Airspeed Indicator (ASI): IAS, CAS, TAS, and Pitot-Static Errors

022-02-06Pablo Asensio Martínez2026-03-271 min

The Airspeed Indicator (ASI) measures the speed of the aircraft through the air by comparing total pressure and static pressure.

Principle

  • Formula: $\text{Dynamic Pressure} (q) = \text{Total Pressure} (P_t) - \text{Static Pressure} (P_s)$.
  • Mechanism: Pitot pressure ($P_t$) is fed to the capsule; static pressure ($P_s$) is fed to the case. The expansion of the capsule represents dynamic pressure ($1/2 \rho V^2$), which is calibrated to airspeed.

Airspeed Definitions

  1. IAS (Indicated): The direct reading from the dial.
  2. CAS (Calibrated): IAS corrected for Instrument and Position errors.
  3. EAS (Equivalent): CAS corrected for Compressibility error (significant > 300 kts / Mach 0.4).
  4. TAS (True): EAS corrected for Density error (Altitude/Temperature). TAS is the actual speed through the air.
    • Rule of Thumb: TAS increases with altitude for a constant IAS. ($TAS \approx EAS \times \sqrt{\rho_0 / \rho}$).

Blockages (PUDSOD)

Errors depend on which port is blocked and the phase of flight:

  • Pitot Blocked:
    • Under-reads in Descent (and over-reads in climb). Acts like an altimeter.
  • Static Blocked:
    • Over-reads in Descent (and under-reads in climb).

Color Coding

  • White Arc: Flap operating range ($V_{SO}$ to $V_{FE}$).
  • Green Arc: Normal operating range ($V_{S1}$ to $V_{NO}$).
  • Yellow Arc: Caution range ($V_{NO}$ to $V_{NE}$).
  • Red Line: Never Exceed speed ($V_{NE}$).
  • Blue Line: Best single-engine rate of climb ($V_{YSE}$) on light twins.

Unreliable Airspeed

Recognized by mismatched ASI readings, IAS disagree alerts, or abnormal pitch/power relationships. Pilots must cross-check with Ground Speed (GNSS/INS) and pitch/power tables.