The Airspeed Indicator (ASI) — Page 39, Lesson 49

I want to walk you through the Airspeed Indicator now — specifically its calibration and the errors that define how we interpret what it shows. This is the heart of why the ASI reading is not simply "your speed," and why we have a whole chain of corrections before we trust it. Let's start with calibration, because everything else hangs off it. The ASI works by sensing dynamic pressure — the pressure you feel from the air rushing past. But here's the catch: dynamic pressure depends not only on the speed of the aircraft, but also on the air density. If the air is denser, the same speed produces more dynamic pressure. If it's thinner, less. And that density varies with temperature and pressure, and therefore with altitude. So the manufacturer has to pick one reference condition to calibrate the instrument against. The ASI is calibrated to read true airspeed for an air density of 1225 grams per cubic metre. That's the density you'd get under International Standard Atmosphere conditions at Mean Sea Level — a pressure of 1013.25 hPa and a temperature of +15°C, in dry air conditions. So the instrument is built assuming that density, and no allowance is made in the calibration for the change in density that occurs with change of altitude. That's the key point: the ASI is only calibrated at MSL under ISA conditions. Fly at any other height, and the reading will carry errors. That's not a defect — it's a design choice, and it's why we need corrections. Now let's look at the errors themselves. There are three families you need to know. First, Instrument Error. This comes from manufacturing imperfections and from usage — wear and tear over time. These are small errors, and they're determined on the ground under laboratory conditions, by comparing the instrument against a reference, a datum instrument. From that, a correction card can be produced for the speed range of the instrument. So on the ground, we know exactly how far off this particular ASI is at various speeds. Second, Position Error — which is alternatively known as 'pressure' error. This arises mainly from the sensing of incorrect static pressure. Think about it: the static port is supposed to sample undisturbed ambient pressure, but the airflow around the aircraft can disturb it, so the pressure we sense isn't quite the true static pressure. That's the root of position error. It's described more fully in the section on Pressure Heads, but for now, know that it's a pressure-sensing problem. Position errors throughout the speed range are determined by the aircraft manufacturer during the test flying programme, for a particular aircraft type. So it's type-specific — each aircraft model has its own position error signature. Now, it's not unusual to compile a joint correction card for position and instrument errors together, and place it in the aircraft near the ASI concerned. So the pilot has one card that combines both corrections. Third, Manoeuvre-induced Errors. These are associated chiefly with manoeuvres involving a change in angle of attack. When you change angle of attack, you get transient errors and a lag in the indication of changes in airspeed. So during a manoeuvre, the needle doesn't instantly settle on the new speed — it lags and may overshoot briefly before stabilising. Now, the payoff — Calibrated Airspeed, or CAS. This is the bridge between what you see and what you can trust. The pilot corrects the Indicated Airspeed — the IAS — for Instrument and Position Error, using the correction card, to give Calibrated Airspeed. So for a combined correction card, you read your IAS, apply the correction from the card, and the result is CAS. That's the speed that has had the instrument's own quirks and the pressure-sensing errors removed. So the chain is: IAS is what the needle shows. Apply instrument and position corrections from the card, and you get CAS. And CAS is the foundation for everything that comes next — true airspeed, and all the performance calculations. But that's where we'll go next.