Let’s pick this up right where the correction card comes in — Page 39, Lesson 50

Let’s pick this up right where the correction card comes in. You’ve got the IAS — that’s the raw number you read straight off the airspeed indicator, before any corrections. Now, the correction card on the instrument panel gives you the combined correction for Instrument Error and Position Error. When you apply that correction to IAS, you get Calibrated Airspeed — CAS. Here’s the practical example. Say the correction card says the combined correction is plus 2 knots. If you want to fly at 100 knots CAS, you don’t set 100 on the dial. You fly at 98 knots IAS, because 98 plus the 2-knot correction gives you your 100 knots CAS. So the correction is added to IAS to get CAS. Now, a bit of terminology history. In some older publications, and on the Pooley CRP-5 Navigation Computer, Calibrated Airspeed is called Rectified Airspeed — RAS. That term is no longer in general use, but you’ll still see it on that computer, so don’t be surprised by it. Let me give you the precise definition. CAS is IAS corrected for Instrument and Position Error. Position Error is sometimes called Pressure Error — same thing, the error caused by the static pressure at the static ports not being exactly the ambient pressure, especially at angle of attack or sideslip. Now, here’s the deeper reason CAS matters, and this is the heart of the matter. Aircraft stress limits and stall limits are determined by dynamic pressure, not by speed. Dynamic pressure is the pressure you feel from the air rushing past — it’s what bends the wings and what keeps the airflow attached over the wing. The quoted limiting “speeds” — the V-speeds for overstress, for flap extension, for undercarriage — those are really functions of dynamic pressure. As pressures, they could be quoted in bars, psi, or pascals. They are not speeds. But here’s the clever part. The airspeed indicator is calibrated in knots, even though knots is only a true speed at one specific air density. So we measure dynamic pressure in knots of CAS. And that’s why CAS is equally valid as a measure of stall or stress limits, no matter what the aircraft’s true airspeed is, no matter what altitude you’re at, and no matter what the air density is. True airspeed changes with density, but CAS — as a measure of dynamic pressure — stays tied to the aerodynamic limits. So, what does the display actually show you? A simple airspeed indicator displays IAS. An Air Data Computer displays IAS, or, if it has a computed correction for Instrument and Pressure Error built in, it displays CAS. That’s the whole point — the display is arranged so that the pilot sees a value that directly relates to the aerodynamic limits of the aircraft. Let me just tie it together. IAS is what you read. Apply the correction card — Instrument and Position Error — and you get CAS. CAS is your measure of dynamic pressure, and that’s what governs stall and stress limits. That’s why we bother with the correction at all.