Position error is one of the errors affecting the systems in an aircraft for measuring airspeed and altitude.[1][2] It is not practical or necessary for an aircraft to have an airspeed indicating system and an altitude indicating system that are exactly accurate. A small amount of error is tolerable. It is caused by the location of the static vent that supplies air pressure to the airspeed indicator and altimeter; there is no position on an aircraft where, at all angles of attack, the static pressure is always equal to atmospheric pressure.