Plain English Meaning

A two-part speed plan for the descent. Up high, the pilot flies a set Mach number (here, 0.80). Lower down, where Mach is no longer the limiting factor, the pilot switches to a set indicated airspeed (here, 280 knots) and holds that the rest of the way down.

Full Definition

A descent speed schedule for jet aircraft expressed as a Mach number followed by an indicated airspeed in knots. Mach .80/280 means the aircraft descends at Mach 0.80 in the upper altitudes, then transitions to 280 knots indicated airspeed once the descent reaches the altitude where 280 KIAS and Mach 0.80 produce the same true airspeed (the crossover altitude). The aircraft holds Mach until the crossover, then holds the indicated airspeed for the remainder of the descent.