altitude hold is used for this. Pitch servos are moved to remedy the fault by being controlled by the signals. The signals are constantly delivered to the pitch servos by an altitude choose function until a predetermined height is reached. The aeroplane then uses altitude hold signals to keep the predetermined altitude.

Yaw Dampening

While flying at a fixed heading, many aircraft have a propensity to oscillation about their vertical axis. Input from the rudder must be nearly constant to offset this impact. This motion is adjusted by using a yaw damper. It could be a separate unit or a component of an autopilot system. The turn coordinator rate gyro sends error signals to the yaw damper. Rudder movement, which is automatically made by the rudder servo(s) in reaction to the polarity and magnitude of the error signal, counteracts oscillating yaw motion.

The Automatic Flight Control System (AFCS)

Automatic flight control systems are aircraft autopilots with a variety of capabilities and many autopilot-related technologies combined into a single system (AFCS). Until recently, only high-performance aircraft had them. Currently, modern aircraft of any size may feature AFCS thanks to advancements in digital aviation technology.

The AFCS capabilities differ between systems. The degree of programmability, the degree of integration of navigational aids, the integration of flight director and autothrottle systems, and the combining of the command elements of these various systems into a single integrated flight control human interface are some of the advancements beyond ordinary autopilot systems.

An autothrottle system is connected into the flight director and autopilot systems with glide scope modes at the AFCS level of integration to enable auto landings. It's possible that small general aviation aircraft with AFCS won't have throttle-dependent features.

A comprehensive integration of digital attitude heading and reference systems (AHRS) with navigational aids like glideslope is a feature of contemporary general aviation AFCS. Additionally, they have a more up-to-date computer architecture than the analogue autopilot systems mentioned above for the autopilot (and flight director systems). A number of interconnected computers are used to perform various functions, and part of the error correction computations are performed by intelligent servos. There is no central autopilot computer; instead, the servos connect with specialized avionics computers and display unit computers via a control panel.

Flight Director Systems

An instrument system called a flight director system is made up of electronic parts that compute and display the aircraft attitude needed to achieve and maintain a chosen flying state. The pilot can see how much and in which direction the aircraft's attitude needs to be modified by looking at a command bar on the attitude indicator. Many of the mental calculations necessary for instrument flights, such as interception angles, wind drift correction, and rates of climb and descent, are taken care of by the computed command indications, which free up the pilot.

A flight director system is essentially an autopilot system with no servos. The same measurements and calculations are conducted, but the pilot controls the aircraft and executes maneuvers by paying attention to the instrument panel's directives. Flight director systems may be an independent component of an autopilot system or be present on aircraft without complete autopilot systems. A flight director display can frequently be engaged or disengaged in autopilot systems.

On the instrument that shows the aircraft's attitude, information about the flight director is visible. The procedure is carried out using a visual referencing method. The flight director places a command bar in the appropriate spot so that a maneuver can be performed by inserting a symbol for the aircraft there. Depending on the manufacturer, several symbols are used to symbolize the aeroplane and the command bar. The goal is always to fly the aeroplane symbol into the command bar symbol, regardless of circumstances.

A flight director indicator (FDI), attitude director indicator (ADI), or electronic attitude director indicator is the name of the device that shows the flight director directives (EADI). Even the term "artificial horizon with flight director" may be used to describe it. Together with the other main elements of the flight director system, this display piece forms the system. These are made up of the sensing components, a computer, and an interface panel, just like an autopilot.

The instrument that displays flight director directives is known as a flight director indicator (FDI), attitude director indicator (ADI), or electronic attitude director indicator (EADI). It may even be referred to as a "artificial horizon with flight director." This display piece makes up the flight director system together with the other essential components. Similar to an autopilot, these consist of the sensing elements, a computer, and an interface panel.

The characteristics and level of sophistication of flight director systems vary. Many have functions like pitch hold, altitude hold, and altitude choose. However, flight director systems are built to be most helpful when making an instrument approach. The computer receives signals from the ILS localizer and glideslope, which are then given as command indications. This enables the pilot to use the flight director system to guide the aircraft down the best approach path to the runway.

When the altitude hold function is used, level flight can be kept throughout the approach's maneuvering and procedural turn phases. When the glideslope is intercepted, altitude hold automatically disengages. The flight director's command signals are kept in a zeroed or centered condition once the localizer is inbound. The command pitch indicator indicates downward when the glideslope is intercepted. A fly-up or fly-down command indicator is produced if the glideslope path deviates from the intended course. The only requirement for the pilot is that the aeroplane icon remain within the command bar.

Airbus A320 Autopilot

The Airbus A320 aircraft autopilot is integrated with many sensors, computers, and control systems. A short summary of the Airbus A320 Autopilit is as below:

Two similar Flight Management Guidance Computers (FMGC) that typically operate together perform the main processing. The flight management guidance system is what they are collectively referred to as (FMGS). Two Multipurpose Control and Display Units (MCDUs) on the centre pedestal, along with a flight control unit (FCU) on the glare shield, are used by the pilots to input data into the FMGS.

The FMGS's flight management section oversees display management, performance optimization, navigation, and flight planning. The flight guiding component issues commands for the flight director, autopilot, and auto-thrust. The flight augmentation portion offers a variety of yaw operations as well as computation of the flight envelope, maneuvering speed, windshear detection, and floor protection.

The FMGCs often gather data from "on-side" sensors and exchange it with one another to verify information. Both FMGCs are fed by the FCU.

Autopilot controls for an A320

The Flight Control Unit (FCU), which is placed above the instrument panel, houses the controls for the A320 autopilot. Fundamentally, the flight management guidance system's calculations or the handling pilot's direct inputs set the target parameters that the autopilot and autothrottle strive to acquire or maintain (FMGS). The word "managed" is used to describe the target parameter when it is set by the FMGS. The word "chosen" is used when the target parameter is set by the flight crew.

You can push or pull the buttons for choosing your speed and altitude. When pressed, the altitude selector knob enables the FMGS to "handle" any intermediate altitude limits entered into a route. When rotated, the altitude selector knob adjusts the goal height (or flight level). The altitude selector knob, when turned, starts a "open" climb or fall. When a route is described as "open," any intermediate altitude constraints that are part of the active route as determined by the FMGS are disregarded. When in descent, idle thrust is also chosen, and the elevators are in charge of regulating airspeed.

Similar operations are controlled by the speed selection control. When pushed, the FMGS computation determines the target "managed" speed and blanks the speed selection window. Pulling it causes the FCU SPD/MACH window to display the current speed, which the pilot can then use to turn the knob to change the target speed. Additionally, there is a switch button in the speed selection options that allows the goal speed to be specified as either a Mach number or a calibrated airspeed. (In this context, indicated airspeed (IAS) and calibrated airspeed (CAS) are extremely comparable.)

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