Mach appears on the PFD at which threshold speed or altitude?

• A. 0.40 Mach or 25,000 ft
• B. 0.45 Mach or 31,600 ft
• C. 0.50 Mach or 40,000 ft
• D. 0.55 Mach or 50,000 ft

Answer: (B)

As you climb, the primary speed reference on a modern PFD shifts from indicated airspeed to Mach, because at higher altitudes the air is thinner and the local speed of sound changes, making Mach a more meaningful measure of how the airplane is moving through the air. The system is designed to start showing Mach once you reach a practical threshold, which in this case is 0.45 Mach, roughly corresponding to 31,600 feet. This means the Mach reading on the PFD becomes active when you hit about Mach 0.45 or when you’re around 31,600 ft, whichever condition is met, so pilots have a consistent, performance-relevant speed reference in the cruise and descent phases.