True airspeed (TAS) is the actual speed of an aircraft relative to the undisturbed air mass surrounding it. It accounts for the effects of altitude and temperature on air density, giving a more accurate picture of how fast the aircraft is truly moving through the air.
How It Works#
Your airspeed indicator measures indicated airspeed (IAS), which is based on the pressure difference between ram air and static air. At sea level on a standard day, IAS and TAS are nearly equal. But as you climb, the air thins out. The same pressure difference now represents a higher actual speed, so TAS increases above IAS.
Two factors drive this difference: altitude and temperature. Higher altitude means lower air density, which increases TAS for a given IAS. Warmer air also reduces density, pushing TAS higher. Colder air does the opposite.
A useful rule of thumb: TAS increases roughly 2% above IAS for every 1,000 feet of altitude. At 10,000 feet on a standard day, an IAS of 120 knots becomes approximately 138 knots TAS. At cruise altitudes above FL300, the gap is dramatic.
Pilots calculate TAS using a flight computer (E6B), an onboard avionics system, or a simple formula. Modern glass cockpits display TAS directly. Older analog setups require the pilot to compute it manually.
Example in Aviation#
A Cessna 172 climbs to 8,000 feet and holds an indicated airspeed of 110 knots. The outside air temperature is standard for that altitude. The pilot uses an E6B to calculate TAS and gets 126 knots. That 16-knot difference matters when filing a flight plan, because ATC and navigation calculations depend on accurate groundspeed estimates.
The pilot enters 126 knots TAS into the flight plan, then factors in wind to determine groundspeed. Using IAS instead would cause the estimated time en route to be noticeably off.
Why It Matters#
TAS is the foundation for accurate flight planning. Wind correction, fuel burn estimates, and time-en-route calculations all start with TAS. If you plug in IAS instead, every downstream number is wrong.
TAS also affects aircraft performance. Stall speed in terms of IAS stays roughly constant, but TAS at stall increases with altitude. Understanding this distinction helps pilots recognize that the aircraft is moving faster through the air at altitude, even when the airspeed indicator looks familiar.
Key Takeaways#
- TAS is the aircraft's real speed through the surrounding air mass.
- IAS and TAS diverge as altitude and temperature change air density.
- TAS runs roughly 2% higher than IAS per 1,000 feet of altitude gained.
- Accurate flight planning requires TAS, not IAS.
- Modern avionics display TAS directly; older systems require manual calculation.