The Atmosphere — Page 38, Lesson 43

Let's pick up right where the airspeed story gets interesting. We've established that the airspeed indicator is really a pressure gauge, and its needle responds to changes in dynamic pressure, which we write as ½ ρ V². That's half times rho times V squared — rho being air density, V being the true speed of the aircraft through the air. Now, here's the catch that defines everything else. The instrument is calibrated assuming standard sea level density, which is 1.225 kilograms per cubic metre. But you're almost never flying at standard sea level conditions. So the speed the needle shows will be different from your actual speed through the air, unless you happen to be in those exact standard conditions, which is unlikely. Let me give you the precise names for these two speeds, because they're the backbone of this whole topic. The actual speed of the aircraft relative to the free stream — that is, relative to the undisturbed air ahead of it — is called true airspeed, abbreviated TAS, and we denote it with the symbol V. That's the V in our dynamic pressure formula. Now, the speed recorded by the airspeed indicator, calibrated as we described, assuming no other errors exist, is called equivalent airspeed, abbreviated EAS. So EAS is what the instrument reads when the only difference between reality and calibration is the density of the air you're flying through. You might think it's a drawback that the instrument gives you equivalent rather than true airspeed. But here's why it's actually a feature. You can always work out true airspeed from equivalent airspeed. More importantly, many of the handling characteristics of an aircraft — things like stall behaviour, control feel, structural limits — depend mainly on dynamic pressure, which is exactly what equivalent airspeed reflects. So having a direct reading of EAS is often more useful than having TAS. Now, the instrument isn't perfect even beyond that density effect. It's subject to other errors, and we need to name them properly. First, instrument error. This arises from imperfections in the design and manufacture of the instrument itself, and it varies from one instrument to another. In modern practice this error is usually very small and for all practical purposes can be disregarded. But where any instrument error does exist, it's incorporated into the calibrated airspeed correction chart for that particular aeroplane. So the correction is handled on the chart, not by you mentally. Second, position error, also called pressure error. And this is where the excerpt cuts off, so let me set it up for you. This error comes from the fact that the static pressure — the ambient pressure the instrument uses as a reference — may not be exactly the true free stream static pressure at the point where it's sensed on the aircraft. The airflow around the aircraft disturbs the pressure field, and that disturbance creates this error. We'll pick up right there with the details of how it works and how it's corrected.