TheAirspeedIndicator (Asl) — Page 1, Lesson 5

I want to walk you through the Airspeed Indicator and the errors that affect it. Let's start with compressibility error. Air is compressible. That means when air rams into the pitot tube, it gets squashed, and the pressure produced inside the pitot tube is actually higher than it would be if air were an ideal incompressible fluid. For an ideal incompressible fluid, dynamic pressure is given by the formula one-half rho V squared — that's ½ρV². But because air is compressible, the real pressure is different. The Airspeed Indicator, or ASI, is calibrated to allow for this complex compressible flow formula. It assumes a specific air density: 1225 grams per cubic metre. That's the density of air in the International Standard Atmosphere at Mean Sea Level — ISA at MSL. If the actual air density is any lower than that, the standard correction for compressibility built into the instrument will be in error. So what happens because of this? The instrument will over-read. That means Indicated Airspeed, or IAS, and Calibrated Airspeed, or CAS, will both be too high. To fix that, you have to apply a subtractive compressibility correction — you subtract a value to bring the reading down to the correct figure. This correction can be quite large. If True Airspeed, or TAS, is near the speed of sound, the correction exceeds 20 knots. You can get the correction from graphs or tables, or you can apply it using high-speed navigation computers. However, if TAS is less than 300 knots, the error is small enough that you can ignore it when calculating TAS from IAS. Now let's look at how these corrections are applied in sequence. What you see on the instrument face is called Indicated Airspeed — IAS. First, you apply Instrument Error and Pressure Error — also called Position Error — to IAS, and that gives you Calibrated Airspeed, or CAS. In practice, we don't have separate correction cards for Instrument Error and 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, 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 — the value the ASI is calibrated for. But in practice, as an airline pilot, you are unlikely to deal with EAS. It is rarely encountered outside scientific and test flying. Remember, the ASI is already calibrated to allow for compressibility at ISA at MSL, and under those conditions no compressibility correction is necessary. Also, compressibility correction is small at True Airspeeds lower than 300 knots, and no correction is considered necessary there either. So here is the practical sequence you will 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 talk about limiting speeds. Some of these are usually shown on the ASI itself. Vne is the Never Exceed speed — you must never fly faster than this. 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 — that means a configuration other than landing, like takeoff or clean. Vfe is the maximum Flap Extension speed — you must not extend flaps above this speed. And Vrse is the best rate of climb speed with one engine failed — that's your single-engine best rate-of-climb speed.