An aircraft's indicated stall speed is 130 KIAS at any altitude, with no compressibility correction needed. The airplane stalls at 130 KIAS at sea level and again at 130 KIAS at FL350, where the density ratio (sigma) is approximately 0.31. Compared to the sea-level stall, the true airspeed at the FL350 stall is approximately:
- A130 KTAS, the same as at sea level since IAS is constant
- B81 KTAS, lower, because reduced air density reduces drag and required speed
- C234 KTAS, substantially higher, because lower air density requires more true airspeed for the same dynamic pressure
- DImpossible to determine without knowing the aircraft's Mach number at FL350
Show answer & explanationAnswer & explanation
Correct answer: C. 234 KTAS, substantially higher, because lower air density requires more true airspeed for the same dynamic pressure
Indicated airspeed reflects dynamic pressure; true airspeed relates to IAS by TAS = IAS / sqrt(sigma). At FL350, sigma ≈ 0.31, so sqrt(0.31) ≈ 0.556. TAS = 130 / 0.556 ≈ 234 KTAS. Even though the indicated stall speed stays the same, the actual (true) speed through the air at altitude is much higher because thinner air requires more true speed to generate the same dynamic pressure/lift.
Why the other options are wrong
- A. Incorrect — TAS is not constant with altitude even though IAS is.
- B. This reverses the relationship; TAS increases, not decreases, with altitude for constant IAS.
- D. TAS can be found from IAS and density ratio alone without needing Mach number.
True Stall Speed vs Altitude
Indicated stall speed remains essentially constant with altitude (absent compressibility effects), but true airspeed at the stall increases with altitude because TAS = IAS/√σ, where σ decreases with altitude.
- TAS = IAS / √(density ratio σ)
- σ decreases as altitude increases
- Indicated stall speed stays roughly constant
- True stall speed increases significantly at altitude
Memory trick: Thin air means you must go 'true-ly' faster to feel the same push