This instrument provides the pilot with an indication of the airspeed past his or her airplane. It is an indication of airspeed or dynamic pressure not corrected for any error like non standard temperature or pressure. It is called IAS or indicated airspeed.
All other aircraft speeds are derived from this IAS, such as TAS (True airspeed) and GS, Ground speed. Follow the next link for more detailed information on these important aircraft speeds.
On this page we are going to take a look inside this airspeed indicator and see how it operates and what limitations and possible errors these instruments have and of which the pilot should be aware of during his day to day flying.
Indicated Airspeed Operation
As already explained in our previous article on the aircraft pitot static system, the airspeed indicator is connected to the static and dynamic port. See also the image to the right.
The pitot connection supplies dynamic and static pressure, thus for a correct indication the static pressure has to be subtracted from the pitot pressure. The pitot pressure is fed to a diaphragm inside a sealed chamber. This chamber is then connected to the static port. The result is that when the diaphragm moves it acts against the static pressure thereby subtracting it.
Via a geared system the needle on the outside of the instrument is able to move, the instrument scale on the outside is calibrated against either knots, km/h or mph. Most aircraft have the scale calibrated in knots with sometimes a smaller subscale in mph. Some East-European aircraft can be found with the airspeed indicator in km/h.
There are a number of instrument errors we have to deal with and these can be categorized by density, instrument, installation and we discuss them below.
Variations in air pressure and temperature will result in outside air conditions that are almost never equal to the well known ISA standard atmosphere. Some airspeed indicators have a knob to compensate for density error by dialing the OAT against pressure altitude. The instrument then shows true airspeed (TAS).
To do this: set the altimeter at standard pressure (1013 or 29.92) and read pressure altitude (do not forget to reset to the old value). Now, read OAT and dial this temperature against the pressure altitude on the airspeed indicator. You can then read true airspeed from the white subscale.
Caused by wear and tear of the instrument and the manner of installation in the aircraft and cockpit, it is usually very small. Although wear and tear will increase with the age of the instrument/aircraft.
Determined by the accuracy with which the pitot tube and static port(s) can measure the actual dynamic and static pressures around the aircraft. Slipping or skidding or flying with a high angle of attack result in a airflow coming in under an angle into the pitot tube or even air pressure entering the static ports.
As a result of this we can see that the aircraft should be in balanced unaccelerated flight for proper airspeed indication.
Some aircraft have a PEC, position error correction chart in the POH of flight manual. This shows in detail the magnitude of these errors.
This is only a factor above 200 kts indicated airspeed. As air is a compressible medium higher speeds will cause the air to be compressed inside the pitot tube. Most noticable above pressure altitudes of 10000 feet and over 200 kts IAS. This error will result in a over-read of the instrument. This is, of course, only a concern to pilots flying in higher speed and altitude flying aircraft.
Flight manuals will contain a correction chart or table on these higher performance aircraft.