How the airspeed indicator works

What does the airspeed indicator show?

It shows indicated airspeed: the speed read straight off the dial. This is the speed you fly by. The stall speeds and other speed limits in an aircraft’s handbook are given as indicated airspeed.

It is speed through the air, not over the ground. A headwind or tailwind changes how fast you cover the ground, but not what this dial reads.

How do you read the colour arcs?

The dial is marked every 5 knots from 40 to 200, with a number every 20. The coloured arcs show the speed ranges for the aircraft. On the SkyDeck dial they are:

These numbers belong to the SkyDeck gauge, drawn to match the 172 model in X-Plane 12. A real aircraft’s limits are in its own handbook, and that is the one to trust.

Some important speeds have no colour on the dial, for example the design manoeuvring speed (V_A) and the best climb speeds (V_X and V_Y). Pilots learn them from the handbook.

The small movable ring near the top, with the PRESS ALT and TEMP windows, is for working out true airspeed from the pressure altitude and the outside temperature. On this page it stays at sea level on a standard day, so it reads the same as the needle.

How does it work inside?

It is the only flight instrument that uses both the pitot tube and the static port. The pitot tube faces forward into the airflow and catches ram air pressure. The static port, on the side of the aircraft, feels the still air pressure around it.

1. 1
   Ram air from the pitot tube flows into a thin, flexible metal diaphragm inside the instrument.
2. 2
   The sealed case around the diaphragm is filled with static air pressure from the static port.
3. 3
   The faster the aircraft flies, the more the ram pressure beats the static pressure, and the more the diaphragm swells.
4. 4
   Small gears and levers turn that swelling into needle movement on the dial.

Indicated, calibrated, true and ground speed: what is the difference?

• Indicated airspeed (IAS) is what the needle shows.
• Calibrated airspeed (CAS) is indicated airspeed corrected for small errors in the instrument and in how it is fitted to the aircraft.
• True airspeed (TAS) is your real speed through the air. It is calibrated airspeed corrected for altitude and non-standard temperature. Higher up, the air is thinner, so true airspeed is higher than indicated.
• Ground speed (GS) is true airspeed corrected for wind: how fast you move over the ground.

What can go wrong?

• A blocked pitot tube with the drain hole open (ice is a common cause): the ram air leaks away and the needle drops to zero.
• A blocked pitot tube and drain hole: the trapped air makes the gauge act like an altimeter. It shows more speed in a climb and less in a descent, whatever the real speed.
• A blocked static port: the airspeed reads lower than it should above the height where it blocked, and higher than it should below it.
• Reading it as ground speed: wind does not move the needle. Use a GPS or your flight plan for ground speed.