What it is: A Mach value shows how fast you are travelling compared with the speed of sound in the surrounding air. Because the speed of sound changes with air temperature, the same true airspeed can produce a different Mach reading as you climb or descend.
In practice: In many airliners and faster jets, climb, cruise and descent speeds are managed in Mach once you reach higher altitude. A typical flight might start in indicated airspeed, then switch to a target such as Mach 0.78 or 0.82 during cruise. For simmers, this matters because aircraft performance limits, FMC schedules, autopilot behaviour and overspeed protection are often tied to Mach rather than knots. If Mach gets too high, you can run into high-speed buffet or MMO limits even when the indicated airspeed does not look excessive. That is why the speed tape in modern cockpits often shows both IAS and Mach, with the changeover happening automatically.
Key considerations:
- At altitude, watching Mach is often more useful than watching knots, especially in pressurised jets.
- MMO is a Mach limit; exceeding it in a sim can trigger warnings, autopilot reactions or handling problems depending on the aircraft.
