The Atmosphere — Page 38, Lesson 45

Right, let's pick this up. We've just finished looking at how the airspeed indicator works, and now I want to show you how all those different speed values you'll see in the cockpit actually relate to each other. This is where we tie the theory of dynamic pressure to the practical speeds you'll fly. So, we established that the airspeed indicator is really just a pressure gauge. It measures dynamic pressure, which we write as Q, and that's equal to half rho times V squared — Q = ½ ρ V². Now, the raw reading you get from that instrument, before any corrections, is your indicated airspeed, your IAS. But here's the key point: when you correct that indicated airspeed for instrument errors, for position errors — that's the pressure error from where the static port is mounted — and for compressibility errors at high speed, you arrive at what's called the equivalent airspeed, the EAS. And the EAS is literally defined as the speed that gives you that same dynamic pressure, ½ ρ V². So the EAS is the speed that corresponds to the actual dynamic pressure the aircraft feels. Now, let's talk about V speeds. You're going to see a whole alphabet of them: VS, which is the stall speed; V1, the take-off decision speed; VR, rotation speed; V2, take-off safety speed; VMD, minimum drag speed; VMC, minimum control speed; VYSE, which is the best rate-of-climb speed with one engine inoperative — and many others. Here's the crucial thing: all of these V speeds are calibrated airspeeds, CAS. Why? Because they relate to aircraft operations at low speed. At low speed, compressibility isn't a factor, so the corrections are straightforward. The appropriate corrections are made, and these speeds are supplied to the pilot in the Flight Manual as IAS. So when you open your Flight Manual and see V1 or VR listed, you're reading them as indicated airspeeds. But there's one exception, and it's important. VMO — the maximum operating IAS — is actually an EAS, an equivalent airspeed. It's a high speed, so compressibility effects become significant, and that's why it's treated differently. But again, even though it's technically an EAS, it's supplied to the pilot in the Flight Manual as an IAS. So the pattern is: all V speeds are CAS except VMO which is EAS, but every single one of them is presented to you in the Flight Manual as an IAS. Now let me give you the summary of how this all hangs together, because this is the part that really matters for your understanding of the atmosphere's effect on flight. Dynamic pressure, Q, is affected by changes in air density. And air density itself — let's trace what changes it. Air density decreases if atmospheric pressure decreases. Air density decreases if air temperature increases. And air density decreases if humidity increases. So three things reduce density: lower pressure, higher temperature, higher humidity. Now, picture the aircraft on the ground, ready for take-off. If you're taking off from an airfield with low atmospheric pressure, and/or high air temperature, and/or high humidity — all of which mean low density — then you will require a higher true airspeed, a higher TAS, to achieve the same dynamic pressure, the same IAS. Think about it: Q = ½ ρ V². If rho is small, you need a bigger V to get the same Q. So on a hot, high, humid day, your true airspeed on take-off will be higher for the same indicated airspeed. For general understanding, here's the golden rule: a constant IAS will give you constant dynamic pressure. That's the whole point of the instrument — it's telling you the dynamic pressure, not the true speed. Now, with the aircraft airborne, as you climb, altitude increases and air density decreases because of decreasing static pressure. So as altitude increases, a higher TAS is required to maintain that constant dynamic pressure. And here's the beautiful thing: maintaining a constant IAS will compensate for changes in air density. That's why you fly IAS — it automatically accounts for the density changes, so the aerodynamic behaviour of the aircraft stays predictable. And finally, the most important conceptual point of all: there is only one speed, and that's the speed of the aircraft through the air — the true airspeed, the TAS. All the other so-called speeds — IAS, CAS, EAS — they are pressures. They're pressure readings, not true speeds. The airspeed indicator is a pressure gauge, and it's measuring dynamic pressure. So when you see a speed on that instrument, remember: it's a pressure, and the only real speed is TAS. So to tie it all together: aircraft V speeds are CAS, except VMO which is an EAS, but all of them are presented to the pilot in the Flight Manual as IAS. That's the complete picture of how the speed values relate to each other and to the atmosphere.