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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