Definition
The Mach number — the airplane's speed expressed as a fraction of the local speed of sound — that the airplane is flown at during the cruise phase of flight. In jet and high-performance turboprop operations, cruise speed at altitude is managed by Mach number rather than indicated airspeed, because the speed of sound and the airplane's aerodynamic limits both depend on temperature and altitude.
Plain English
The cruising speed of a high-altitude airplane, given as a percentage of the speed of sound rather than in knots. For example, a cruise Mach of 0.78 means the airplane is flying at 78% of the speed of sound for the air it is flying through.
Context Anchor
Seen in high-altitude cruise and descent planning, especially when a pilot or flight plan uses Mach number rather than a simple airspeed number.
Derivation
Mach is named after Ernst Mach, a 19th-century Austrian physicist who studied how objects move through air at high speeds. 'Cruise' comes from the Dutch 'kruisen,' meaning to cross or travel steadily — the steady, level portion of a flight between climb and descent. So 'cruise Mach' simply means the Mach number used during the steady part of the flight.
Why Pilots Care
It determines the top-of-descent point needed to meet crossing restrictions and fuel-efficient arrival routing.
Grounding Statement
At high altitude, a pilot may think of speed as a percentage of the speed of sound because that better matches how the airplane and air behave there.
Intuition Check
Cruise does not mean a casual or approximate speed here. It means the planned speed used in the cruise portion of the flight. Mach is not a fixed miles-per-hour number; it depends on the speed of sound in the air around the airplane.
Example Sentence 1
The crew set a cruise Mach of 0.78 and used it to calculate the top-of-descent point for the arrival.
Example Sentence 2
Accurate descent planning begins with knowing the planned cruise Mach so the top-of-descent point can be calculated correctly.