We’re starting a brand-new chapter now: the Airspeed Indicator, the ASI — Page 39, Lesson 46

We’re starting a brand-new chapter now: the Airspeed Indicator, the ASI. This is one of the most important instruments on your panel, and it’s the one that tells you how fast you’re moving through the air. Let me walk you through the structure of this chapter first, because it’s laid out in a very logical order, and then we’ll dive into the first real topic. The chapter opens with the Principle of Operation — that’s the basic physics of how the instrument works. Then we move to Construction — what’s physically inside the case. After that comes Calibration, which is how the instrument is set up to read correctly. Then we get into the meat of it: ASI Definitions and Errors. This is where you’ll meet the key speed terms — Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), and True Airspeed (TAS) — and understand the differences between them. That’s followed by Airspeed Indicator Errors – Application of Corrections, which is about how you fix the readings for those errors. Then we have ASI Colour Coding — the coloured arcs on the dial face that tell you the operating limits. After that, the chapter covers Pitot and Static Blockages and Pitot and Static Leaks — these are the failure conditions you need to recognise. Finally, there’s Serviceability Checks — how you confirm the instrument is working before flight — and then the Questions and Answers at the end. Now, before we go any further, I want to make sure you’ve got the foundation. The ASI works on the difference between two pressures: pitot pressure and static pressure. Pitot pressure is the ram air pressure you get from the forward motion of the aircraft — it’s picked up by a tube facing into the airflow. Static pressure is the ambient atmospheric pressure around the aircraft, picked up from static ports. The difference between these two is what drives the indicator. That’s the principle of operation in a nutshell, and we’ll build on it as we go through the chapter. Let’s start with the Principle of Operation in detail. The ASI is essentially a pressure gauge that measures the difference between pitot pressure and static pressure. When the aircraft moves forward, the air rams into the pitot tube, increasing the pressure there. The static pressure stays at the ambient level. The greater your speed, the greater the pressure difference, and the further the needle moves. That’s the core idea — the instrument converts a pressure difference into a speed reading. Now, the Construction. Inside the instrument case, you have a diaphragm — a flexible capsule — that’s connected to the pitot pressure. The case itself is connected to the static pressure. So the diaphragm expands or contracts based on the pressure difference between the pitot and static sides. That movement is transmitted through a mechanical linkage to the pointer on the dial. The dial is calibrated so that the pointer position corresponds to a speed. That’s the construction — a pressure-sensing capsule, a linkage, and a pointer. Next, Calibration. The instrument is calibrated so that the pressure difference is converted into a speed reading. But here’s the key point: the calibration is based on a standard atmosphere — a specific set of conditions. The instrument assumes a particular air density at sea level. So the reading you get directly from the instrument is not necessarily your true speed through the air; it’s an indicated value that needs correction. That’s where the definitions come in. Let me introduce the first definition: Calibrated Airspeed (CAS). This is the indicated airspeed corrected for instrument and installation errors. In other words, the raw reading on the dial, adjusted for any errors in the instrument itself or in how the pitot and static systems are installed on the aircraft. CAS is what you get after those corrections are applied. Then we have Equivalent Airspeed (EAS). This is the CAS corrected for compressibility effects. At high speeds, the air compresses in front of the pitot tube, which can make the pressure difference read higher than it should. EAS accounts for that. So EAS is a more accurate representation of the dynamic pressure acting on the aircraft. Finally, True Airspeed (TAS). This is the EAS corrected for air density. At altitude, the air is less dense, so for the same dynamic pressure, your actual speed through the air is higher. TAS is your actual speed relative to the air mass. So the chain is: Indicated Airspeed → CAS → EAS → TAS, with each step correcting for a different error. Now, the Airspeed Indicator Errors – Application of Corrections. This is where you apply those corrections in practice. The instrument has inherent errors — position error from where the static ports are mounted, instrument error from the mechanical parts, and compressibility error at high speeds. You apply corrections to move from the indicated reading to CAS, then to EAS, then to TAS. Each correction is a step in the chain. Then we have ASI Colour Coding. The dial face has coloured arcs that mark the operating limits. The white arc is the flap operating range, the green arc is the normal operating range, the yellow arc is the caution range, and the red line is the never-exceed speed. These are critical for safe operation — you need to know what each colour means. Next, Pitot and Static Blockages. If the pitot tube gets blocked — say, by ice or debris — the pressure inside the diaphragm is trapped, and the ASI will behave in a specific way. If the static port gets blocked, the case pressure is trapped, and the reading will change with altitude. You need to recognise these failure modes because they affect the reading differently. Then Pitot and Static Leaks. A leak in the pitot line will cause the pressure to bleed off, giving a lower reading. A leak in the static line will cause the case pressure to change, affecting the reading in a different way. These are subtle failures that can be hard to detect, so the Serviceability Checks are important. Finally, the Serviceability Checks. Before flight, you check that the ASI is reading correctly — for example, that it reads zero on the ground, and that the pitot and static systems are clear and free of leaks. These checks ensure the instrument is serviceable before you rely on it. That’s the whole chapter laid out. We’ll go through each section in detail as we work through it. Let’s start with the principle of operation and construction in depth, because everything else builds on that foundation.