True Airspeed

True airspeed (TAS) is the actual speed of the aircraft through the air mass — indicated airspeed (IAS) corrected for both position/instrument error (CAS) and air density. TAS is used for flight planning, navigation, groundspeed calculations, and performance analysis.

TAS and Altitude

As altitude increases, air density decreases. The airspeed indicator measures dynamic pressure, which decreases with density. At the same TAS, the IAS will be lower at higher altitude — the aircraft is flying at the same actual speed through the air but the indicator shows less. TAS = IAS × (density at sea level / density at altitude)^0.5, approximately.

TAS Rule of Thumb

TAS increases approximately 2% per 1,000 ft of altitude above sea level. At FL120 (12,000 ft): TAS ≈ IAS × 1.24. At FL180: TAS ≈ IAS × 1.36. For navigation, always use TAS (converted to groundspeed using the wind correction) — using IAS for navigation will systematically under-estimate arrival times.

On the TC Written Exam

TAS is used in all navigation calculations. Know: TAS > IAS at altitude; for flight planning, compute TAS from IAS and altitude/temperature; use TAS + wind to find groundspeed.

Related Terms

See also: IAS, CAS, Groundspeed.