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TAS with given ALT and CAS

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  • TAS with given ALT and CAS

    Hi guys,

    As I work in an airline, I need to perform an excel file automated and to find a TAS speed with a given altitude and a given CAS (like this link , EAS/TAS/CAS conversions )

    Could you please help by given me the formula in order to put in Excel ?

    Thanks for your help

  • #2
    Dear LFPO26,

    **EDITED: This is only valid for low speed and low level flight. see further response on following post.

    The only thing you need to calculate TAS from CAS is density at the height you are interested at. Dividing your CAS by the square root of the relative density at that height you will get TAS.

    If you are only looking for TAS from CAS, you can use the following (this is only valid at ISA conditions and for altitudes bellow 11000 m):

    a) Calculate relative temperature Rt:

    Rt = 1 - 22,557 * (h *10- 6); where h is the altitude in meters

    b) Once Rt is calculated, calculate Relative density at that height from the temperature:

    Rd = Rt 4,256; where Rd is relative density

    c) Once you have relative density, it is easy to calculate TAS, as:

    TAS = CAS / SqRoot (Rd);


    - For relative density calculation: Aerodin
    Last edited by FreePilot; 26-11-2018, 22:00.


    • #3
      Unfortunately the previous poster is wrong. It is correct to state:

      TAS = EAS / SqRoot (Rd)

      However the calculation of TAS from CAS is more complex due to compressibility error.

      I have made up a Excel spreadsheet which does all of the air data conversion calculations (EAS / CAS / TAS / Mach number / SAT / TAT) up to FL 500. I have given this free of charge to ATPL students.

      I would be happy to help your airline with this project, but for commercial work I would have to charge a fee. Please PM me if interested.


      • #4
        Dear Ortac,

        You are absolutely right, reviewing the references, I found *Assuming IAS=EAS. thus, my statement is only valid for low altitude and low level flight and wrong for general cases.

        I do apologise if this lead to further confusion to the community.

        Thanks for your correction!

        I will try to be more careful next time.

        Kind regards,