FAA Aircraft Dispatcher (ADX)Flight Planning and PerformanceHard
A turbojet climbs from sea level to FL350 while the pilot maintains a constant calibrated airspeed (CAS) of 250 knots. As altitude increases, what happens to the aircraft's true airspeed (TAS)?
- ATAS decreases because air density decreases
- BTAS remains equal to CAS at all altitudes
- CTAS increases only if Mach number decreases
- DTAS increases because air density decreases
Show answer & explanationAnswer & explanation
Correct answer: D. TAS increases because air density decreases
As altitude increases, air density decreases, so a given CAS corresponds to a higher TAS to produce the same dynamic pressure (indicated airspeed). This is why TAS diverges increasingly above CAS at higher altitudes.
Why the other options are wrong
- A. TAS increases, not decreases, as density drops.
- B. TAS only equals CAS near sea level under standard conditions; they diverge with altitude.
- C. TAS increase is driven by density change, not directly tied to Mach number decreasing.
TAS vs CAS with Altitude
As altitude increases and air density decreases, true airspeed (TAS) becomes progressively greater than calibrated airspeed (CAS) for the same dynamic pressure reading.
- CAS reflects instrument-corrected dynamic pressure
- TAS = CAS adjusted for air density (via EAS)
- Divergence between CAS and TAS increases with altitude
Memory trick: 'Thin air, fast plane' — as density drops, TAS climbs above CAS.