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 an airspeed indicator that measures dynamic pressure, but here's the catch: dynamic pressure doesn't just depend on your true airspeed, it also depends on the density of the air you're flying through. The instrument is calibrated for one specific density: 1225 grams per cubic metre. That's the calibration density, and it only occurs near sea level under standard conditions. So unless the air around the aircraft is exactly at that density, the ASI cannot correctly indicate true airspeed, or TAS. Here's the physics. Dynamic pressure is proportional to density. At altitude, the air is thinner, density is less. So for a given true airspeed, the dynamic pressure you generate is less than it would be for the same true airspeed at sea level. Less dynamic pressure means the capsule inside the ASI expands less, so the needle shows a lower speed. The indicated speed is less than the true speed. So to summarise: at altitude, because density is less than 1225 grams per cubic metre, the ASI under-reads the true speed. We call this discrepancy 'density error'. And the reverse: if density is greater than ISA at mean sea level, the ASI will over-read the true speed. Now, the relationship that ties it all together is this: EAS plus Density Error equals TAS. EAS is equivalent airspeed — that's the speed the ASI would show if there were no density error, the speed corrected for instrument and position error. Add the density error on top, and you get true airspeed. Here's the practical problem. There is no single instrument that gives you a direct reading of density. Density has to be calculated using the interaction of pressure and temperature. But pressure is a function of altitude. So by combining altitude and temperature information, the density correction is derived by the navigational computer. And note this: the major factor in the density calculation is the pressure value. One last thing to keep you honest for the exam: the calculation of these errors using a navigation computer is not required for the Instrumentation EASA exam. If you're curious about the full calculation details, they're in the Oxford ATPL Book 10, General Navigation, Chapter 6. But for our purposes, you need to know the concept, the relationship, and the direction of the error.