In the newest aircraft, autopilots are of paramount importance, but how are they designed? What is the hardware used to run them (I guess it is quite different from my laptop)? What is the software (language) used and what are its characteristics (regular clock counter, robust, ...)? This question concerns mainly civil aircraft since I guess that it will be hard to get answers for military ones!

EDIT To make this question answerable and less broad, I would like to have answers focusing on turbojet engine aircraft used as civilian carriers (A320 seems a good candidate to me but I am opened to any other airplane fitting the description).

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    $\begingroup$ Even ignoring military aircraft, this question is much too broad to be answered successfully. $\endgroup$ – Steve V. Dec 30 '13 at 22:02
  • $\begingroup$ @SteveV. I am not looking for the details about the coding or the way an autopilot works just which hardware is used (hard drive? mother board?) and the main features of the software (C++, Fortran, Python and why this choice instead of another). I could split this question in two, but it made sense to me to ask them together and this is exactly the kind of questions I hoped to get answers to in this platform. $\endgroup$ – Ludovic C. Dec 30 '13 at 22:08
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    $\begingroup$ Perhaps it could be divided into several questions: a) What are the main components of an autopilot system in a piston engine GA airplane? b) How the autopilot in a piston single or twin differ from turboprop/jet aircraft? c) What is the difference between single-, dual- and three-axis autopilots? d) What have been the top three selling autopilot systems in piston GA aircraft in the last 10 years? e) What is the embedded software used to program the top selling piston GA autopilot system? Is it different than the software used in second-best selling system? $\endgroup$ – Canuk Dec 30 '13 at 22:14
  • $\begingroup$ @ludovic - what makes you think that all autopilots use the same hard drive, or the same programming language? $\endgroup$ – Steve V. Dec 30 '13 at 22:16
  • $\begingroup$ even just considering piston GA the newer agar in autopilots versus say an STEC are pretty wildly different. I think to be answerable this really needs to be broken down into component questions about specific autopilots/implementations.... $\endgroup$ – voretaq7 Dec 30 '13 at 22:48

Most sophisticated autopilot systems used in modern airliners are integral parts of a larger Flight Management System (FMS). FMS systems include multiple independant computers communicating over one or more common data busses which allow for expandability and upgradability. Examples of the various 'customer facing' independent parts for the B-737, each it's own computer, are one or more Control Display Units (CDUs), Mode Control Panel (MCP), and autopilot logic computers. There are also multiple independent and redundant computers for the various air data and navigation position systems that feed into the FMS.

Each computer is optimized for it's specific task so it is very difficult to answer your question. Characteristically, there is just enough computing horsepower to get the job done and nothing more (and occasionally just a bit less...).


Hopefully the following information will prompt a more specific question (or series of questions):

I looked up S-TEC autopilots because they have an STC (supplemental type certificate) for their autopilot system for over 1,600 models of aircraft.

An autopilot is an avionics system designed to serve two primary purposes:

  • Enhance a pilot’s flight control capabilities
  • Reduce cockpit workload by putting the airplane in an automatic flight mode

An autopilot consists of three basic components:

  1. Mode selector/programmer (The device that allows the pilot to program the autopilot to achieve the intended flight profile. This may interact with other avionics, such as a GPS unit)

  2. Computer (This receives the information from the programmer or avionics and sends the appropriate signal to the actuators)

  3. Actuators (The servos or motors which physically move the aircraft's control surfaces)

An autopilot system can be either position-based or rate-based.

Position or attitude autopilots typically use the pilot’s attitude indicator as a gyroscope signal source.

Rate-based autopilot systems use rate gyros and/or accelerometers as primary sensors.

There are many features and functions of an autopilot which are outlined in the referenced PDF file below, but to give you an overview:

  • Wing Stabilizer (Wing Leveler).
  • Heading Mode.
  • Course Tracking and Coupling.
  • Course Deviation Warning.
  • Dual Mode Intercept.
  • Roll Steering.
  • Altitude Hold.
  • Vertical Speed.
  • Pre-Selected Altitude Capture.
  • Glideslope Coupling.
  • Trim.
  • Control Wheel Steering.

Fundamentally, autopilots typically perform the following tasks:

  • Maintaining Assigned Altitude and Heading.
  • Holding a heading
  • Tracking a VOR, GPS, LORAN or LOC signal
  • Intercepting the course

Each one of these aspects of an autopilot could provide several ideas for questions that could help you get a fuller picture of all the technologies involved in autopilots, and perhaps help you limit the scope of your question to something that might not require textbook-length answer :)

Hope this helps in your quest!

S-TEC Autopilot Overview: http://sharepoint.s-tec.com/Documentation/Shared%20Documents/Autopilot%20Selection/AutoPilotBook.pdf

S-TEC Pilot Operating Handbooks: http://sharepoint.s-tec.com/Documentation/default.aspx

Garmin also makes a GA autopilot called the GFC 700 which integrates into their Garmin 1000 flight deck. More info here: http://www8.garmin.com/learningcenter/training/gfc700/

  • $\begingroup$ Thanks for trying to answer my un-answerable question ;) It definitely helps and provides a basis for further questions. Just to make sure, an autopilot also acts on the propulsion part because you just mentioned control surfaces or no? $\endgroup$ – Ludovic C. Dec 30 '13 at 23:45
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    $\begingroup$ Control surfaces is just a broad way of encompassing things like the ailerons, elevators and rudder. (There are other control surfaces like canards). The propulsion is the engine, and where an autopilot would fit it is on autothrottles where the autopilot moves the throttle which increases or decreases thrust/power of the engine. This is standard in commercial jets and rare (possibly nonexistent) in small piston aircraft. $\endgroup$ – Canuk Dec 31 '13 at 9:58

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