The Airspeed Indicator (ASI) — Page 49, Lesson 53

Let's pick up with the Airspeed Indicator and the errors that affect it. We've already covered the basic principle of how the instrument measures dynamic pressure, so now we're going to look at what happens when the real world doesn't match the instrument's ideal assumptions. First, let's talk about Compressibility Error. Air is compressible. When air rams into the pitot tube at high speed, it gets squashed, and that produces a pressure that is higher than it would be for an ideal, incompressible fluid. For an ideal fluid, the dynamic pressure is given by the formula ½ρV² — that's one-half times rho, the air density, times V squared, the velocity squared. But because air compresses, the actual pressure is higher than that simple formula predicts. The ASI is calibrated to allow for this complex compressible flow formula, but it assumes a specific density: 1225 grams per cubic metre. That's the density of the ISA atmosphere at Mean Sea Level. If the actual air density is any lower than that, the standard correction for compressibility will be in error. Here's the consequence: because of this, the instrument will over-read. Both Indicated Airspeed and Calibrated Airspeed will be too high, and you'll have to apply a subtractive compressibility correction — meaning you subtract a value to bring the reading down to the truth. How big is this correction? It can exceed 20 knots if the True Airspeed is near the speed of sound. You can get the correction from graphs or tables, or it can be applied automatically by high-speed navigation computers. But here's a useful rule of thumb: if the True Airspeed is less than 300 knots, the error is small enough to be ignored when calculating TAS from IAS. Now let's look at how all these corrections fit together in practice. What you see on the instrument face is called Indicated Airspeed, or IAS. To get from IAS to Calibrated Airspeed, or CAS, you apply two corrections: Instrument Error and Pressure Error, which is also called Position Error. In practice, we don't carry separate correction cards for each of those two errors. It's simpler to combine both corrections onto a single card. So IAS is corrected for both Instrument and Pressure Error by that one correction card, and the result is CAS. Next, CAS is corrected for compressibility error to give Equivalent Airspeed, or EAS. This correction allows for the fact that the density of air is rarely precisely 1225 grams per cubic metre — that value the ASI is calibrated for. Now, as an airline pilot, you are unlikely to deal with EAS. It's rarely encountered outside scientific and test flying. The ASI is already calibrated to allow for compressibility at ISA at Mean Sea Level, and under those exact conditions, no compressibility correction is necessary. And as we said, the compressibility correction is small at True Airspeeds lower than 300 knots, so no correction is considered necessary there either. So here's the practical sequence you'll follow. You will always calculate the Density Error correction first to give True Airspeed, or TAS. If the TAS you find is 300 knots or less, no further correction is necessary. If the TAS is greater than 300 knots, then you must apply the Compressibility Error correction. Finally, let's look at the Limiting Speeds — the operational limits that keep the aircraft within its safe envelope. Some of these are usually shown on the ASI itself, often as coloured arcs. Let me define each one precisely. VNO is the maximum normal operating limit speed. This is the top of the green arc — you shouldn't exceed it except in smooth air. VNE is the Never Exceed speed. This is the red line — you must never exceed this speed under any circumstances. VSO is the stall speed, or the minimum steady flight speed, in the landing configuration. That's with flaps and gear down. VS1 is the stall speed, or the minimum steady flight speed, in a specified configuration — a defined configuration other than landing, typically the clean configuration. VFE is the maximum Flap Extension speed — the fastest you can fly with flaps extended. And VYSE is the best rate of climb speed with one engine failed — the speed that gives you the best climb performance on a single engine. That figure shows you how these speeds appear as coloured arcs on the ASI face — the yellow arc denotes the caution range, which sits between VNO and VNE. So you can see the whole picture: the white arc for flap speeds, the green arc for normal operation, the yellow caution range, and the red line at VNE. So to summarise the correction chain: IAS plus Instrument and Pressure Error gives CAS, CAS plus Compressibility Error gives EAS, and then Density Error correction gives TAS — with the compressibility step only mattering above 300 knots.