As can be seen in the correction chart, the difference between the IAS and CAS is greater at lower indicated airspeeds, however when the IAS increases, the difference is minimal. enter image description here

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    $\begingroup$ Only from the title on hot network questions, I though it was referring to close air support, with the answer being you have more time on target. :)) $\endgroup$
    – vsz
    Commented Feb 19 at 5:37
  • $\begingroup$ Especially since this is on HNQ (Hot Network Questions), could you please edit to define at least the less-well-known terms like CAS (in the question body; the title can be short)? I know that IAS is Indicated AirSpeed (and the question title does define it) but I don't remember coming across CAS. (or anyone else could edit, perhaps @vsz) I know I can just look it up, but so will multiple future readers. (Is IAS without instrument error the same as CAS? was in the "related questions" sidebar: Calibrated AirSpeed.) $\endgroup$ Commented Feb 19 at 15:23

1 Answer 1


Near stall speed the angle of attack is high and the pitot tube is pointed less into the relative wind. Thus it registers less pressure and shows a lower value.

It also makes sense to calibrate the airspeed indicator so it is most accurate at speeds in the middle of the envelope where you spend the vast majority of your flight time.

  • $\begingroup$ As a non-pilot, it seems to me that it would make the most sense to calibrate it for the most critical parts of flight - take off & landing, instead of cruise. Again, IANAP... $\endgroup$
    – FreeMan
    Commented Feb 19 at 18:09
  • $\begingroup$ @FreeMan As long as the airspeed indicates consistently in these phases (it does), you don't really care about the exact values. This calibration is mostly useful for determining your true airspeed for determining how long it will take you to get to your destination. Being off by 20% for a few seconds during the flare doesn't really make a significant difference. $\endgroup$
    – Chris
    Commented Feb 19 at 18:20
  • $\begingroup$ Ah, that makes sense. thanks! $\endgroup$
    – FreeMan
    Commented Feb 19 at 18:23
  • $\begingroup$ Therefore, the table shown in the question is technically valid only for 1g flight conditions. At 2g, for example, angle of attack would higher for a given speed. $\endgroup$
    – AeroAndy
    Commented Jun 5 at 2:52

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