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

Right, let's get into the Airspeed Indicator. We've covered the basic pitot-static principle, so now we're looking at the errors that make the instrument read something other than the truth, and then the corrections we apply to fix that. First, the big one: Compressibility Error. Air is compressible. When air is rammed into the pitot tube at high speed, it gets squashed, and that squashing produces a pressure that is higher than it would be for an ideal, incompressible fluid. For that ideal fluid, the dynamic pressure is given by the formula ½ρV² — that's one half times rho times V squared, where rho is the air density and V is the true airspeed. But because air compresses, the actual pressure in the pitot tube is higher than that ideal formula predicts. Now, the ASI is calibrated to allow for this complex compressible flow formula, and it's calibrated assuming a specific density: 1225 grams per cubic metre. That's the density of the ISA atmosphere at Mean Sea Level. If the actual density is any lower than that, the standard correction for compressibility will be in error. Because of this, the instrument will over-read. Both IAS and CAS will be too high, and a subtractive compressibility correction will have to be applied — you have to subtract a value to bring the reading down to the truth. How big is this correction? It exceeds 20 knots if your TAS is near the speed of sound. You can get the correction from graphs or tables, or it can be applied by high-speed navigation computers. But here's the good news: if the TAS is less than 300 knots, the error is small enough to be ignored in the calculation of TAS from IAS. Now let's look at the application of corrections — the order in which we fix the readings. What you see on the instrument is called Indicated Airspeed, or IAS. The first two errors to be applied are Instrument Error and Pressure, or Position, Error. Applying those gives you Calibrated Airspeed, or CAS. In practice, we don't have separate cards for the Instrument Error and the Pressure Error. It's simpler to combine both corrections onto a single card. So IAS is corrected for both Instrument and Pressure Error by that correction card, to give CAS. Next, CAS is corrected for compressibility error to give Equivalent Airspeed, or EAS. This allows for the fact that the density of air is rarely precisely 1225 grams per cubic metre — the value for which the ASI is calibrated. 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 MSL, and under those conditions, no compressibility correction is necessary. And as I said, the compressibility correction is small at TAS lower than 300 knots, so no correction is considered necessary. So here's the practical sequence you will always 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 the Compressibility Error correction must be applied. Finally, let's look at the limiting speeds — the coloured arcs and markings on the ASI face. These are the speeds you must respect. VNO is the maximum normal operating limit speed. VNE is the Never Exceed speed. VSO is the stall speed, or the minimum steady flight speed, in the landing configuration. VS1 is the stall speed, or the minimum steady flight speed, in a specified configuration. VFE is the maximum Flap Extension speed. And VYSE is the best rate of climb speed with one engine failed. So to tie it all together: the ASI reads IAS, we correct for instrument and position error to get CAS, then compressibility to get EAS, then density error to get TAS — and the compressibility step only matters above 300 knots TAS. That's the full chain of corrections.