Autopilot

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The autopilot control panel of a Boeing 747-200 aircraft FMS B747-cockpit.jpg
The autopilot control panel of a Boeing 747-200 aircraft

An autopilot is a system used to control the trajectory of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator's control of the vehicle, allowing the operator to focus on broader aspects of operations (for example, monitoring the trajectory, weather and on-board systems). [1]

Contents

When present, an autopilot is often used in conjunction with an autothrottle, a system for controlling the power delivered by the engines.

An autopilot system is sometimes colloquially referred to as "George" [2] (e.g. "we'll let George fly for a while"). The etymology of the nickname is unclear: some claim it is a reference to inventor George De Beeson, who patented an autopilot in the 1930s, while others claim that Royal Air Force pilots coined the term during World War II to symbolize that their aircraft technically belonged to King George VI. [3]

First autopilots

A World War II-era Honeywell C-1 autopilot control panel Honeywell C-1 Autopilot Control Panel.jpg
A World War II-era Honeywell C-1 autopilot control panel

In the early days of aviation, aircraft required the continuous attention of a pilot to fly safely. As aircraft range increased, allowing flights of many hours, the constant attention led to serious fatigue. An autopilot is designed to perform some of the tasks of the pilot.

The first aircraft autopilot was developed by Sperry Corporation in 1912. The autopilot connected a gyroscopic heading indicator and attitude indicator to hydraulically operated elevators and rudder. (Ailerons were not connected as wing dihedral was counted upon to produce the necessary roll stability.) It permitted the aircraft to fly straight and level on a compass course without a pilot's attention, greatly reducing the pilot's workload.

Lawrence Sperry (the son of famous inventor Elmer Sperry) demonstrated it in 1914 at an aviation safety contest held in Paris. Sperry demonstrated the credibility of the invention by flying the aircraft with his hands away from the controls and visible to onlookers. Elmer Sperry Jr., the son of Lawrence Sperry, and Capt Shiras continued work on the same autopilot after the war, and in 1930 they tested a more compact and reliable autopilot which kept a US Army Air Corps aircraft on a true heading and altitude for three hours. [4]

In 1930, the Royal Aircraft Establishment in the United Kingdom developed an autopilot called a pilots' assister that used a pneumatically-spun gyroscope to move the flight controls. [5]

The autopilot was further developed, to include e.g. improved control algorithms and hydraulic servomechanisms. Adding more instruments such as radio-navigation aids made it possible to fly at night and in bad weather. In 1947 a US Air Force C-53 made a transatlantic flight, including takeoff and landing, completely under the control of an autopilot. [6] [7] Bill Lear developed his F-5 automatic pilot and automatic approach control system, and was awarded the Collier Trophy for 1949. [8]

In the early 1920s, the Standard Oil tanker J.A. Moffet became the first ship to use an autopilot.

The Piasecki HUP-2 Retriever was the first production helicopter with an autopilot. [9]

The lunar module digital autopilot of the Apollo program was an early example of a fully digital autopilot system in spacecraft. [10]

Modern autopilots

The modern flight control unit of an Airbus A340 A340 FCU.jpg
The modern flight control unit of an Airbus A340

Not all of the passenger aircraft flying today have an autopilot system. Older and smaller general aviation aircraft especially are still hand-flown, and even small airliners with fewer than twenty seats may also be without an autopilot as they are used on short-duration flights with two pilots. The installation of autopilots in aircraft with more than twenty seats is generally made mandatory by international aviation regulations. There are three levels of control in autopilots for smaller aircraft. A single-axis autopilot controls an aircraft in the roll axis only; such autopilots are also known colloquially as "wing levellers," reflecting their single capability. A two-axis autopilot controls an aircraft in the pitch axis as well as roll, and may be little more than a wing leveller with limited pitch oscillation-correcting ability; or it may receive inputs from on-board radio navigation systems to provide true automatic flight guidance once the aircraft has taken off until shortly before landing; or its capabilities may lie somewhere between these two extremes. A three-axis autopilot adds control in the yaw axis and is not required in many small aircraft.

Autopilots in modern complex aircraft are three-axis and generally divide a flight into taxi, takeoff, climb, cruise (level flight), descent, approach, and landing phases. Autopilots that automate all of these flight phases except taxi and takeoff exist. An autopilot-controlled landing on a runway and controlling the aircraft on rollout (i.e. keeping it on the centre of the runway) is known as a CAT IIIb landing or Autoland, available on many major airports' runways today, especially at airports subject to adverse weather phenomena such as fog. Landing, rollout, and taxi control to the aircraft parking position is known as CAT IIIc. This is not used to date, but may be used in the future. An autopilot is often an integral component of a Flight Management System.

Modern autopilots use computer software to control the aircraft. The software reads the aircraft's current position, and then controls a flight control system to guide the aircraft. In such a system, besides classic flight controls, many autopilots incorporate thrust control capabilities that can control throttles to optimize the airspeed.

The autopilot in a modern large aircraft typically reads its position and the aircraft's attitude from an inertial guidance system. Inertial guidance systems accumulate errors over time. They will incorporate error reduction systems such as the carousel system that rotates once a minute so that any errors are dissipated in different directions and have an overall nulling effect. Error in gyroscopes is known as drift. This is due to physical properties within the system, be it mechanical or laser guided, that corrupt positional data. The disagreements between the two are resolved with digital signal processing, most often a six-dimensional Kalman filter. The six dimensions are usually roll, pitch, yaw, altitude, latitude, and longitude. Aircraft may fly routes that have a required performance factor, therefore the amount of error or actual performance factor must be monitored in order to fly those particular routes. The longer the flight, the more error accumulates within the system. Radio aids such as DME, DME updates, and GPS may be used to correct the aircraft position.

Control Wheel Steering

Servomotor for Autopilot applications EBACE 2019, Le Grand-Saconnex (EB190664).jpg
Servomotor for Autopilot applications

An option midway between fully automated flight and manual flying is Control Wheel Steering (CWS). Although it is becoming less used as a stand-alone option in modern airliners, CWS is still a function on many aircraft today. Generally, an autopilot that is CWS equipped has three positions: off, CWS, and CMD. In CMD (Command) mode the autopilot has full control of the aircraft, and receives its input from either the heading/altitude setting, radio and navaids, or the FMS (Flight Management System). In CWS mode, the pilot controls the autopilot through inputs on the yoke or the stick. These inputs are translated to a specific heading and attitude, which the autopilot will then hold until instructed to do otherwise. This provides stability in pitch and roll. Some aircraft employ a form of CWS even in manual mode, such as the MD-11 which uses a constant CWS in roll. In many ways, a modern Airbus fly-by-wire aircraft in Normal Law is always in CWS mode. The major difference is that in this system the limitations of the aircraft are guarded by the flight computer, and the pilot cannot steer the aircraft past these limits. [11]

Computer system details

The hardware of an autopilot varies between implementations, but is generally designed with redundancy and reliability as foremost considerations. For example, the Rockwell Collins AFDS-770 Autopilot Flight Director System used on the Boeing 777 uses triplicated FCP-2002 microprocessors which have been formally verified and are fabricated in a radiation-resistant process. [12]

Software and hardware in an autopilot are tightly controlled, and extensive test procedures are put in place.

Some autopilots also use design diversity. In this safety feature, critical software processes will not only run on separate computers and possibly even using different architectures, but each computer will run software created by different engineering teams, often being programmed in different programming languages. It is generally considered unlikely that different engineering teams will make the same mistakes. As the software becomes more expensive and complex, design diversity is becoming less common because fewer engineering companies can afford it. The flight control computers on the Space Shuttle used this design: there were five computers, four of which redundantly ran identical software, and a fifth backup running software that was developed independently. The software on the fifth system provided only the basic functions needed to fly the Shuttle, further reducing any possible commonality with the software running on the four primary systems.

Stability augmentation systems

A stability augmentation system (SAS) is another type of automatic flight control system; however, instead of maintaining the aircraft on a predetermined attitude or flight path, the SAS will actuate the aircraft flight controls to dampen out aircraft buffeting regardless of the attitude or flight path. SAS can automatically stabilize the aircraft in one or more axes. The most common type of SAS is the yaw damper which is used to eliminate the Dutch roll tendency of swept-wing aircraft. Some yaw dampers are integral to the autopilot system while others are stand-alone systems.

Yaw dampers usually consist of a yaw rate sensor (either a gyroscope or angular accelerometer), a computer/amplifier and a servo actuator. The yaw damper uses yaw rate sensor to sense when the aircraft begins a Dutch roll. A computer processes the signals from the yaw rate sensor to determine the amount of rudder movement that is required to dampen out the Dutch roll. The computer then commands the servo actuator to move the rudder that amount. The Dutch roll is dampened out and the aircraft becomes stable about the yaw axis. Because Dutch roll is an instability that is inherent to all swept-wing aircraft, most swept-wing aircraft have some sort of yaw damper system installed.

There are two types of yaw dampers: series yaw dampers and parallel yaw dampers. The servo actuator of a parallel yaw damper will actuate the rudder independently of the rudder pedals while the servo actuator of a series yaw damper is clutched to the rudder control quadrant and will result in pedal movement when the system commands the rudder to move.

Some aircraft have stability augmentation systems that will stabilize the aircraft in more than a single axis. The Boeing B-52, for example, requires both pitch and yaw SAS in order to provide a stable bombing platform. Many helicopters have pitch, roll and yaw SAS systems. Pitch and roll SAS systems operate much the same way as the yaw damper described above; however, instead of dampening out Dutch roll, they will dampen pitch and roll oscillations or buffeting to improve the overall stability of the aircraft.

Autopilot for ILS landings

Instrument-aided landings are defined in categories by the International Civil Aviation Organization, or ICAO. These are dependent upon the required visibility level and the degree to which the landing can be conducted automatically without input by the pilot.

CAT I – This category permits pilots to land with a decision height of 200 feet (61 m) and a forward visibility or Runway Visual Range (RVR) of 550 metres (1,800 ft). Autopilots are not required. [13]

CAT II – This category permits pilots to land with a decision height between 200 feet (61 m) and 100 feet (30 m) and a RVR of 300 metres (980 ft). Autopilots have a fail passive requirement.

CAT IIIa -This category permits pilots to land with a decision height as low as 50 feet (15 m) and a RVR of 200 metres (660 ft). It needs a fail-passive autopilot. There must be only a 10−6 probability of landing outside the prescribed area.

CAT IIIb – As IIIa but with the addition of automatic roll out after touchdown incorporated with the pilot taking control some distance along the runway. This category permits pilots to land with a decision height less than 50 feet or no decision height and a forward visibility of 250 feet (76 m) in Europe (76 metres, compare this to aircraft size, some of which are now over 70 metres (230 ft) long) or 300 feet (91 m) in the United States. For a landing-without-decision aid, a fail-operational autopilot is needed. For this category some form of runway guidance system is needed: at least fail-passive but it needs to be fail-operational for landing without decision height or for RVR below 100 metres (330 ft).

CAT IIIc – As IIIb but without decision height or visibility minimums, also known as "zero-zero". Not yet implemented as it would require the pilots to taxi in zero-zero visibility. An aircraft that is capable of landing in a CAT IIIb that is equipped with autobrake would be able to fully stop on the runway but would have no ability to taxi.

Fail-passive autopilot: in case of failure, the aircraft stays in a controllable position and the pilot can take control of it to go around or finish landing. It is usually a dual-channel system.

Fail-operational autopilot: in case of a failure below alert height, the approach, flare and landing can still be completed automatically. It is usually a triple-channel system or dual-dual system.

Radio-controlled models

In radio-controlled modelling, and especially RC aircraft and helicopters, an autopilot is usually a set of extra hardware and software that deals with pre-programming the model's flight. [14]

See also

Related Research Articles

Fly-by-wire system that replaces the conventional manual flight controls of an aircraft with an electronic interface

Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control computers determine how to move the actuators at each control surface to provide the ordered response. It can use mechanical flight control backup systems or use fully fly-by-wire controls.

Instrument landing system Ground-based visual aid for landing

In aviation, the instrument landing system (ILS) is a radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a 12 mile of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. Bringing the aircraft this close to the runway dramatically improves the weather conditions in which a safe landing can be made. Later versions of the system, or "categories", have further reduced the minimum altitudes.

Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Articles related to aviation include:

Flight control surfaces Surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

Rockwell-MBB X-31

The Rockwell-Messerschmitt-Bölkow-Blohm X-31 was an experimental jet fighter designed to test fighter thrust vectoring technology.

Aircraft flight control system How aircraft are controlled

A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed.

Spoiler (aeronautics)

In aeronautics, a spoiler is a device which intentionally reduces the lift component of an airfoil in a controlled way. Most often, spoilers are plates on the top surface of a wing that can be extended upward into the airflow to spoil the streamline flow. By so doing, the spoiler creates a controlled stall over the portion of the wing behind it, greatly reducing the lift of that wing section. Spoilers differ from airbrakes in that airbrakes are designed to increase drag without affecting lift, while spoilers reduce lift as well as increasing drag.

Dutch roll Aircraft motion combining rolling and yawing

Dutch roll is a type of aircraft motion consisting of an out-of-phase combination of "tail-wagging" (yaw) and rocking from side to side (roll). This yaw-roll coupling is one of the basic flight dynamic modes. This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping as airspeed decreases and altitude increases. Dutch roll stability can be artificially increased by the installation of a yaw damper. Wings placed well above the center of gravity, sweepback and dihedral wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies; this is why high-winged aircraft often are slightly anhedral, and transport-category swept-wing aircraft are equipped with yaw dampers. A similar phenomenon can happen in a trailer pulled by a car.

Aerobatic maneuver

Aerobatic maneuvers are flight paths putting aircraft in unusual attitudes, in air shows, dogfights or competition aerobatics. Aerobatics can be performed by a single aircraft or in formation with several others. Nearly all aircraft are capable of performing aerobatics maneuvers of some kind, although it may not be legal or safe to do so in certain aircraft.

Yoke (aeronautics)

A yoke, alternatively known as a control wheel or a control column, is a device used for piloting some fixed-wing aircraft.

Autoland

In aviation, autoland describes a system that fully automates the landing procedure of an aircraft's flight, with the flight crew supervising the process. Such systems enable airliners to land in weather conditions that would otherwise be dangerous or impossible to operate in.

Trim tab Small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls

Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a particular desired attitude without the need for the operator to constantly apply a control force. This is done by adjusting the angle of the tab relative to the larger surface.

The dynamic stability of an aircraft refers to how the aircraft behaves after it has been disturbed following steady non-oscillating flight.

A yaw damper is a system used to reduce the undesirable tendencies of an aircraft to oscillate in a repetitive rolling and yawing motion, a phenomenon known as the Dutch roll. A large number of modern aircraft, both jet-powered and propeller-driven, have been furnished with such systems.

Aircraft principal axes

An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down; pitch, nose up or down about an axis running from wing to wing; and roll, rotation about an axis running from nose to tail. The axes are alternatively designated as vertical, transverse, and longitudinal respectively. These axes move with the vehicle and rotate relative to the Earth along with the craft. These definitions were analogously applied to spacecraft when the first manned spacecraft were designed in the late 1950s.

Flight control modes

A flight control mode or flight control law is a computer software algorithm that transforms the movement of the yoke or joystick, made by an aircraft pilot, into movements of the aircraft control surfaces. The control surface movements depend on which of several modes the flight computer is in. In aircraft in which the flight control system is fly-by-wire, the movements the pilot makes to the yoke or joystick in the cockpit, to control the flight, are converted to electronic signals, which are transmitted to the flight control computers that determine how to move each control surface to provide the aircraft movement the pilot ordered.

Several aviation incidents and accidents have occurred in which the control surfaces of the aircraft became disabled, often due to failure of hydraulic systems or the flight control system. Other incidents have occurred where controls were not functioning correctly prior to take-off, either due to maintenance or pilot error, and controls can become inoperative from extreme weather conditions. Aircraft are not designed to be flown in such circumstances, however a small number of pilots have had some success in flying and landing aircraft with disabled controls.

This is a list of the acronyms and abbreviations used in avionics.

In signal processing, a washout filter is a stable high pass filter with zero static gain. This leads to the filtering of lower frequency inputs signals, leaving the steady state output unaffected by unwanted low frequency inputs.

The gyroscopic autopilot was a type of autopilot system developed primarily for aviation uses in the early 20th century. Since then, the principles of this autopilot has been the basis of many different aircraft control systems, both military and civilian.

References

  1. "Automated Flight Controls" (PDF). faa.gov. Federal Aviation Administration. Retrieved 20 February 2014.
  2. "George the Autopilot". Historic Wings. Thomas Van Hare. Retrieved 18 March 2014.
  3. Baker, Mark (1 April 2020). "PRESIDENT'S POSITION: GIVING GEORGE A BREAK". aopa.org. Aircraft Owners and Pilots Association . Retrieved 16 May 2020.
  4. "Now – The Automatic Pilot" Popular Science Monthly, February 1930, p. 22.
  5. "Robot Air Pilot Keeps Plane on True Course" Popular Mechanics , December 1930, p. 950.
  6. Stevens, Brian; Lewis, Frank (1992). Aircraft Control and Simulation. New York: Wiley. ISBN   978-0-471-61397-8.
  7. Flightglobal/Archive
  8. Collier Trophy awards
  9. "HUP-1 Retriever/H-25 Army Mule Helicopter". boeing.com. Boeing . Retrieved 1 November 2018.
  10. William S. Widnall, vol 8, no. 1, 1970 (October 1970). "Lunar Module Digital Autopilot, Journal of Spacecraft". doi:10.2514/3.30217 . Retrieved 7 September 2019.Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
  11. https://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/steering-wheel-controls.htm.Missing or empty |title= (help)
  12. "Rockwell Collins AFDS-770 Autopilot Flight Director System". Rockwell Collins. 3 February 2010. Archived from the original on 22 August 2010. Retrieved 14 July 2010.
  13. "Aeronautical Information manual". faa.gov. FAA. Retrieved 16 June 2014.
  14. Alan Parekh (14 April 2008). "Autopilot RC Plane". Hacked Gadgets. Archived from the original on 27 July 2010. Retrieved 14 July 2010.