As you climb, what happens to the Mach number?

• A. It remains constant
• B. It increases because of the increase in true airspeed
• C. It decreases due to lower air density
• D. It becomes unpredictable

Answer: (B)

Mach number is the ratio of true airspeed to local speed of sound. As you climb, the air gets colder, so the speed of sound drops. At the same time, to maintain the same indicated airspeed in thinner air, the true airspeed tends to rise. With TAS increasing and the speed of sound decreasing, the Mach number climbs. So, the correct idea is that Mach increases because true airspeed increases (in this context) as you ascend, with the accompanying drop in speed of sound amplifying the effect. Density doesn’t directly set Mach, and Mach isn’t unpredictable.