Instruments · Technical General

How an airspeed indicator works

Two small holes, one thin diaphragm, and a number that is never quite your real speed. The story of IAS, TAS and the coloured arcs.

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A car knows its speed by counting how fast its wheels turn. An aircraft has nothing to count. All it can do is feel the air pushing against it, and that push depends on two things: how fast you are going, and how thick the air is.

The pitot tube faces into the airflow and feels the still air pressure plus the push of the moving air. The static port feels only the still air. The airspeed indicator puts one inside a diaphragm and the other around it, so the diaphragm swells with the difference: dynamic pressure.

Dynamic pressure grows with the square of speed. Double the speed and the push is four times as strong. Slide from 40 to 80 kt above and watch the dynamic pressure readout: it does not double, it quadruples.

Indicated is not true

The dial is calibrated for sea-level air in the ISA. Climb, and each cubic metre holds fewer molecules. To feel the same push, you must fly faster through it. The number on the dial, indicated airspeed, stays the same. Your real speed through the air, true airspeed, grows.

The airspeed indicator only feels how hard the air hits it, and thin air hits more softly. So at altitude it shows the same IAS while the aircraft moves through the air at a higher true airspeed.

Pilots shortcut this with the 2% rule: add 2% to IAS for every 1,000 ft. Try it in the figure. It is very good below 10,000 ft, which is where you will use it, and drifts at airliner heights, where compressibility also starts to matter.

There is a good reason the dial shows IAS and not TAS, though. The wing feels the air exactly as the airspeed indicator does. It stalls at the same indicated airspeed whether you are at sea level or at 8,000 ft. One number on the dial, valid everywhere.

Reading the arcs

That is also why the important limits are painted straight onto the dial. They are all indicated airspeeds, so they never move.

The markings on a Cessna 172S airspeed indicator. Slide through the speeds and read what each band allows.

Now test itPitot-static instrumentsEight questions, with explanations. About five minutes.