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

Let’s pick this up right where the correction card leaves off. You’ve just taken the raw Indicated Airspeed, or IAS, and applied the correction card to remove Instrument Error and Position Error. The result of that correction is a new, more accurate speed value, and that value has a very specific name: Calibrated Airspeed, or CAS. Let me give you the exact definition, because it’s a cornerstone of everything we do with the airspeed indicator. Calibrated Airspeed is IAS corrected for Instrument and Position (Pressure) Error. So the correction card you’re reading in the cockpit isn’t just a random table—it’s the tool that converts your IAS into CAS. Here’s a concrete example to make it stick. Suppose the correction card tells you there’s a combined correction of plus 2 knots. If you, as the pilot, want to fly at 100 knots CAS, you don’t set the ASI to read 100. You set it to read 98 knots IAS, because 98 plus the 2-knot correction gives you exactly 100 knots CAS. The instrument shows IAS; the correction card bridges the gap to CAS. Now, a bit of history, because you’ll see this in older documents and on the Pooley CRP-5 Navigation Computer. Calibrated Airspeed is also called Rectified Airspeed, abbreviated RAS. That term appears in older publications, but I want you to know it’s no longer in general use. So if you see RAS on an old chart or on the CRP-5, understand it means the same thing as CAS. But in modern practice, we say CAS. Why does this matter so much? Here’s the critical concept: aircraft stress limits and stall limits are determined by dynamic pressure, not by speed itself. Let me unpack that. Dynamic pressure is the pressure you feel from the air rushing past the aircraft—it’s what actually bends the wings, loads the flaps, and causes a stall. The book gives you quoted limiting “speeds” to avoid overstressing the aircraft as a whole, and also to avoid overstressing specific services like flaps and undercarriage. There are also stall “speeds” for various configurations, like flaps down or gear down. But here’s the subtle point: these are all functions of dynamic pressure. As pressures, they could be quoted in bars, psi, or pascals. They are not true speeds in the everyday sense. The reason we call them “speeds” is that the ASI is calibrated in knots. And that calibration is only a true speed for one specific air density. So, to make dynamic pressure measurable on a cockpit instrument, we express it in knots of CAS. And this is the beautiful part: 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. That’s why we use it as the reference for all those limits. True airspeed changes with altitude and temperature, but CAS, as a measure of dynamic pressure, stays a reliable reference for what the aircraft structure and aerodynamics actually feel. Now, how does this show up in the cockpit hardware? A simple ASI displays IAS. That’s it—just IAS. But an Air Data Computer, which is the more advanced system, can display IAS as well. However, if the Air Data Computer has a computed correction for Instrument and Pressure Error built in, then it displays CAS instead. The reason for this is exactly what we just discussed: displaying CAS allows the pilot to compare the reading directly against the stress and stall limits, which are quoted in CAS, without having to do the correction card math in the air. So to tie it all together: IAS is what the instrument shows raw. You correct it for Instrument and Position Error using the card, and you get CAS. CAS is the measure of dynamic pressure in knots, and it’s the value that governs your stall and stress limits regardless of altitude or density. The simple ASI gives you IAS; the Air Data Computer gives you CAS when it has that built-in correction. That’s the whole chain, and it’s the foundation for how you’ll interpret airspeed in every phase of flight.