Now, let's talk about the airspeed indicator and the corrections we apply to… — Page 109, Lesson 102

I want to walk you through the Navigation Computer and how we use it for TAS and altitude conversions. This is a core skill for any professional pilot, so let's take it step by step. First, let me outline what the Navigation Computer can do for us in this area. It handles five specific functions: calculating True Airspeed from Calibrated Airspeed, correcting for compressibility error, calculating True Airspeed from Mach Number, calculating True Altitude from Indicated Altitude, and calculating Density Altitude from Pressure Altitude. Now, let's talk about the airspeed indicator and the corrections we apply to it. What you see directly on the instrument is called Indicated Airspeed, or IAS for short. That's the raw reading straight off the dial. But IAS isn't the final value we work with. We have to apply corrections. The first two corrections are Instrument Error and Pressure Error — sometimes called Position Error. When you apply both of these to IAS, you get Calibrated Airspeed, or CAS. This is the modern, generally accepted term. However, you should know that many older documents use the term Rectified Airspeed, or RAS, for the same thing. So CAS and RAS are essentially the same value. In practice, we don't have separate correction cards for Instrument Error and Pressure Error. Instead, it's simpler to combine both corrections onto a single card that sits beside the instrument. So when you read IAS, you go straight to that correction card, apply both Instrument and Pressure Error together, and that gives you CAS. Now, strictly speaking, there's another step. After you have CAS, you should correct it for compressibility error to get Equivalent Airspeed, or EAS. Why do we need this? Because the airspeed indicator is calibrated for a specific air density — 1225 grams per cubic metre. That's the density of air in the ISA atmosphere at Mean Sea Level. But in reality, the air density is rarely exactly that value. The compressibility correction accounts for that difference. However, here's the practical reality for you as an airline pilot: you are unlikely to deal with EAS. It's rarely encountered outside scientific flying and test flying. The ASI is already calibrated to allow for compressibility at ISA at Mean Sea Level, and under those conditions, no compressibility correction is needed. Let me show you the breakdown visually. This figure shows the order in which these corrections are applied — from IAS, through Instrument and Pressure Error to CAS, then through compressibility error to EAS. So the key takeaway for your daily flying: you start with IAS, apply the correction card to get CAS, and from there you can go directly to calculating True Airspeed using the Navigation Computer. We'll cover exactly how to do that calculation next.