We're starting a brand-new chapter now — Chapter 4, The Airspeed Indicator… — Page 39, Lesson 46

We're starting a brand-new chapter now — Chapter 4, The Airspeed Indicator, the ASI. This is one of the six basic flight instruments, and it's the one that tells you how fast the aeroplane is moving through the air. Let me give you the roadmap of what this chapter covers, because the structure itself teaches you how a professional thinks about this instrument. First, we open with the principle of operation — how the instrument actually works. Then construction — what's physically inside the case. Then calibration — how we make the readings meaningful. After that come the definitions and errors, and this is the heart of the chapter: we define four different airspeeds, each one a step in a chain of corrections. We start with Indicated Airspeed, correct it for instrument and installation errors to get Calibrated Airspeed, CAS. Then we correct CAS for compressibility to get Equivalent Airspeed, EAS. Then we correct EAS for density altitude to get True Airspeed, TAS. That's the whole ladder, and you'll need every rung. Then we apply those corrections in practice, we look at the colour coding on the dial — the white, green, yellow, and red arcs that mark the operating limits. After that, the failure modes: pitot and static blockages, then pitot and static leaks. These are the things that can go wrong in flight, and you must know what the instrument does when they fail. Finally, serviceability checks — how you confirm the instrument is healthy before you trust it. The chapter closes with practice questions and answers. Now, before we dive into the principle of operation, I want you to hold onto one idea: the ASI is a pressure instrument. It doesn't measure speed directly. It measures the difference between two air pressures — the pitot pressure and the static pressure — and it converts that pressure difference into a speed reading. Everything in this chapter, every error, every blockage, every leak, comes back to that one fact. So when we start with the principle of operation, that's exactly where we'll begin.