Engineering & Scientific
Pitot-Static Tube Velocity: Equivalent Airspeed (EAS) vs True Airspeed (TAS)
Convert Pitot-static differential pressure dP = P_total - P_static into sea-level calibrated Equivalent Airspeed and altitude-density-corrected True Airspeed V_TAS = SQRT(2 * dP / rho).
Why sea-level Pitot gauges systematically under-read true flow velocity at altitude or high temperature
Henri Pitot’s stagnation probe combined with a wall static port measures incompressible dynamic pressure dP = P_total - P_static = 0.5 * rho * V^2 (Pascals). Standard mechanical airspeed indicators and uncompensated HVAC Pitot manometers assume ISA sea-level air density rho_0 = 1.225 kg/m^3, reporting Equivalent Airspeed V_EAS = SQRT(2 * dP / 1.225).
When an aircraft climbs or stack flue gas heats up so actual local density drops to rho = 0.750 kg/m^3, the exact same 1,500 Pa Pitot differential corresponds to V_EAS = 49.49 m/s on a sea-level gauge while actual True Airspeed is V_TAS = SQRT(2 * 1500 / 0.750) = 63.25 m/s—a SQRT(1.225 / 0.750) = 1.2780x (+27.80%) velocity gap!
| Pitot Anemometry Quantity (dP = 1,500 Pa) | Formula | Result (rho = 0.75 kg/m3) |
|---|---|---|
| Sea-Level Equivalent Speed V_EAS | SQRT(2 * dP / 1.225) | 49.49 m/s |
| Density-Corrected True Speed V_TAS | SQRT(2 * dP / rho_local) | 63.25 m/s (122.95 knots) |
| Density Correction Factor | SQRT(rho_0 / rho_local) | 1.2780 (+27.80% over EAS) |
Local-Density True Airspeed / Duct Velocity V_TAS Formula (m/s)
Guard rho <= 0, clamp negative transducer zero-drift via MAX(0, B2), and evaluate SQRT(2 * dP / rho).
=IF(B3 <= 0, "INVALID_DENSITY", ROUND(SQRT((2 * MAX(0, B2)) / B3), 2))Sea-Level Equivalent Speed V_EAS & TAS/EAS Ratio
Compare V_TAS against sea-level SQRT(2 * dP / 1.225) and verify they match identically (49.49 m/s) when local density equals 1.225 kg/m^3.
=ROUND(SQRT(B4 / B3), 4)Try the example
Use the original values or change them to test your understanding.