True Airspeed & Mach

A pitot-static system measures impact pressure, the difference between total and static pressure. Calibrated airspeed is the speed at which sea-level standard air would produce that impact pressure: q_c = p₀ × [(1 + 0.2 (CAS ÷ a₀)²)^3.5 − 1]. At altitude the same q_c against a lower static pressure p means a higher Mach number, M = √(5 × [(q_c ÷ p + 1)^(2/7) − 1]), and true airspeed is Mach times the local speed of sound, √(γRT) at the outside air temperature. Static pressure is the ISA pressure at the pressure altitude.

Equivalent airspeed is TAS × √σ, the speed that gives the same dynamic pressure at sea level; it differs from CAS only by compressibility, a fraction of a knot below Mach 0.3 and more above. The rule of thumb — TAS ≈ CAS + 2 % per 1,000 ft — assumes a standard day and ignores compressibility; the page shows its figure and the difference.

Given a true airspeed the page runs the same relations backwards to the calibrated airspeed you would see — useful for a cruise speed quoted in TAS. The relations are subsonic; above Mach 1 the page declines.

Indicated airspeed is taken as calibrated: the position and instrument error table in the aircraft handbook comes first, and can be several knots at low speed. A planning aid, not a substitute for the pilot’s operating handbook, the aircraft’s limitations or the rules that apply to the flight. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.