IAS → CAS → TAS

For navigation you need your true airspeed (TAS), but the dial shows IAS (indicated airspeed). Two corrections bridge the gap, in order:

• IAS → CAS (calibrated airspeed). Look up the number on the ASI correction card and apply it, usually a few knots either way. This covers instrument and position error in the pitot-static system.
• CAS → TAS. Then correct for thin air: TAS = CAS · √(ρ0/ρ), where ρ is air density and ρ0/ρ = (P0/P) · (T/T0). The 0 subscripts are the ISA sea-level references (P0 = 29.92 in.Hg, T0 = 288.15 K); P is the ambient pressure at your pressure altitude and T the actual OAT, in kelvin.

The pitot tube measures dynamic pressure, not speed. In thinner air (higher up, or hotter) each knot of true speed pushes less air into it, so the dial reads low. So TAS is more than CAS whenever the air is thinner than sea level (essentially always in cruise): you multiply by the density ratio, never divide. Quick check: TAS runs about 2% above CAS per 1000 ft of altitude (rule of thumb, not in the CARs/TC AIM).

If OAT is quoted in °F, convert to °C (then kelvin) before the density term: °C = (°F − 32) · 5/9. Feeding the °F number straight in overstates temperature and TAS.

Trap: reading IAS straight off as TAS, skipping the temperature term, or leaving OAT in °F.

Practice