IAS → CAS → TAS
Navigation needs TAS (true airspeed). Dial shows IAS (indicated airspeed). Bare airspeed means IAS (AC 100-001).
Two corrections, in order:
| Step | Fix | Method |
|---|---|---|
| IAS → CAS (calibrated) | Instrument and position error in pitot-static system, usually a few knots either way | Apply ASI correction card value |
| CAS → TAS | Thin air | TAS = CAS · √(ρ0/ρ) |
- Table also runs in reverse: POH performance speed given in CAS, find IAS to fly (TP 1102, obstacle clearance approach).
Density term: ρ0/ρ = (P0/P) · (T/T0)
| Symbol | Meaning |
|---|---|
ρ |
Air density |
P0 |
ISA sea-level pressure, 29.92 in.Hg |
T0 |
ISA sea-level temperature, 15°C = 288.15 K |
P |
Ambient pressure at pressure altitude |
T |
Actual OAT, in kelvin |
(0 subscript = ISA sea level; TC AIM AIR 1.5.1)
Why TAS > CAS:
- Pitot tube measures dynamic pressure, not speed.
- Thinner air (higher or hotter): each knot of true speed pushes less air in, dial reads low.
- TAS > CAS whenever air thinner than sea level (essentially always in cruise).
- Multiply by density ratio, never divide.
- Quick check: TAS about 2% above CAS per 1000 ft (rule of thumb, not in CARs/TC AIM).
Trap: reading IAS straight off as TAS, or skipping the temperature term.
References
- AC 100-001 glossary (IAS, TAS definitions)
- TP 1102 (obstacle clearance approach: CAS to IAS via the airspeed correction table)
- FAA-H-8083-25 PHAK, glossary (calibrated and true airspeed definitions)
- TC AIM AIR 1.5.1 (ICAO Standard Atmosphere: 29.92 in.Hg, 15°C at mean sea level)
- #engine/atmosphere.js (ICAO standard-atmosphere sea-level references)