In order to determine the speed and altitude of an aircraft in flight it is necessary to measure the total pressure, static pressure and temperature of the air in which the aircraft is flying. To measure these quantities an air data system is used, which typically comprises total and static pressure sensors, pressure or data transmission lines and instruments. (A temperature sensor is also required.)

The difference between static pressure, at a pressure sensor on an aircraft, and the ambient (free stream) static pressure has traditionally been referred to as "position error" (or pressure error). The error is caused by the presence of the aircraft disturbing the air mass through which it is flying, hence changing the local static pressure in the area of the sensor, and any influence of the sensor itself. The pressure adjustment required to correct this error is frequently termed the "position error correction".

Other errors in the pressure system include instrument error and lag error. Instrument errors can be minimised by good build quality of mechanical instruments or the use of electronic pressure transducer systems where these errors tend to be negligible. The magnitude of lag is dependent on the position of the transducer in the system and the volume of the system.

This Data Item considers various flight test techniques used to determine the corrections required to measured static pressure. In addition to the flight test aircraft some of these techniques require such dedicated resources as radar tracking facilities or pacer aircraft. Other techniques are more independent, requiring only instrumentation carried by the test aircraft, for example, those using trailing static cones or GPS.

The assessment of pressure errors considered is for subsonic flight conditions where the air data sensors are not influenced by shock waves.