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

I want to walk you through the correction of compressibility error on the navigation computer. This is an important refinement to the TAS calculation we've been doing. First, let's be clear about what compressibility error is. The airspeed indicator, or ASI, is calibrated to allow for compressibility effects at one specific atmospheric condition: an air density of 1225 grams per cubic metre. That's exactly the density of the International Standard Atmosphere at mean sea level. But here's the thing — we almost never fly at that exact density. At any other density, which is nearly always the case in real operations, compressibility error must be corrected. Now, when does this correction actually matter? Compressibility correction is insignificant at airspeeds less than 300 knots TAS. So if your initial TAS calculation comes out below 300 knots, you don't need to worry about it. But you must apply the correction if that initial calculation gives you a TAS of more than 300 knots. Let me walk you through a worked example so you can see exactly how this works on the CRP-5 computer. We have: Flight Level 350, COAT — that's Corrected Outside Air Temperature — of minus 47 degrees Celsius, and a CAS of 280 knots. We want to find the true TAS. The first step is to calculate TAS from CAS in the usual way — the same method you've already been using. In the AIRSPEED window on the computer, you set Flight Level 350 against minus 47 degrees Celsius. Then you find 280 knots CAS on the inner scale. Now read off the value on the outer scale — that gives you 500 knots TAS. But wait — 500 knots is above 300 knots, so we need to apply the compressibility correction. The example shows you the initial calculation, and then the correction would follow. The figures referenced — Figure 6.8 showing FL370 against minus 47 degrees Celsius, and Figure 6.9 showing 280 KCAS against 500 KTAS — illustrate these settings on the computer. So the key takeaway here is a two-step process. First, do your normal TAS calculation. Second, check if the result exceeds 300 knots TAS. If it does, you must then apply the compressibility correction. If it's below 300 knots, you're fine to use that TAS value directly.