A pilot is conducting a high-altitude cruise flight. What is the primary reason that maintaining a constant Mach number is often preferred over maintaining a constant indicated airspeed (IAS) for optimal performance at high altitudes?
- ATo maximize lift-to-drag ratio.
- BTo maintain a constant true airspeed (TAS).
- CTo prevent exceeding the aircraft's structural load limits.
- DTo ensure efficient engine operation and avoid compressibility effects.
Show answer & explanationAnswer & explanation
Correct answer: D. To ensure efficient engine operation and avoid compressibility effects.
At high altitudes, air density decreases significantly, and the speed of sound also decreases. Maintaining a constant Mach number helps keep the aircraft within its efficient operating range relative to the speed of sound, preventing issues like shock waves and excessive drag due to compressibility effects, which are critical for high-altitude, high-speed flight.
Why the other options are wrong
- A. While important, L/D ratio is not the primary reason for favoring Mach over IAS at high altitude.
- B. Maintaining constant Mach number does not necessarily mean constant TAS, as TAS varies with temperature.
- C. Structural limits are related to IAS and load factor, not primarily Mach number for cruise.
Mach vs. IAS at High Altitude
At high altitudes, maintaining a constant Mach number is often preferred over constant IAS to manage compressibility effects and ensure engine efficiency, as the speed of sound decreases with altitude.
- Mach number is TAS divided by the local speed of sound.
- Speed of sound decreases with temperature (and thus altitude).
- Compressibility effects (e.g., shock waves, drag rise) become significant at high Mach numbers.
- Engines are designed for specific Mach operating ranges at high altitudes.
Memory trick: At altitude, Mach keeps your engines happy and avoids speed-of-sound surprises, unlike IAS which gets tricky.