For flight planning at high altitude, Mach number relates TAS to what factor?

• A. The speed of sound at altitude
• B. Air density changes
• C. Wind speed
• D. Time of day

Answer: (A)

Mach number is the ratio of true airspeed to the local speed of sound. At altitude, the speed of sound varies with air temperature, so the same TAS corresponds to a different Mach depending on how warm or cold it is up there. That’s why Mach is used for high‑altitude flight planning: it ties performance and aerodynamic behavior to the actual speed of the air the aircraft is moving through, which changes with altitude. The other factors listed don’t define Mach—air density, wind, and time of day affect other aspects like indicated or groundspeed, but Mach specifically relates TAS to the speed of sound at that altitude.