General Principles - Cruise — Page 241, Lesson 287

Let's pick up right where the pressure probe corrections leave off. We've already dealt with instrument and position errors to get from indicated airspeed to calibrated airspeed. Now I want to walk you through the next two corrections, because they give us the speeds we actually use for performance and navigation. First, the Equivalent Airspeed, abbreviated EAS. This is the correction for a physical effect called compressibility. Here's what happens: typically, at speeds beyond 220 knots, the air ahead of the aeroplane doesn't have time to move out of the way. Instead, it starts to build up and compress right in front of the aircraft. That build-up of air is the compressibility effect, and it distorts the pressure readings at the probes. When we correct the probe data for this compressibility error, on top of the position and instrument errors we already corrected, the speed we get is called equivalent airspeed. EAS is the most accurate of all the speeds derived from dynamic pressure. Now, for most of this performance book, we're going to assume that indicated airspeed, calibrated airspeed, and equivalent airspeed are all the same. But unless I state otherwise, assume any reference to aeroplane speed means indicated airspeed. Now the big one for navigation: True Airspeed, abbreviated TAS. True airspeed is the equivalent airspeed corrected for density error. And as the name suggests, it's the true speed of the aircraft relative to the air through which it's flying. Let me give you a very simplified formula to show the relationship: TAS is proportional to EAS divided by DENSITY. So if we hold equivalent airspeed constant and the density falls, the true airspeed increases. That's exactly what happens with increasing altitude — at a constant equivalent airspeed, true airspeed goes up as you climb. That relationship is shown in Figure 5.7. Now, how do we actually get true airspeed in practice? We can calculate it using the tables in the aeroplane flight manual, we can use a flight navigation computer, or even use a calibration scale on the airspeed indicator itself. And the key point for you as a pilot: true airspeed is mainly used for navigation and flight planning purposes. That's the speed you'll use to work out your groundspeed and fuel planning, because it's the actual speed of the aircraft through the air, not just a probe reading. So to tie it all together: we start with indicated airspeed from the instruments, correct for instrument and position errors to get calibrated airspeed, correct for compressibility to get equivalent airspeed, and finally correct for density to get true airspeed. Each step gets us closer to the real physical speed of the aircraft through the air.