General Principles - Cruise — Page 241, Lesson 287

Let’s pick this up right where the speed definitions get refined. We’ve already dealt with indicated airspeed and calibrated airspeed — the raw probe readings and the corrections for position and instrument error. Now I want to walk you through the next two speeds in the chain: equivalent airspeed and true airspeed. First, equivalent airspeed, abbreviated EAS. This is the final correction applied to the pressure probe data, and it compensates for the effect of compressibility. Here’s the physics: typically, at speeds beyond 220 knots, the air ahead of the aeroplane does not move out of the way in time. Instead, the air starts to build up and compress in front of the aeroplane. That build-up of air is called the compressibility effect. So if the probes are corrected for the compressibility error, in addition to the position and instrument errors we already corrected for, the speed you obtain is called equivalent airspeed — EAS. Now, an important point: equivalent airspeed is the most accurate of the speeds which are obtained from dynamic pressure. Dynamic pressure is the pressure you feel from the air rushing past — that’s what the pitot probe senses. So EAS is the best of the dynamic-pressure-derived speeds. For the most part, this performance book will assume that indicated airspeed, calibrated airspeed and equivalent airspeed are the same. But unless otherwise stated, assume any reference to aeroplane speed as being indicated airspeed. So when you see a speed in this book, default to IAS unless told otherwise. Next, true airspeed, abbreviated TAS. True airspeed is the equivalent airspeed corrected for density error. And as the name suggests, it is the true speed of the aircraft relative to the air through which the aeroplane is flying. So EAS corrects for compressibility; TAS takes that and corrects for the density of the air. Here’s a very simplified formula showing the relationship: TAS is proportional to EAS divided by density. Let me read that carefully — TAS is proportional to EAS ÷ DENSITY. So if you hold equivalent airspeed constant and the density falls, the true airspeed would increase. That’s the key relationship. Think about what happens with altitude: as you climb, air density decreases. So with increasing altitude at a constant equivalent airspeed, true airspeed increases. That’s exactly what Figure 5.7 shows — the relationship between EAS and TAS with altitude. At sea level, where density is highest, TAS and EAS are nearly the same. As you climb and density drops, TAS grows larger than EAS for the same indicated speed. Now, how do you actually get true airspeed in practice? You can calculate it using the tables in the aeroplane flight manual, using a flight navigation computer, or even using a calibration scale on the airspeed indicator. And here’s why it matters operationally: true airspeed is mainly used for navigation and flight planning purposes. That makes sense — navigation is about your actual speed through the air mass, not the pressure-derived reading. So when you’re planning a route, working out groundspeed, or doing fuel planning, you work in TAS. Let me tie the whole chain together for you. The pressure probes give you indicated airspeed. Correct for position and instrument errors and you get calibrated airspeed. Correct for compressibility and you get equivalent airspeed. Correct for density and you get true airspeed. Each step refines the reading toward the actual speed of the aeroplane relative to the air. And remember the default: unless stated otherwise, any speed reference in this book means indicated airspeed.