Aviation numbers
Mach, a ratio and not a speed.
Mach compares an aircraft to the air it is flying through. Because that air changes with height, so does the answer — which is exactly why airliners use it.
- 661 ktThe speed of sound at sea level on a standard day.
- 589 ktThe same sound at 30,000 ft. The yardstick shrinks as the aircraft climbs.
- ≈573 ktAbove the tropopause near 36,000 ft it stops falling and stays there.
A ratio, not a speed
Every other speed on an aircraft is a distance over a time. Mach is not: it is the aircraft's true airspeed divided by the speed of sound in the air the aircraft is flying through. It has no units, and it changes when nothing about the aircraft changes.
Mach number = true airspeed ÷ local speed of sound.
The speed of sound depends on temperature alone: a = 38.97 × √T, with T in kelvin and the answer in knots.
Why the same Mach is a different speed at every level
Air cools as you climb — about 2 °C per thousand feet up to the tropopause near 36,000 ft, and then it stops. Colder air carries sound more slowly, so the yardstick shrinks as the aircraft climbs. At sea level on a standard day sound travels at 661 kt; at 30,000 ft it is 589 kt; above the tropopause it settles at about 573 kt and stays there.
The consequence is the one that catches people out. An aircraft holding Mach 0.85 all the way up is not holding a speed at all — it is slowing down through the air even while the Mach meter does not move.
Why airliners talk in Mach at all
Because the thing that hurts is not speed, it is compressibility. Long before an aircraft reaches the speed of sound, air accelerating over the curved upper surface of the wing gets there first. Shock waves form locally, drag rises steeply and the handling changes. That threshold tracks Mach number, not knots — so the limit that matters is written in Mach, and the crew flies the number that keeps the wing out of trouble.
That is why cruise Mach numbers cluster where they do. Turboprops sit low, jets in the 0.74 to 0.85 band, and the region from roughly Mach 0.8 to 1.2 — transonic, where some of the flow is supersonic and some is not — is the one that commercial design works to avoid living in.
What it means for a charter quote
An aircraft's published cruise is often a Mach number, and turning that into an arrival time needs an altitude, a temperature and a wind. A Mach figure on its own tells you what class of aircraft you are dealing with; it does not tell you when the freight lands.
Temperatures from the ICAO Standard Atmosphere: 15 °C at sea level, a lapse of 1.9812 °C per 1,000 ft to the tropopause at 36,089 ft, constant at −56.5 °C above. Speed of sound from a = √(γRT) with γ = 1.4 and R = 287.05 J/kg·K. Real air deviates from the standard every day, which is what the ISA deviation field is for.