IAS is what the airspeed indicator shows, CAS corrects it for instrument and position error, TAS corrects for air density, and ground speed adds the wind. Climb to 8,000 ft at 120 kt calibrated on a standard day and your true airspeed is about 135 kt.
The four speeds at a glance
| Speed | What it is | Corrected for | Where you use it |
|---|---|---|---|
| IAS (indicated) | Raw reading of the airspeed indicator | Nothing | Flying the aircraft: V-speeds, approach speeds |
| CAS (calibrated) | IAS fixed for instrument and pitot-static position error | Instrument and position error | Performance charts, TAS calculations |
| TAS (true) | Your real speed through the air mass | Air density (altitude and temperature) | Flight planning, flight plans, navigation |
| GS (ground speed) | Your speed over the ground | Wind | Time en route, fuel, ETA |
At jet speeds there is one more step, EAS (equivalent airspeed), which corrects CAS for air compression in the pitot tube. It matters above roughly 200 kt and 10,000 ft. For most piston and turboprop flying, CAS to TAS is enough.
IAS to CAS
The pitot tube and static ports never sense airflow perfectly, especially at high angles of attack and with flaps down. Your POH has an airspeed calibration table. At cruise the difference is often only 1–3 kt; near the stall it can be much more.
CAS to TAS: why air density matters
The airspeed indicator measures dynamic pressure. Thinner air gives less pressure for the same true speed, so as you climb or the air warms, IAS reads lower than your real speed through the air.
TAS equals CAS divided by the square root of the density ratio (σ):
TAS = CAS ÷ √σ
σ comes from pressure altitude and outside air temperature. The true airspeed calculator does this for you.
Rule of thumb: TAS is about CAS plus 2% per 1,000 ft of altitude. 120 kt at 8,000 ft gives 120 + 16% ≈ 139 kt. The rule assumes a standard temperature; on a cold day it over-estimates, on a hot day it under-estimates.
Worked examples
| CAS | Pressure altitude | OAT | TAS (calculated) | 2% rule |
|---|---|---|---|---|
| 100 kt | 6,000 ft | +25 °C | 114 kt | 112 kt |
| 120 kt | 8,000 ft | 0 °C | 136 kt | 139 kt |
| 120 kt | 8,000 ft | +15 °C | 139 kt | 139 kt |
The hot day shows why density altitude matters: same indicated speed, higher true speed, longer take-off and landing roll.
TAS to ground speed
Ground speed is TAS plus or minus the wind. A 136 kt TAS into a 20 kt headwind gives 116 kt over the ground; with a 20 kt tailwind, 156 kt. Crosswinds also reduce ground speed and need a heading correction, which the wind correction angle calculator works out.
Use ground speed, never TAS, for time and fuel. The time, speed and distance guide shows the E6B method.
Which speed for which job
- Flying the aircraft: IAS. Stall and approach speeds stay almost the same in IAS at any altitude, because the wing also responds to dynamic pressure.
- Filing a flight plan: TAS at cruise.
- ETA and fuel: ground speed.
- High altitude jets: Mach number, because the limit becomes the speed of sound, which falls as the air cools.
Common mistakes
- Planning time en route with TAS and forgetting the wind.
- Using indicated altitude instead of pressure altitude for the TAS calculation.
- Ignoring temperature: the 2% rule can be off by several knots on a very hot or cold day.
- Landing at a hot, high airport at normal IAS and being surprised by the higher ground speed.
FAQ
Is true airspeed higher than indicated airspeed?
Above sea level, yes. TAS rises about 2% per 1,000 ft for the same indicated speed.
What speed do I put in a flight plan?
True airspeed at your planned cruise level.
Why does my GPS ground speed differ from TAS?
Wind. A headwind lowers ground speed; a tailwind raises it.
Do I fly approach speeds in IAS or TAS?
IAS. The aircraft stalls at about the same indicated speed at any altitude.
Reference only. Use your aircraft flight manual and official sources for real flight planning.
Sources and further reading
- FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25), flight instruments chapter
- FAA Pilot/Controller Glossary (definitions of airspeeds)
- Try it: the USFITools true airspeed calculator and wind correction angle calculator
Sources checked November 2026. For real flights, use your aircraft flight manual, your operator’s procedures and the current publications of the authority that applies to you.