The Navigation Computer - TAS and Altitude Conversions — Page 109, Lesson 109

Let’s pick up with the compressibility correction on the navigation computer. This is the step that refines your TAS calculation once you’ve already got a basic TAS from the airspeed window. Here’s the core idea. Your airspeed indicator, the ASI, is calibrated to allow for compressibility at one specific atmospheric density: 1225 grams per cubic metre. That’s the density of the International Standard Atmosphere at mean sea level. But in practice, you’re almost never flying at that exact density. At any other density — which is nearly always — compressibility error must be corrected. Now, when is this correction actually significant? The book is clear: compressibility correction is insignificant at airspeeds less than 300 knots TAS. But you must apply it if your initial calculation gives a TAS of more than 300 knots. So the trigger is the TAS you get from the first step, not your CAS. Let me walk you through the example. Flight Level 350, COAT is -47°C, and your CAS is 280 knots. What’s the TAS? First, you calculate TAS from CAS in the usual way. That means you go to the AIRSPEED window on the CRP-5. You set FL350 against -47°C. Then you find 280 knots CAS on the inner scale. Reading off the outer scale, you get 500 knots TAS. Now here’s the key point. That 500 knots TAS is above 300 knots, so compressibility correction must be applied. The book shows you the setup in the figures — Figure 6.8 shows FL370 against -47°C, and Figure 6.9 shows 280 KCAS against 500 KTAS. So you’re aligning the flight level with the temperature, then reading CAS on the inner scale against TAS on the outer scale. The important thing to remember is the threshold: below 300 knots TAS, you don’t bother with the correction. Above that, you do. And the correction is applied after you’ve done the basic TAS calculation, not before. So the sequence is: get your initial TAS from the airspeed window, check if it exceeds 300 knots, and if it does, apply the compressibility correction. That’s the whole point of this step — it’s the refinement that makes your TAS accurate at high speeds and high altitudes where compressibility really starts to bite.