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Compressibility effects at altitude?

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  • Compressibility effects at altitude?

    Can someone explain why "compressibility effects" with higher altitude increase the stall speed? I.e. what is the mechanism of action here? All the BGS explanations do not offer this detail. e.g. question 810578

  • #2
    The short answer is: It is because compressibility causes a discrepancy between IAS and EAS.
    IAS becomes a higher figure than EAS, as discussed in ATPdigital Instruments Lesson 5 page 8, and PoF Lesson 2 page 26.
    I will improve the explanation to QID 810578, along these lines:

    Stall speed (Vs) is the minimum IAS at which the pilot can maintain horizontal flight.
    Lift is created by dynamic pressure. The IAS displayed on our ASI is basically a measurement of dynamic pressure. But when Mach number has increased to a point when air starts to compress in the Pitot system, this measurement becomes less accurate, and the ASI starts to over-read.

    The result of this compressibility effect is that, as speed reduces, the pilot will observe a higher IAS reading at which the value of dynamic pressure required for level flight is no longer sufficient. In other words, the pilot will encounter the stall when his ASI is showing a higher IAS.

    The scientists require an accurate value of dynamic pressure for their calculations and apply the necessary corrections to produce EAS (Equivalent Airspeed).
    But pilots fly using IAS, not EAS.


    • #3
      Ben is right of cause, but that's only part of the reason.

      The other part is that the value of CLmax reduces with Mach number and at high altitude you stall at a higher Mach number. Even if you had your PFD speed tape programmed to read EAS there would still be an increase in indicated stall speed at high altitude, just less so than for CAS.

      Also remember that the difference between CAS and EAS is not caused by compressibility per se. It is due to the fact that at high altitude the calibration of a mechanical ASI does not correctly account for compressibility. At sea level there is no difference between CAS and EAS even at high Mach number.

      AFIK the Eclipse business jet is the only civil aircraft which displays EAS on its PFD. Ideally all jets would do so.