Definition
A concept describing the distinction between Mach number, which expresses an aircraft's speed as a ratio to the local speed of sound, and airspeed, which expresses speed in knots through the surrounding air. At high altitudes, the speed of sound decreases as air temperature drops, so a constant indicated airspeed corresponds to a higher Mach number as altitude increases. Conversely, holding a constant Mach number while descending into warmer air results in an increasing indicated airspeed. Pilots of high-altitude, high-speed aircraft must monitor both because aerodynamic limits at altitude are governed by Mach number, while structural and handling limits at lower altitudes are governed by airspeed.
Plain English
Mach number tells you how fast you're going compared to the speed of sound. Airspeed tells you how fast the air is flowing past the aircraft in knots. They are not the same thing, and the relationship between them changes with altitude and temperature. High up where the air is cold, the speed of sound is slower, so the same airspeed gives you a higher Mach number.
Context Anchor
Seen in high-speed flight, jet cruise, climb planning, descent planning, and discussions of Mach limits.
Derivation
Mach is named after Ernst Mach, an Austrian physicist who studied the behavior of objects moving at high speeds through air. Airspeed is simply 'speed through air.' The pairing of the two terms reflects that pilots of fast aircraft cannot rely on either measurement alone.
Why Pilots Care
Using the wrong reference can lead to exceeding critical Mach, producing shock waves, buffet, and loss of control effectiveness even when indicated airspeed appears normal.
Analogy
Airspeed is like knowing how fast you are driving. Mach number is like knowing how close that speed is to a changing limit. If the limit changes, the same speed can become a larger or smaller share of it.
Grounding Statement
Climb at a constant indicated airspeed and the Mach number creeps up; descend at a constant Mach number and the indicated airspeed creeps up. The two move in opposite directions as altitude changes.
Intuition Check
Do not assume Mach number is just another name for airspeed. Airspeed is a speed through the air; Mach number is that speed compared with the local speed of sound.
Example Sentence 1
As the jet climbed through the mid-thirties, the crew transitioned from holding a constant indicated airspeed to holding a constant Mach number for the rest of the climb.
Example Sentence 2
At low altitude the pilot flew by indicated airspeed, but once above the transition level the focus shifted to keeping Mach number below the aircraft limit.