Definition
The airspeed and power setting that produces the greatest distance flown per unit of fuel burned, giving the airplane its maximum still-air range. It is typically a slower cruise speed than the one used for best time, because flying slower reduces fuel flow more than it reduces ground covered, up to a specific optimum point.
Plain English
The cruise speed that lets the airplane travel the furthest on the fuel it has on board. It is the most fuel-efficient way to cover distance, even though it is not the fastest way to get there.
Context Anchor
Seen in high-speed and jet airplane discussions, especially when learning about speed margins near the upper end of an airplane’s operating range.
Derivation
M comes from Mach, named after physicist Ernst Mach, whose name is used for speed compared with the speed of sound. CR means critical, because this is the important point where sonic airflow first appears on part of the airplane.
Why Pilots Care
Operating below this speed with an engine failure can result in loss of directional control and an unrecoverable yaw toward the failed engine.
Intuition Check
MCR does not mean the whole airplane is flying faster than sound. It means some airflow over the airplane has reached the speed of sound first.
Example Sentence 1
With a strong headwind and a long leg ahead, the pilot set power for maximum range cruise to stretch the available fuel.
Example Sentence 2
During the engine failure demonstration, the instructor had the student identify and hold MCR before beginning recovery.