The Airspeed Indicator (ASI) — Page 49, Lesson 51

I want to walk you through the Airspeed Indicator now, and I want to start by placing it in the bigger picture. The whole point of the ASI is to let pilots fly safely without overstressing or stalling the aircraft. And I should mention that in modern aircraft, the Air Data Computer is what actually calculates and indicates speed, altitude, temperature, and other air parameters — the ASI is the instrument that presents that speed to you. Now, here's a key distinction I want you to hold onto. For navigation and flight planning, the Calibrated Airspeed — the CAS — is of no significant use. What you actually need as a pilot is the speed of the aircraft relative to the ground. In the absence of any wind — and that wind effect is covered in General Navigation, book 10 — that speed relative to the ground is called the True Airspeed, the TAS. Let me now introduce the concept of Equivalent Airspeed, the EAS. Here's the relationship I want you to remember: whenever Density Error is present, Compressibility Error also exists. So it's possible to correct CAS just for Compressibility Error, without correcting for Density Error. The resultant dynamic pressure that you get from that correction is called Equivalent Airspeed. So in precise terms: Equivalent Airspeed is CAS corrected for Compressibility Error only. Now, why does EAS matter so much? Because EAS is the most accurate value of dynamic pressure. It has been corrected for Instrument error, Pressure error, and Compressibility error — and all of those are forms of measurement error. So EAS is the most accurate measure of the dynamic pressure over the wing. In practice, the difference between EAS and CAS is not great unless altitude becomes significant. But here's the practical consequence: at high elevation airports, particularly with high-performance airliners that have high take-off and landing speeds, the CAS — and therefore the IAS — is higher for the same EAS. Let me give you the operational significance of that. At constant weight, regardless of altitude, an aircraft always lifts off at a constant EAS. That's a beautiful, clean fact — the lift-off condition is tied to EAS, not to what the dial reads. And here's the design logic: all limit speeds are calculated from EAS, and then the errors are re-introduced to display the speeds as IAS. So the instrument you actually read is built backwards from EAS. Now let's move to True Airspeed, the TAS, and the Density Error. Unless the air around the aircraft is at the calibration density of 1225 grams per cubic metre — and that can only occur near sea level — the ASI... and that's where the excerpt cuts off, so we'll pick that thought up next. But hold on to this: 1225 grams per cubic metre is the calibration density, and it's the reference point against which density error is measured.