Let me set the scene — Page 38, Lesson 44

I want to walk you through the airspeed indicator errors now — this is the part where we stop treating the ASI as a perfect instrument and start treating it like the real, imperfect device it is. Let me set the scene. The airspeed indicator measures dynamic pressure — the pressure you feel from the air rushing past — and converts it into a speed reading. But that reading is only as good as the pressures it's fed. And those pressures are corrupted by several distinct errors. We've just covered the instrument error, which is the mechanical imperfection inside the instrument itself. Now I want to show you the two big ones that come from outside the instrument — from the aeroplane and from the air itself. First, the Position Error, which is also called the Pressure Error. This one splits into two kinds. One kind relates to the static pressure measurement, and the other relates to the pitot pressure measurement — that's the total pressure, the pressure you get from the pitot tube facing directly into the airstream. Here's the problem. The pitot tube and the static port are mounted somewhere on the aircraft — they're not floating in free air. And the flow at that mounting position is affected by the presence of the aircraft itself. It's also affected by changes in configuration — that means flaps, and possibly the landing gear — and by proximity to the ground, which we call ground effect. So if the static port is sitting in disturbed flow, the static pressure it records is the local value, not the free stream value. That's a crucial distinction: local pressure versus free stream pressure. The free stream is the undisturbed air far ahead of the aeroplane; the local value is what's actually happening right at the port, which may be wrong. Now the pitot side. The pitot pressure can be under-recorded because of incorrect alignment. The pitot tube is supposed to face directly into the airstream. But if the tube is inclined to the airstream instead — and that happens with changes in angle of attack, particularly at low speeds — then it doesn't capture the full total pressure. It under-records. And here's the key relationship to remember: the magnitude of these errors generally depends on the angle of attack, and hence on the speed of the aircraft. So the error isn't a fixed number — it changes as the aeroplane changes speed and attitude. Now the second big error: Compressibility Error. This one appears at high speeds. The dynamic pressure — the pressure that the ASI actually senses — is normally given by the formula ½ ρ V². That's one half times rho, the air density, times V squared, the velocity squared. But at high speeds, the dynamic pressure is not simply ½ ρ V². It exceeds that value, by a factor determined by the Mach number. Mach number is the ratio of the aircraft's speed to the speed of sound. So because the dynamic pressure is higher than the simple formula predicts, the airspeed indicator will over-read. It shows a higher speed than is actually there. So now, because of all these errors — instrument, position, compressibility — the speed recorded on the airspeed indicator is generally not the equivalent airspeed. Instead, it's given a different name: the indicated airspeed. That's the raw reading you see on the dial, before any corrections. Now, how do we fix this? The corrections to rectify the instrument error and the position error are determined experimentally. In flight, using special instruments, measurements are taken over the whole range of speeds and configurations. From all those measurements, a calibration curve is obtained. That curve gives the corrections appropriate to each indicated airspeed. And this is where your excerpt connects to the title of this section — that calibration data is incorporated in the calibrated airspeed correction chart for the particular aeroplane. Every aeroplane type has its own chart, because every aeroplane has its own position error characteristics. The compressibility error correction, though, is different — it may be obtained by calculation, rather than by flight testing. Because compressibility is a function of Mach number, it can be computed. So let me pull it together. You have three errors: instrument error, inside the dial; position error, from the mounting of the pitot and static ports in disturbed flow; and compressibility error, from the high-speed breakdown of the simple dynamic pressure formula. The raw reading is indicated airspeed. Apply the instrument and position corrections from the calibration chart, apply the compressibility correction by calculation, and you arrive at the corrected speed. That's the chain you'll be working with every time you read an airspeed indicator in the cockpit.