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Thread: Is IAS indicated wrongly by FS?

  1. #1

    Default Is IAS indicated wrongly by FS?

    Hi all,
    hope you can shed some light on an issue I've been wondering about for some time now...

    It really seems there is an error in the way FS (I have 2004) calculates IAS.

    I had thought about hijacking a recent discussion relating to a similar question, but it's probably better to open an own thread.

    Mexy correctly (AFAIK) states in that discusstion that at sea level IAS and TAS should be very closely the same (and wisely indicates that the PICD speed correction terms can have a small effect).

    However, as CRJ has observed, IAS indicates much higher than TAS at sea level at high speeds in the sim.
    CRJ observes an IAS of 613 kts and a GS of 562 kts at sea level. IAS is 51 knots, or 9%, higher than TAS (=GS assuming no wind).

    Even taking into account the PICD speed correction terms there is no explanation for this large discrepancy. Position (P) and instrument (I) errors to apply to IAS are typically set to zero for most planes in FS, and at sea level density (D) and compressibility (C) correction terms to apply are zero as well.

    As far as I understand there's no physical effect which would explain why there would be this large difference between IAS and TAS at sea level.

    I'd highly appreciate if anybody could point out an explanation/physical effect that would account for this large discrepancy.



    PS: We have discussed this in the FSDeveloper forum in the past, but without resolution.
    I'm copying a message from there.

    Maybe somebody in the larger audience here can shed some light on this issue.

    "I did some more tests with SF8 F-86E and default B737-400 in FS2004, and Microsoft may really have got something wrong here...


    2. Bad news is: The Problem is with IAS, (the IAS speed) displayed is much too high at high IAS compared to TAS.
    Up to 200 KIAS there is no significant error.
    At higher IAS the error sharply increases and has to be considered when determining TAS from IAS indicated in the sim.

    3. Good news is: IMHO this error can be treated as an instrument error (large at high IAS).
    According to the tests the error seems directly to depend on IAS.

    When calculating TAS from IAS the ICE-T steps can be followed:

    Indicated air speed (IAS)

    + Instrument error - this accounts for the wrong indication. See attached graph from fs9 test data.
    This correction is much larger than would be expected for a normal instrument or position error, but applying this correction seems to allow to correctly correlate IAS and TAS (considering the additional corrections below).
    Below 200 KIAS this correction can be neglected.

    (Note: The difference observed by CRJ between IAS and TAS at sea level is 51 kts, exactly what would be expected considering the graph at 613 KIAS.)

    = Calibrated air speed (CAS)

    + compressibility correction. See graph at
    Below 0.5 Mach this correction can be neglected.

    = Equivalent air speed (EAS)

    * Density correction. Perform with E6-B.

    = True air speed (TAS)

    Alternative methods to determine TAS from CAS:

    - Graph from F/A-18A/B/C/D FLIGHT MANUAL, SECTION XI PERFORMANCE DATA, for Low Mach and High Mach region
    Graphs attached.

    - Online Atmospheric Properties Calculator

    Comments on IAS correction:

    + Instrument error: This is where the the apparent error in the IAS displayed by FS can be accounted for.
    For the correction see the data in the attached graph, derived from flight tests in fs2004.
    The SF8 F-86E and default B737-400 have the same display error as function of IAS.
    Note that when going from IAS to CAS the correction is negative, i.e. CAS is smaller than IAS.

    It's hard to believe that MS got this wrong indeed, but I didn't find any source mentioning a correction factor that would fit the observed deviation for IAS.
    I'd highly appreciate if anybody could point out an explanation/physical effect that would account for this correction term.

    I have done tests with two planes only, but results indicate the same correction terms for both.
    I'd be interested to hear if this correction term does not work for other planes.



    Attached Images Attached Images  
    Last edited by teson1; 12-22-2010 at 12:02 PM.

  2. Default

    yes, even at 100ft my plane was flying that much faster with no wind...but the technology might not be quite great 10 years ago in FS 2000!
    shouldn't 100 ft be pretty much the exact same as sea level?

    when i downloaded the planes i transfered the 737-400 panel to the 717, 747, CRJ etc...maybe FS did that panel with speeds wrong...i dunno, but other guys; please answer this 'cause it's a great question.

  3. #3


    I believe that Microsoft is correct because it seems to have taken compressibility effects into account.

    Theoretically, a pitot-static system measures the impact pressure which varies with Mach number according to the series:

    Qimpact/Qdynamic = 1 + M^2 / 4 + M^4 / 40 + M^4 / 40 + M^^ / 1600 + .........

    where Qdynamic is the usual 0.5 *Density * Speed^2 applicable at low speeds. Clearly at low Mach numbers the ratio tends to 1 and the two pressures have effectively the same value.

    The example given is 613 kts IAS and 562kt G/S at sea-level. The latter is equivalent to 0.85M. Applying the formula gives a ratio of 1.207. The speed depends on the square root of the pressure and hence of the ratio. The square root of 1.207 is 1.0985. Multiplying this by the G/S gives an IAS of 617 kts. This seems close enough to the original figure of 613 kts IAS to account for the difference.
    Last edited by mgh; 12-23-2010 at 06:25 AM.

  4. #4


    thanks for these thoughts. I'll have to think about these and will come back.

    However, I still believe that FS is wrong.

    By definition the (calibrated) airspeed indicator should be calibrated (!) to indicate TAS at sea level and standard atmospheric conditions.
    p. 35
    "4.8 Calibrated Airspeed
    ... The gauge is “calibrated” to read true airspeed at sea level standard
    pressure and temperature."

    Furthermore all the references regarding compressibility correction I have found indicate compressibility correction is 0 for sea level (and standard atmospheric conditions to be precise).


  5. #5


    Last edited by mgh; 12-24-2010 at 10:54 AM.

  6. #6



    you have exactly figured out how MS is calculating their "IAS". That helped a lot to understand what's going on.

    Here's how FS is calculating "IAS"; unfortunately it's wrong.

    "IAS FS" = sqrt(RHOa/RHOssl) * sqrt(Qc/Q) * TAS = sqrt(Pa/Pssl * Tssl/Ta) * sqrt(1+M^2/4+M^4/40) * M * Aa

    I suppose they thought about a density correction term (sqrt(RHOa/RHOssl), which is correct if assuming air is incompressible) and a compressibility correction term (sqrt(Qc/Q), which is simply false...).

    -> "IAS FS" = "CAS FS" = sqrt(Pa/Pssl) * sqrt(1+M^2/4+M^4/40) * M * Assl

    a: ambient
    ssl: standard atmosphere, sea level
    RHOa: ambient density
    RHOssl: standard sea level density
    Qc: impact pressure (taking into account compressibility)
    Q: dynamic pressure (assuming air is incompressible)
    P: pressure
    T: temperature
    A: speed of sound

    This equation exactly fits the "IAS"/TAS test data in FS2004.
    However, this equation does not give the correct CAS/IAS !
    As an illustration, even at sea level and standard atmospheric conditions "IAS FS" > TAS. This is plain wrong!

    Correct relation between TAS (=M*Aa) and CAS is the following:

    CAS = sqrt( ( Pa/Pssl * ( (1 + 0.2*M^2)^3.5 - 1 ) + 1 )^(1/3.5) - 1 ) / 0.2 ) * Assl

    (valid for M<1)

    Derived from equations 2.23 and 2.30 in Naval Test Pilot School Flight Test Manual USNTPS-FTM-No. 108 FIXED WING PERFORMANCE Gerald L. Gallagher et al.

    Sanity check: At sea level and standard atmospheric conditions with this equation CAS = TAS, as it should be.

    Strange that Microsoft made this seemingly stupid error (the equations are out there in the literature, and simple ones at that)...
    Last edited by teson1; 12-28-2010 at 03:50 PM.

  7. #7
    Join Date
    Mar 2005
    Seattle, USA.


    guess i need to delete my FS now and sell all addon and hardware ... the game is worthless ... lol

    D.Scobie - new 'rig' is Core2 Duo E8400 at 3GHz w/2GB, Win. XP w/SP3, nVIDIA 9600GT 512MB

  8. #8

  9. #9


    Quote Originally Posted by wdscobie View Post
    guess i need to delete my FS now and sell all addon and hardware ... the game is worthless ... lol
    Great idea! What addons do you have to sell?

    It does have an impact in rare circumstances only that's for sure.
    There's only any significant deviation above about 300 KIAS. So limited to fast flying subsonic (flown at pretty low altitudes), and supersonic aircraft.
    In particular for landing and stalls, where correct indication of IAS is of highest importance, there is no issue due to the low speeds during these phases.

    However, there are some situations where the wrong indication of IAS may have an impact on the sim experience though:

    - Calculating TAS from IAS with the E6-B for dead-reckoning navigation yields false results at speeds above about 300 KIAS.
    That's actually how I first experienced and got bothered by the false indication of IAS. Since some time I've become a buff for pioneer style, non-radio navigation, using almost exclusively pilotage, dead reckoning (or celestial navigation) on my flights, and the E6-B for calculating flight parameters. On a recent flight around the pacific rim with the F-86 by dead reckoning, the calculation of TAS from IAS never provided reasonable flight speeds, whereas results were perfect in previous similar trip with slower plane.

    - For fast planes at low altitudes, where the barberpole is at Vne (limited by IAS, and not limited by Mach) the maximum attainable flight speed (TAS, GS) in the sim is lower than it should be, as IAS is indicated too high.

    - There may be other situations where there may be a small impact on the sim experience or the way somebody flies.

    And hey, it's a primary flight instrument!

  10. #10


    You can adjust it in the aircraft.cfg, for example:
    airspeed_indicator.0=0.9827, -3.0

    How it performs on altitude is to find out.

    // ADD to aircraft.cfg so IAS reads same as in FS2K2 !
    [airspeed_indicators] //IAS adjustment parameters
    airspeed_indicator.0 = 0.9826, 0.0 //Appropriate values for Jet AC

    These values will change '255 kts' IAS at Mach 0.745, FL 350
    to '250 kts'. Which is essentially CAS

    airspeed_indicator.0 = 1.111, -10.0 //Appropriate for C182, C208, etc.
    IAS displays 100 kts at CAS = 100 kts

    The "1.111" is equivalent to the "Weighting Factor" scaled integer
    In REC 1101 of the AIR file. The "-10.0" is the IAS "Base Offset"
    Last edited by Johan_Dees; 12-29-2010 at 03:55 AM.

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