Airspeed Indicators
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 aircraft speeds.
On this page we are going to take a look inside this airspeed indicator and see how it operates and what limitations it has.
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.
Speed calibration
Via a gearing system a needle on the outside of the instrument can move and the instrument scale is calibrated against either knots, km/h or mph.
Instrument errors
These can be categorized by:
Density error
Variations in air pressure and temperature will result in outside conditions that are almost never equal to the 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. Read true airspeed from the white subscale.
Instrument error
Caused by wear and tear of the instrument and the installation in the aircraft and cockpit, it is usually very small.
Position error
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 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.
Compressibility error
This is only a factor above 200 kts airspeed. As air is a compressible medium higher speeds will cause the air to be compressed inside the pitot tube. Noticable above pressure altitudes of 10000 feet and over 200 kts IAS. This error will result in a over read.
Flight manuals will contain a correction chart or table on applicable aircraft.
