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Assuming the plane is on autopilot: for example, if a plane is flying high (over 20000 feet) and its autopilot receives a positive value of external temperature, which is clearly abnormal or impossible.

What will the autopilot do with such a obviously wrong information?

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    $\begingroup$ Hello mouniir oouli, welcome to Aviation.SE. Your question is not very clear. Could you explain what you mean by too high (about 20 tousand feets)? What do you mean by enter in a new procedure or ask itself that it is not normal ? $\endgroup$ – DeltaLima Mar 25 '15 at 20:11
  • $\begingroup$ Hello Mouniir. Are you asking what happens when the outer air temperature changes and about autopilot behavior? $\endgroup$ – SentryRaven Mar 25 '15 at 20:26
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    $\begingroup$ Autopilot does not receive temperature at all. Only autothrust does, in Mach mode, indirectly, since Mach number depends on temperature. $\endgroup$ – Jan Hudec Mar 26 '15 at 5:43
  • $\begingroup$ You may want to open your question to all flight automatisms, rather than autopilot. Or you may want to know what would happen if the autopilot received wrong data from the units processing temperature sensor data (e.g. ADIRS). $\endgroup$ – mins Apr 13 '15 at 6:45
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The autopilot does not receive information about the outside temperature, it has no (direct) relation to what the autopilot controls.

The autopilot receives aircraft state inputs like airspeed, Mach number, vertical speed, attitude, accelerations etc.

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I can not tell you what happens if there is a positive value for the temperature above 20000ft, but here is some general information:

For this issue I will be talking about Airbus planes, as they are known to be very "smart" planes. There are hundreds of algorithms that try to prevent wrong data input from being an issue. But those do not always work.

Take Qantas Flight 72 for example: According to Wikipedia the airplane all of the sudden pitched down violently, as if it was trying to prevent a stall from happening. This was because the average of the two angle of attack sensors was way out the box. Though there was a (very smart) algorithm rechecking the values, it could not deal with multiple wrong values with a certain amount of time - and made the airplane pitch down.

This being an example where the plane actually acted by itself due to a faulty value, here is the general case: If the airplane notices odd values, which it will usually recognize (e.g. a positive temperature), it blanks out the display so that a pilot can not be mislead.

When flying smaller airplanes, you often practice "partial panel flying", where your flight instructor covers up some instruments and you have to fly the aircraft with alternative instruments. As you can see this is well practiced and shouldn't be a problem. Unfortunately though, this makes flying a lot harder which can cause crashes to happen (AF447,QZ8501, and more).

Furthermore, whenever the autopilot finds itself in a situation that it can't handle, for example when the pitot static system fails, it will disconnect and refuse to connect, giving the pilot full control (or rather as full control as possible on an Airbus) of the aircraft. This disengagement ensures the plane not to do something stupid due to wrong inputs.

Conclusion: For redundancy most systems exist two or three times working parallel to provide backup, same goes for sensors. If all data inputs report something obviously wrong, the pilot will not see this wrong information so he can concentrate on what he has.

The answers to this question can also give you a little more information about computer redundancy on aircraft

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  • $\begingroup$ I don't see how this answers the question. I would also welcome input from someone qualified as to whether OAT is used at all by the autopilot. I don't think it is. $\endgroup$ – Simon Mar 25 '15 at 22:14
  • $\begingroup$ Well, as I said I cannot be specific, but I've listed a couple of scenarios what the autopilot will do with wrong inputs. The autopilot itself doesn't use the Oct but it is used for altitude and speed calculations by the system, so if they are wrong the autopilot might still get wring data. $\endgroup$ – Maverick283 Mar 25 '15 at 22:17
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    $\begingroup$ Oh and @Simon calling someone unqualified though he gave an answer based on examples and documentation is quite rude, and I'd really appreciate removing that part of the comment. $\endgroup$ – Maverick283 Mar 25 '15 at 22:20
  • $\begingroup$ I didn't mean to imply that you are not qualified and apologise if you perceive it as rude, which is never my intention. I mean big iron certified aircrew. Nor was I criticising you or your post. Simply stating that I don't believe that it answers the question is not criticism, just a statement of opinion on a Q&A. Cheers $\endgroup$ – Simon Mar 25 '15 at 22:44
  • $\begingroup$ Okay, cheers buddy $\endgroup$ – Maverick283 Mar 25 '15 at 22:46

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