The Airspeed Indicator (ASI) — Page 49, Lesson 51

I want to walk you through the Airspeed Indicator now, and we're going to build this up carefully because the ASI is where a lot of the air data concepts start to come together. Let me start with the big picture. The whole point of the airspeed system is to let pilots fly their aircraft safely without overstressing or stalling them. That's the fundamental job. And I want you to notice something important right away: the modern airliners you'll fly don't actually do this with a simple mechanical gauge anymore. Air Data Computers calculate and indicate speed, altitude, temperature, and other air parameters. So the ASI you see on the panel is really the output of a computer system, but the principles we're about to cover are exactly what that computer is doing internally. Now, here's a key distinction I need you to grasp. For navigation and flight planning, the Calibrated Airspeed — the CAS — is of no significant use. Think about why. CAS is a measure of what the air is doing to the aircraft, but when you're navigating, what you actually need is the speed of the aircraft relative to the ground. That's the True Airspeed, the TAS. And I'll add a qualifier: in the absence of any wind, the speed relative to the ground is known as the True Airspeed. The wind effect itself is discussed in General Navigation, book 10, so we won't go deep into that here, but you need to know that TAS is your ground-referenced speed only when there's no wind. Let me now introduce you to a speed you may not have met before: Equivalent Airspeed, or EAS. Here's the relationship I want you to lock in. Whenever Density Error is present, Compressibility Error exists. Those two errors travel together. Now, it is possible to correct CAS just for Compressibility Error without correcting for Density Error. When you do that — when you take CAS and remove only the compressibility error — the resultant dynamic pressure is called Equivalent Airspeed. So the precise definition: Equivalent Airspeed is CAS corrected for Compressibility Error only. Let me unpack what that means physically, because this is the heart of it. EAS is the most accurate value of dynamic pressure. It has been corrected for three things: Instrument error, Pressure error, and Compressibility error. All of those are forms of measurement error. So when you strip those out, what you're left with is the most accurate measure of the dynamic pressure over the wing. That's why EAS matters so much — it's telling you the true aerodynamic loading on the wing. Now, in practice, the difference between EAS and CAS is not great unless altitude becomes significant. But here's where it gets operationally important. At high elevation airports — think places like Mexico City or Denver — particularly with high-performance airliners that have high take-off and landing speeds, the CAS, and therefore the IAS, is higher for the same EAS. Let me make sure you understand that direction. At a high-altitude airport, the air is less dense, so to generate the same dynamic pressure over the wing, the aircraft has to move faster through that thin air. So the indicated speed reads higher for the same actual aerodynamic load. And this leads to one of the most beautiful, clean rules in all of aviation: at constant weight, regardless of altitude, an aircraft always lifts off at a constant EAS. Think about what that means. The wing needs a certain dynamic pressure to generate enough lift to get the aircraft off the ground. That dynamic pressure is EAS. So no matter how high the airport is, at a given weight, the EAS at lift-off is always the same. The IAS will read differently at altitude, but the EAS — the true aerodynamic condition — is constant. Here's the final piece of the puzzle, and it's a really elegant design philosophy. All limit speeds are calculated from EAS. The engineers work out the structural and aerodynamic limits in terms of EAS, because that's the true measure of the loads. Then, the errors are re-introduced to display the speeds as IAS. So the pilot's instruments show IAS, which is what the pilot actually flies, but the underlying limits that those markings represent were all derived from EAS. That's why the speed limits on your ASI are valid — they've been converted back from the true aerodynamic values. Now, let me bring in True Airspeed properly, because we touched on it but we need the full definition. TAS involves the Density Error. Unless the air round the aircraft is at the calibration density of 1225 grams per cubic metre — and I want you to remember that number, 1225 grams per cubic metre, which can only occur near sea level — the ASI is going to be affected. That calibration density is the standard sea-level air density that the instrument is designed around. When the air isn't at that density, which is almost always the case once you climb, you get a density error, and correcting for that density error is what takes you from EAS to TAS. So let me give you the complete chain, because this is the mental model you need to carry. You start with the raw indicated airspeed, IAS, which has instrument error. Correct that and you get CAS. Correct CAS for compressibility error and you get EAS. Correct EAS for density error and you get TAS. Each step removes one more layer of error, and each speed has its own job: IAS is what you fly, CAS is the calibrated value, EAS is the true dynamic pressure over the wing, and TAS is your speed relative to the air mass, which becomes your ground speed when there's no wind. And I want to show you the coloured arcs on the ASI, because this ties the theory to what you'll actually see on the panel. The Yellow Arc denotes the caution range. That's the range of speeds where you need to be careful — you're above the normal operating range but below the never-exceed speed. The coloured arcs are the practical expression of all those EAS-derived limits we just talked about, displayed to you as IAS so you can fly them directly. So the takeaway from this whole picture: the ASI isn't just a gauge telling you how fast you're going. It's the end product of a chain of corrections, each one removing a specific error, and each speed in that chain — IAS, CAS, EAS, TAS — has a distinct and important role in how you fly and how the aircraft is designed.