Which statement is true regarding IAS and Mach number for high-altitude planning?

• A. IAS remains the primary reference for planning at all altitudes.
• B. Mach number and IAS are unrelated.
• C. IAS measures vertical speed.
• D. Mach number relates true airspeed to the speed of sound at altitude; IAS is used for controls; at high altitude Mach becomes the key reference.

Answer: (D)

At high altitude, the speed reference that matters for planning shifts from indicated airspeed to Mach number because the local speed of sound changes with altitude and temperature, and the aerodynamic regime you’re in is best described by Mach. Mach is the ratio of true airspeed to the local speed of sound, so it directly tells you how close you are to sonic conditions and how the aircraft will behave in different layers of the atmosphere. Indicated airspeed, while useful for control and ensuring safe margins at lower speeds, is tied to dynamic pressure and air density, which vary with altitude. That means the same IAS can correspond to very different true airspeeds as you climb, making it less reliable for planning high-altitude performance. So the true statement is that Mach relates true airspeed to the speed of sound at altitude, IAS is used for controls, and at high altitude Mach becomes the key reference.