Which statement correctly distinguishes Mach number from indicated airspeed for flight planning?

• A. Mach number is the same as IAS at all altitudes.
• B. IAS increases with altitude while Mach stays constant.
• C. Mach number relates IAS to wind speed.
• D. Mach number relates true airspeed to the speed of sound at altitude; IAS is the instrument reading used for controls and performance; at high altitude Mach becomes the key reference.

Answer: (D)

Understanding the difference between Mach number and indicated airspeed is essential for flight planning, especially as you climb. Mach number is true airspeed divided by the local speed of sound. The speed of sound changes with temperature, which varies with altitude, so Mach provides a consistent reference to how fast you’re moving relative to sonic speed and to the onset of compressibility effects. Indicated airspeed, on the other hand, is the cockpit reading derived from dynamic pressure and calibrated for sea‑level density; it’s what you use to control the airplane and judge performance at lower altitudes, but it doesn’t directly tell you how fast you’re moving through the air when density and temperature change with altitude.

So the correct idea is that Mach number links true airspeed to the speed of sound at altitude, while indicated airspeed is the instrument reading used for control and performance; at high altitude, Mach becomes the key reference for planning and cruise because it accounts for compressibility and the changing sonic conditions.