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Question 220816 - PFD

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  • Question 220816 - PFD

    Hello everybody,

    Here is question 220816:

    The Primary Flight Display (PFD) of an EFIS equipped aircraft displays the following parameters:
    IAS, attitude, altitude, heading.
    Attitude, heading, altitude, systems information.
    Attitude, heading, IAS, navigation map in Plan layout mode.
    Altitude, attitude, heading, engine parameters.

    The expected answer is A.

    I disagree with A as I believe a PFD would show a CAS and not an IAS.
    I answered B, thinking that "systems information" could possibly be associated to the FMA indications (even though I believe this is not what was meant in the first place...but how can I know).

    Is there really a known similar question in the exams? What would you have answered ? Neither answer is perfect...

  • #2
    The way I would remember it is if you look at the mode control panel on say a 737 (google it) there is a window for IAS/MACH, also when you think systems. Think Air conditioning, hydraulics, landing gear, not modes of Autopilot etc. Not sure if that helps at all but the way I remember it. SYSTEMS is the one that stands out for me.


    • #3
      Hi FlyingJim,

      Thank you very much for your input.

      It is true that on the 737 it is written IAS on the MCP (for a reason I don't know !). But I believe the question isn't supposed to be airplane specific. Moreover, still about the 737, the speed displayed on the PFD is CAS, not IAS, as we can see on the picture below:

      I believe that most EFIS aircraft display CAS instead of IAS, as the correction is easy to perform via electronic means.

      I also interpreted the systems like bleed, hydraulics, ... at first, but the IAS instead of CAS in question A made me reconsider my position and try to find another explanation. I thought that maybe the question was badly formulated and that the systems were instead related the the FMA. But that was just because of the IAS in question A otherwise I would have gone for A.

      Anyway this is not a big deal, but I find this question a little bit misleading. CAS instead of IAS in answer A would probably remove the ambiguity !


      • #4
        The difference (if any) between IAS and CAS is due to instrument error and position error. With modern electronic instruments the air data modules which measure the pitot and static pressures are very accurate, so instrument error is negligible. To correct for position error requires angle of attack (AOA) data (in addition to pitot static data). A PFD gets its airspeed from an air data computer which has AOA input and can therefore correct for position error. So on a modern PFD IAS and CAS are the same.

        The integrated standby flight display on Boeing aircraft does not have built in position error correction, so this instrument displays IAS. You will find a position error correction chart for the standby display in the flight manual.

        You may ask why a PFD is not also corrected for compressibility error so as to display EAS? After all an air data computer can just as easily compute EAS as CAS. I think the reason is mainly tradition and so as to have the PFD read the same as would a mechanical ASI.
        Last edited by Ortac; 03-12-2017, 14:20.


        • #5
          Thank you for this complete answer !

          Indeed the difference between IAS and CAS was probably mostly relevant for mechanical ASIs, so from now on I will consider that they are the same when it comes to EFIS instruments.
          I think that the definitions were made for traditional ASIs, as the position error is now corrected internally on new aircraft. The Indicated airspeed is no more a result subject to errors, but a result already corrected. In fact I believe that for EFIS aircraft, we should state that the Indicated Airspeed is the CAS, and forget the traditional definition of the IAS which may lead to some confusion.

          Thank you very much.


          • #6
            Originally posted by greg765 View Post
            Thank you for this complete answer !

            In fact I believe that for EFIS aircraft, we should state that the Indicated Airspeed is the CAS, and forget the traditional definition of the IAS which may lead to some confusion.

            Thank you very much.
            This is true only for a PFD fed by an air data computer which incorporates position error correction.

            The Boeing standby EFIS (ISFD) does not incorporate position error correction. EFIS in light aircraft do not usually incorporate position error correction either. For any EFIS which does not incorporate position error correction, a position error graph should be found in the flight manual so you can make manual correction between IAS and CAS.

            Last edited by Ortac; 07-12-2017, 11:47.