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

Let's start with the calibration of the Airspeed Indicator, because that's the root of why the instrument reads what it reads. The ASI doesn't measure speed directly. It measures dynamic pressure — the pressure you feel when air rams into the pitot tube. And here's the key point I want you to hold onto: dynamic pressure depends not only on the speed of the aircraft, but also on the air density. The same speed through thin air produces less dynamic pressure than through thick air. Now, air density changes with temperature and pressure, and therefore with altitude. So the instrument has to be calibrated against some fixed reference. The ASI is calibrated to read true airspeed for the air density of 1225 grams per cubic metre. That's the density you get at ISA Mean Sea Level conditions — a pressure of 1013.25 hPa and a temperature of +15°C, in dry air conditions. So the instrument is built to give you the correct speed only for that one specific density. And critically, no allowance is made in the calibration for the change in density that occurs with change of altitude. That means the ASI is only accurate at MSL under ISA conditions. Fly at any other height, and the instrument will show errors. That's the fundamental limitation of the basic ASI. Now let's move to the errors themselves. There are three categories I want you to know. First, Instrument Error. This comes from manufacturing imperfections and from usage — wear and tear. These are small errors, and they're determined on the ground under laboratory conditions, by comparing the instrument against a datum instrument — a reference standard. From that, a correction card can be produced for the speed range of the instrument. Second, Position Error, which is alternatively known as pressure error. This arises mainly from the sensing of incorrect static pressure. The static port is supposed to sample the undisturbed ambient pressure, but the airflow around the aircraft disturbs it, so the pressure you sense isn't quite right. This is described more fully in the section on Pressure Heads. Position errors throughout the speed range are determined by the aircraft manufacturer during the test flying programme for that particular aircraft type. And it's not unusual to compile a joint correction card for both position and instrument errors, and place it in the aircraft near the ASI concerned. Third, Manoeuvre-induced Errors. These are associated chiefly with manoeuvres that involve a change in angle of attack. When you pitch the aircraft, the airflow over the static ports changes, and you get transient errors and a lag in the indication of changes in airspeed. So the needle doesn't instantly settle on the new value — it lags behind. Now, how do we use all this? That brings us to Calibrated Airspeed, or CAS. The pilot corrects the Indicated Airspeed — the IAS — for Instrument Error and Position Error, using the correction card, to give Calibrated Airspeed. So CAS is the IAS with those two errors removed. For instance, for a combined correction — and the excerpt cuts off there, but that's the process: you take the indicated value, apply the correction from the card, and you get CAS. So the chain is: IAS, corrected for instrument and position error, gives you CAS. And remember, the manoeuvre-induced errors are transient — they're not part of that steady-state correction.