FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsHard
A transport aircraft is cruising at high altitude and high Mach number. The pitot-static system senses a higher impact pressure than incompressible-flow calibration would predict for the same true airspeed. What correction must be applied to calibrated airspeed (CAS) to account for this?
- AA density correction added directly to CAS to obtain true airspeed
- BA position error correction based on angle of attack
- CNo correction is needed above FL250 since Mach number replaces airspeed indications entirely
- DA compressibility correction, subtracted from CAS to obtain equivalent airspeed (EAS)
Show answer & explanationAnswer & explanation
Correct answer: D. A compressibility correction, subtracted from CAS to obtain equivalent airspeed (EAS)
At high Mach numbers, air compressibility causes the pitot probe to sense a disproportionately higher impact pressure than incompressible calibration assumes. This compressibility error must be subtracted from CAS to obtain equivalent airspeed (EAS), which is then used with density altitude to derive true airspeed (TAS).
Why the other options are wrong
- A. TAS is derived from EAS and density altitude, not directly from CAS with a simple density addition.
- B. Position error relates to sensor location/airflow disturbance, not compressibility, and is a separate correction.
- C. Airspeed indications and their corrections remain relevant and necessary even when Mach is the primary reference.
Compressibility Error / EAS
At high speed and altitude, compressible airflow causes the pitot-static system to over-read impact pressure; a compressibility correction is subtracted from CAS to yield equivalent airspeed (EAS), which is then corrected for density to obtain TAS.
- Compressibility error grows with increasing Mach and altitude
- CAS − compressibility correction = EAS
- EAS + density correction = TAS
Memory trick: CAS shrinks to EAS, then grows to TAS with thin air