Definition
The highest Mach number at which an airplane is certified to be operated. Mach number is the airplane's true airspeed expressed as a fraction of the local speed of sound, so an MMO of 0.84 means the airplane is approved to fly up to 84% of the speed of sound. At higher altitudes, where the speed of sound is lower and compressibility effects become significant, MMO replaces VMO (the maximum operating airspeed in knots) as the limiting speed.
Plain English
The fastest speed, expressed as a fraction of the speed of sound, that the airplane is allowed to fly. Going faster than this risks airflow problems over the wings that the airplane was never designed or tested to handle.
Context Anchor
Seen in the airplane flight manual or pilot’s operating handbook, especially for high-performance airplanes operated at higher altitudes.
Derivation
The 'M' stands for Mach, named after Austrian physicist Ernst Mach, who studied how objects move through air at high speeds. 'MO' stands for Maximum Operating. So MMO literally reads as 'Mach, Maximum Operating.'
Why Pilots Care
Exceeding MMO risks shock-wave formation, loss of elevator effectiveness, and possible structural damage.
Intuition Check
MMO is not the same as a normal airspeed limit shown only in knots. It is a Mach limit, so it matters most when the airplane is high and fast.
Example Sentence 1
Climbing through the upper twenties, the crew transitioned from watching VMO to watching MMO as the Mach limit became the more restrictive of the two.
Example Sentence 2
The multiengine jet's flight manual listed MMO as a hard limitation that must never be exceeded in level flight.