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IAS vs. CAS Stall Speeds

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Aerodynamics

While reviewing the POH for a Cessna 172R, I noticed that the stall speed for the most rearward center of gravity with full full flaps is 33 KIAS and 47 KCAS.  Why is there a 14 knot difference between the two airspeeds?  Is it because not all of the airflow is flowing directly into the pitot tube during a higher angle of attack (stall)?  I would think that would result in a higher calibrated airspeed, but I have been receiving different answers from multiple pilots. 

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1 Answers



  1. Skyfox on Sep 22, 2016

    The difference between IAS and CAS varies greatly from level cruise to slow stall speeds/configurations. CAS is a correction for installation and instrument errors, so there will be more errors (for most airplanes) when the attitude is nose high and slow and that is why the CAS is so much higher than the IAS in your example. Here’s a paragraph pasted from the Pilot’s Handbook of Aeronautical Knowledge:

    “Calibrated airspeed (CAS)—IAS corrected for installation error and instrument error. Although manufacturers attempt to keep airspeed errors to a minimum, it is not possible to eliminate all errors throughout the airspeed operating range. At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds. In the cruising and higher airspeed ranges, IAS and CAS are approximately the same. Refer to the airspeed calibration chart to correct for possible airspeed errors.”

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