I was flying right seat on an arrival back into CLT a while ago, and as we were descending, the controller asked us to “maintain 280 knots in the transition.” That was a new one on me. As is my custom, I looked at the guy in the other seat and said, “huh?” He repeated it to me and I repeated it to ATC.
The guy in the other seat – I can’t remember who it was – pointed to the airspeed tape and said something like we’re descending now at mach .76 going through 34,000 feet. When the airspeed comes up to 280 knots, we’ll start maintaining that instead of our mach number. Pretty impressive that I can remember what a guy said verbatim, but can’t remember who the guy was, right? I knew that at some point, the mach number and the airspeed “met,” but I’d never heard it called “the transition.”
When we’re flying at altitudes above 31,000 or so, we measure our speed in mach. There are tons of resources on the internet where you can get WAY into the weeds on this subject, so I’ll keep it at a basic level. Mach is the ratio of the airplane’s speed relative to the speed of sound in its current chunk of air. At sea level and 59°F, the speed of sound is 660 knots (760mph) but those numbers change with altitude and temperature.
A knot (one nautical mile per hour) is the unit by which we measure airspeed down low. It’s rooted in the longitude/latitude coordinate system and is the standard measure of speed for nautical and aviation navigation. By using a standard unit of measure, the need to convert miles per hour to kilometers per hour or vice versa, depending on where in the world the navigation is taking place, is eliminated.
For the example I’m using here, we’re flying at an altitude of 40,000 feet at a speed of mach 0.76 and 227 knots, which means that 227 knots is 76% of the speed of sound in our current chunk of air. After doing some rudimentary math that I definitely didn’t struggle with at all, I figured that the speed of sound is 298.6 knots (344mph).
So, back to “the transition.”
If you take a look at the STOCR 3 RNAV arrival into KCLT, you’ll see a note at the top of the plate that says to maintain your mach number until intercepting 280 knots.
The purpose of arrivals is to minimize radio communications between airplanes and ATC, and this is just another part of that. This note allows each airplane fly at whatever speed the pilot chooses until they intercept 280 knots “in the transition,” which puts everybody at the same speed at the same point in space in an attempt to keep all the aircraft on that arrival spaced correctly.
As we start to descend at the mach number, the airspeed in knots begins to increase. At a certain point along the descent path, the airspeed in knots is going to increase to the target speed, 280 knots in this case. I diligently labored literally threes of minutes to make this very accurate and professional-looking graph to illustrate what I’m talking about.
Once we capture 280 knots and maintain it in the descent, the mach number will begin decreasing. And, just like that, we’re able to “maintain 280 knots in the transition.”