Mach number is the ratio of true airspeed to the speed of sound.

• A. Ratio of actual to wind speed
• B. Indicated airspeed to true airspeed
• C. True airspeed to speed of sound
• D. Ground speed to airspeed

Answer: (C)

Mach number tells you how fast the aircraft is moving through the surrounding air compared to how fast sound travels in that air. In other words, it’s the ratio of true airspeed to the local speed of sound. The speed of sound isn’t a fixed constant; it changes with air temperature and composition, so the same true airspeed can produce different Mach numbers in different conditions. That’s why Mach is defined using true airspeed over the local speed of sound. The other ideas—using wind speed, comparing indicated to true airspeed, or comparing ground speed to airspeed—don’t characterize how close you are to sonic speed, which is tied to the speed of sound in the air you’re actually flying through.