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.
Plain English
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.
Context Anchor
Seen in jet descent planning, especially when calculating or briefing the descent profile for high-performance aircraft.
Derivation
Mach is named after Ernst Mach, the physicist who studied airflow and the speed of sound. A Mach number is a fraction of the local speed of sound. The slash notation pairs the high-altitude limit (Mach) with the lower-altitude limit (indicated airspeed in knots), reflecting that each is the relevant speed limit in its own altitude band.
Why Pilots Care
Allows efficient high-speed descent while preventing excessive airspeed as the aircraft descends into denser air.
Grounding Statement
At high altitude, jets are often flown by Mach number; lower down, they are usually flown by the speed shown on the airspeed indicator.
Intuition Check
The slash does not mean “choose either Mach .80 or 280 knots.” It means use Mach .80 first, then transition to 280 knots when that becomes the controlling speed for the descent.
Example Sentence 1
The crew programmed a Mach .80/280 descent profile into the FMS before starting down from FL370.
Example Sentence 2
Following the descent profile, they held Mach .80/280 until reaching 280 knots.