Table of Contents
What Is True Airspeed?
True Airspeed (TAS) is the actual speed of an aircraft relative to the air mass through which it is flying. Unlike Indicated Airspeed (IAS) or Calibrated Airspeed (CAS), TAS accounts for the decreased air density at higher altitudes. At sea level in standard conditions, TAS equals CAS, but as altitude increases, TAS becomes progressively larger than CAS.
TAS is critical for flight planning because it determines the actual distance covered over time. Ground speed is TAS corrected for wind. Pilots use TAS for navigation calculations, fuel planning, and filing flight plans with air traffic control.
TAS Formula
Where ρ0 is sea level standard density (1.225 kg/m³), ρ is the actual air density at altitude, T is temperature in Kelvin, and P is pressure. A practical approximation adds about 2% to CAS for every 1,000 feet of altitude.
ISA Standard Atmosphere
| Altitude (ft) | ISA Temp (°C) | Pressure (hPa) | TAS/CAS Ratio |
|---|---|---|---|
| 0 | 15.0 | 1013.25 | 1.000 |
| 5,000 | 5.1 | 843.1 | 1.043 |
| 10,000 | -4.8 | 696.8 | 1.091 |
| 20,000 | -24.6 | 465.6 | 1.206 |
| 35,000 | -54.3 | 238.4 | 1.473 |
Frequently Asked Questions
Why is TAS higher than IAS at altitude?
The airspeed indicator measures dynamic pressure (impact pressure minus static pressure). At higher altitudes, air is less dense, so the aircraft must move faster through the air to generate the same dynamic pressure. TAS is always greater than or equal to CAS.
What is density altitude?
Density altitude is the altitude in the standard atmosphere that has the same air density as the actual conditions. It accounts for both temperature and pressure deviations from standard. High density altitude degrades aircraft and engine performance.
How accurate is the 2% per 1000ft rule?
The rule of thumb is accurate within about 1-2% up to 20,000 feet in standard conditions. At higher altitudes or non-standard temperatures, the simplified formula used in this calculator provides better accuracy.