What is Mach number?
Mach number is an aircraft's speed relative to the speed of sound. M.82 means 82% of the speed of sound. Above roughly FL280, pilots and controllers stop using knots and start using Mach.
Why the switch from knots?
At lower altitudes, airspeed is expressed in knots indicated airspeed (IAS). The pitot tube on the aircraft measures how fast air is hitting it, and that works well near the ground. But as altitude increases, the air gets thinner. An aircraft at FL350 showing 280 knots indicated is actually moving through the air at about 480 knots true airspeed.
The problem is that IAS becomes less meaningful for separation at high altitude. Two aircraft could both show 280 knots IAS but be at different flight levels with very different true airspeeds. Mach number solves this because it directly relates to the physics that matter up there - shock waves, compressibility, structural limits.
The speed of sound itself decreases with altitude because temperature drops. At sea level it's about 661 knots. At FL350 it's roughly 575 knots. So M.82 at FL350 is about 471 knots true airspeed.
Typical cruise Mach speeds
| Aircraft type | Typical cruise Mach | Example |
|---|---|---|
| Regional jets | M.74 - M.78 | CRJ-900, E175 |
| Narrowbody | M.78 - M.82 | B737, A320 |
| Widebody | M.82 - M.86 | B777, A330 |
| Fast widebody | M.84 - M.89 | B787, A350 |
| Business jets | M.80 - M.90 | G650, Global 7500 |
These numbers matter for controllers. If you have a B737 at M.78 following a B787 at M.85, the gap is growing. If a CRJ at M.74 is ahead of an A320 at M.80, the faster jet is catching up. Controllers assign Mach numbers to keep spacing consistent.
How Mach is assigned
A typical instruction sounds like: "maintain Mach point eight two" or "reduce to Mach point seven eight." The decimal point is always spoken as "point." M.82 is "Mach point eight two," not "Mach eighty-two."
Controllers use Mach for miles-in-trail spacing at high altitude. If they need 10 miles between aircraft on the same route, keeping everyone at the same Mach prevents the spacing from changing.
Crossover altitude
The crossover altitude is where a given IAS and Mach number represent the same true airspeed. Below it, the crew follows the IAS. Above it, they follow the Mach number. For a typical climb profile of 280 knots / M.78, the crossover is around FL280-FL310 depending on temperature. Above FL290, RVSM allows 1,000ft vertical separation, making accurate Mach control even more important.
Pilots usually file a speed pair like 280/M.78 in their flight plan. ATC sees both.
In radarcontrol.io
The sim uses IAS (knots) for all speed assignments. You issue commands like s280 to set 280 knots. Mach number is not directly used in commands or displayed on the scope.
In the real world, controllers above FL280 assign Mach numbers rather than knots. The sim simplifies this - when you assign a speed to a high-altitude aircraft, it works in knots regardless of altitude. The aircraft still fly at realistic true airspeeds based on their altitude, so spacing behavior is correct even without explicit Mach control.
This means you won't practice Mach-based speed control, but the fundamentals of speed-based spacing still apply.
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Related: What is a flight level? | What are airways? | How arrivals are sequenced
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