We're starting a new chapter now, and it's a practical one — Page 109, Lesson 102

We're starting a new chapter now, and it's a practical one. We're moving into the Navigation Computer, and specifically how we use it for TAS and altitude conversions. This is where the theory of airspeed errors turns into the actual numbers you'll compute in the cockpit. Let me set the scene. The Navigation Computer, your flight computer, isn't just for speed-distance-time problems. In this chapter, we're using it for five specific functions: calculating TAS from CAS, correcting compressibility error, calculating TAS from Mach Number, calculating True Altitude from Indicated Altitude, and calculating Density Altitude from Pressure Altitude. Before we get to the computer itself, we need to nail down the correction chain for the airspeed indicator. This is the foundation. What you see on the instrument face is called Indicated Airspeed, or IAS. That's the raw reading. Now, to get from IAS to something usable, we apply two corrections. First, Instrument Error, which is the mechanical imperfection of the gauge itself. Second, Pressure Error, which is also called Position Error. That's the error caused by the static port sensing the wrong pressure because of the airflow around the aircraft. When you apply both of those to IAS, you get Calibrated Airspeed, or CAS. Now, a note on terminology. CAS is the modern, generally accepted term. But you will see older documents use Rectified Airspeed, or RAS. Same thing. Don't let that trip you up. In practice, we don't have two separate correction cards. We combine Instrument Error and Pressure Error onto a single correction card, which is usually stuck right beside the instrument. So you read IAS, apply the combined correction from that card, and you get CAS. Now, strictly speaking, the chain doesn't end there. CAS is then corrected for compressibility error to give Equivalent Airspeed, or EAS. This correction exists because the airspeed indicator is calibrated for a specific air density: 1225 grams per cubic metre. That's the density of the ISA atmosphere at Mean Sea Level. But in reality, the air density is rarely exactly that value, so compressibility effects change the reading. However, as an airline pilot, you are very unlikely to deal with EAS. It's rarely encountered outside scientific and test flying. The ASI is already calibrated to allow for compressibility at ISA at MSL, so under those specific conditions, there's no compressibility error to worry about. So the practical chain for you is: IAS, corrected by the card for Instrument and Pressure Error, gives you CAS. And CAS is what you'll feed into the computer to get TAS. Let's look at the diagram that lays this whole sequence out. That's the breakdown of the corrections and the order they're applied. Keep that order in mind: IAS, then instrument and pressure error, to CAS, then compressibility to EAS. For your daily flying, you'll live in the IAS-to-CAS world.