Unless the air round the aircraft is at the calibration density of 1225 grams — Page 49, Lesson 52

Let's pick this up right where the density question starts to bite. We've got the ASI showing us a speed, but I want you to understand exactly what that number means, because it is not your true airspeed. The whole story hinges on one number: the calibration density of 1225 grams per cubic metre. That is the density of air at sea level under the International Standard Atmosphere. The ASI is built and calibrated so that, at that density, its readings are correct. Now, here's the catch. That density of 1225 grams per cubic metre can only occur near sea level. The moment you climb, the air gets thinner. And because dynamic pressure — the pressure the air rams into the pitot tube — is proportional to density, a given true airspeed at altitude generates less dynamic pressure than the same true airspeed at sea level. Less pressure on the capsule means less expansion, so the needle sits lower. The ASI under-reads. It shows you a speed that is less than your true speed. So let me give you the precise name for this: it's called density error. And the rule is simple — at altitude, where density is less than 1225 grams per cubic metre, the ASI under-reads true speed. Flip it around: if the density is greater than ISA at mean sea level, the ASI will over-read the true speed. Now, here's the beautiful little equation that ties it all together: EAS + Density Error = TAS EAS is equivalent airspeed — that's the speed the ASI would show if there were no density error, the reading corrected for instrument and position error. Add the density error to it, and you get true airspeed. That's the actual speed of the aircraft through the air. But here's the practical problem. There is no single instrument that gives you a direct reading of density. You can't just look at a gauge and read "density." Density has to be calculated, and it's calculated using the interaction of pressure and temperature. Pressure is a function of altitude, so by combining altitude information with temperature information, the density correction is derived — and that derivation is done by the navigational computer. One thing I want you to remember clearly: the major factor in that density calculation is the pressure value. Temperature matters, but pressure is the dominant term. Now, a word of reassurance. For the Instrumentation EASA exam, you are not required to calculate these errors using a navigation computer. You need to understand the concept, the equation, and the direction of the error. If you're curious about the full calculation details, they live in the Oxford ATPL Book 10 — General Navigation, Chapter 6. But for our purposes here, you've got the core: density error makes the ASI under-read at altitude, and EAS plus that error gives you TAS.