Editing Autopilot (section)

==Modern autopilots==
{{Unreferenced section|date=September 2017}}
[[File:A340 FCU.jpg|thumb|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 [[airliner]]s 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 [[flight dynamics|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 [[flight dynamics|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 [[flight dynamics|yaw]] axis and is not required in many small aircraft.

Autopilots in modern complex aircraft are three-axis and generally divide a flight into [[taxiing|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 approach to landing on a runway and controlling the aircraft on rollout (i.e. keeping it on the centre of the runway) is known as an Autoland, where the autopilot utilizes an [[Instrument landing system|Instrument Landing System]] (ILS) Cat IIIc approach, which is used when the visibility is zero. These approaches are available at many major airports' runways today, especially at airports subject to adverse weather phenomena such as [[fog]]. The aircraft can typically stop on their own, but will require the disengagement of the autopilot in order to exit the runway and taxi to the gate. 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 [[aircraft flight control system|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===
[[File:EBACE 2019, Le Grand-Saconnex (EB190664).jpg|thumb|[[Servo Motor|Servo motor]] 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 [[Flight control modes#Normal law|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 control computer]], and the pilot cannot steer the aircraft past these limits.<ref>{{cite web |url=https://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/steering-wheel-controls.htm|title=How Steering Wheel Controls Work|date=22 April 2009}}</ref>

===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.<ref>{{cite web | url=http://www.rockwellcollins.com/ecat/at/AFDS-770.html | title=Rockwell Collins AFDS-770 Autopilot Flight Director System| publisher=Rockwell Collins | date=3 February 2010 | access-date=14 July 2010| archive-url= https://web.archive.org/web/20100822151603/http://www.rockwellcollins.com/ecat/AT/AFDS-770.html| archive-date= 22 August 2010 | url-status=live}}</ref>

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.