Editing True airspeed (section)

{{Short description|Speed of an aircraft relative to the air mass through which it is flying}}{{No footnotes|date=January 2009}}[[File:True airspeed indicator-FAA.SVG|thumb|An analog true airspeed indicator for an airplane. The pilot sets the [[pressure altitude]] and [[temperature|air temperature]] in the top window using the knob; the needle indicates true airspeed in the lower left window. Here the speed is displayed both in [[Knot (unit)|knots]] (kn) and [[miles per hour]] (mph).]]

The '''true [[airspeed]]''' ('''TAS'''; also '''KTAS''', for ''knots true airspeed'') of an [[aircraft]] is the [[speed]] of the aircraft relative to the [[air mass]] through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogue [[airspeed indicator|TAS indicator]], but as [[Global Positioning System|GPS]] has become available for civilian use, the importance of such air-measuring instruments has decreased. Since ''indicated'', as opposed to ''true'', airspeed is a better indicator of margin above the [[Stall (fluid dynamics)|stall]], true airspeed is not used for controlling the aircraft; for these purposes the [[indicated airspeed]] &ndash; IAS or KIAS (knots indicated airspeed) &ndash; is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below {{convert|200|kn|km/h}}. Neither provides for accurate [[Ground speed|speed over the ground]], since surface winds or winds aloft are not taken into account.