What is True Airspeed?
True airspeed (TAS) is the actual speed of an aircraft relative to the surrounding air mass. It is what your airspeed indicator would read if the atmosphere were at standard sea-level conditions — 15°C and 29.92 inHg — at every altitude.
As altitude increases, air density decreases. Your pitot-static system senses dynamic pressure, not velocity directly, so your airspeed indicator reads progressively lower than your actual speed through the air. TAS corrects for this by accounting for the reduced air density at your cruising altitude and the deviation from standard temperature.
True airspeed matters for several practical reasons:
- Flight planning and fuel burn — TAS combined with the forecast wind gives you groundspeed, which determines your actual en-route time and fuel required.
- ATC radar — secondary surveillance radar returns ground speed, not airspeed. Knowing your TAS helps you verify that ATC-reported speeds are consistent with your performance.
- Aircraft performance charts — cruise performance tables in your POH are published in TAS. You must know your TAS to determine whether your aircraft is performing as expected.
- Pilot reports (PIREPs) — when filing a PIREP, airspeed is reported as TAS.
How to Calculate True Airspeed
The full calculation derives TAS from calibrated airspeed (CAS) and air density. The practical aviation approximation used by most E6B computers and flight calculators is:
TAS ≈ CAS × (1 + 0.02 × Pressure Altitude in thousands of feet)
This rule of thumb adds approximately 2% per 1,000 ft of pressure altitude and is accurate to within 1–2 knots for most GA cruising speeds and altitudes below FL200.
For a more precise result — particularly when temperature deviates significantly from standard — use the density-based formula:
TAS = CAS × √(ρ₀ / ρ)
Where ρ₀ is standard sea-level air density (1.225 kg/m³) and ρ is the actual air density at your altitude and temperature. The true airspeed calculator at the top of this page uses this full formula, taking pressure altitude, OAT, and CAS as inputs.
Step-by-Step Example
You are cruising at a pressure altitude of 8,770 ft, OAT of +5°C, and your calibrated airspeed is 120 KIAS.
- Pressure Altitude: PA = 8,500 + (29.92 − 29.65) × 1,000 = 8,500 + 270 = 8,770 ft
- ISA Temperature at 8,770 ft: 15 − (8.77 × 2) = 15 − 17.5 ≈ −2.5°C
- Temperature deviation from ISA: +5 − (−2.5) = +7.5°C above ISA — warmer than standard, so air is less dense and TAS will be higher than the rule-of-thumb estimate.
- Rule-of-thumb TAS: 120 × (1 + 0.02 × 8.77) ≈ 120 × 1.175 ≈ 141 KTAS
- With temperature correction: actual TAS ≈ 144 KTAS using the density formula.
Use the true airspeed calculator above for the precise result. For a Cessna 172 at that altitude, a cruise TAS of 120–130 KTAS would be typical; 144 KTAS suggests you may be flying a faster aircraft or the example numbers reflect a higher-performance type.
TAS vs IAS vs CAS vs Groundspeed
Pilots work with four related but distinct airspeed concepts:
Indicated Airspeed (IAS) is the raw reading on your airspeed indicator — uncorrected for instrument or position error.
Calibrated Airspeed (CAS) is IAS corrected for instrument and position error as listed in your POH. At cruise speeds in most GA aircraft, IAS and CAS differ by only 1–3 knots.
True Airspeed (TAS) is CAS corrected for air density (altitude and temperature). It increases with altitude: at FL180 a typical piston will show roughly 30% more TAS than IAS. TAS is your speed through the air mass.
Groundspeed (GS) is TAS adjusted for wind: headwind reduces groundspeed, tailwind increases it. GS = TAS ± wind component. Groundspeed determines your actual progress over the ground and is what your GPS reports.
For aircraft performance — stall speeds, Vx, Vy, Va — you always work in IAS or CAS, because those values are tied to aerodynamic forces which depend on dynamic pressure, not actual speed. For navigation and fuel planning, you always work in TAS and then GS.
Frequently Asked Questions
- What is a true airspeed calculator?
- A true airspeed calculator converts your calibrated (or indicated) airspeed into TAS by accounting for the reduced air density at your cruising altitude and the deviation of the actual temperature from the ISA standard. It takes pressure altitude, OAT, and CAS as inputs and returns TAS in knots.
- Why is TAS always higher than IAS?
- At altitude, air density is lower than at sea level. Your pitot tube collects less air per unit time, so the dynamic pressure — and thus the airspeed indicator reading — is lower than your actual speed through the air. TAS corrects for this: the thinner the air, the larger the gap between IAS and TAS. At sea level on a standard day, TAS and IAS are equal by definition.
- How much does TAS increase per 1,000 ft?
- As a rule of thumb, TAS increases by approximately 2% per 1,000 ft of pressure altitude on a standard day. So if your IAS reads 100 knots at 10,000 ft, your TAS is roughly 120 knots. High temperatures increase the gap further; cold temperatures reduce it slightly.
- Is TAS the same as groundspeed?
- No. TAS is your speed relative to the air mass. Groundspeed is your speed relative to the ground. The difference is wind: a 20-knot headwind subtracts 20 knots from your TAS to give groundspeed; a 20-knot tailwind adds 20 knots. Your GPS displays groundspeed; your airspeed indicator (corrected) gives you TAS.
- Do I need TAS for VFR cross-country planning?
- Yes. To calculate your estimated time en route and fuel required, you need groundspeed — which requires TAS and the forecast wind. Using IAS instead of TAS will underestimate your speed and produce inaccurate fuel and time calculations, especially at higher altitudes where the difference is significant. Always use TAS as the starting point for navigation calculations.