The Air Traffic Ballet

The airspace that was the subject of a coordinated aerial ballet two days ago. The magenta line shows where pilots thought they would be going. The orange line shows where they ended up. (FAA sectional charts via SkyVector.)

During our travels Deb has often mentioned our interest in, and nearly-all-times admiration for, the Air Traffic Controllers with whom we deal. One of her early posts, “Say Souls on Board,” is here; you can find a collection of others from the past three years here.

This past weekend, on our way from western Kansas into Denver, we had another illustration of the unflapped competence with which members of the ATC system can perform. What follows are a lot of details, which put in context the audio clip below. If you want to skip the details, the summary point is: this is a little sample of people doing very high-stakes work calmly, and without anyone particularly noticing except for those they’re directly dealing with.

***

Here is background on three fronts:

  • The permanent landscape. Our destination was Centennial Airport, in Denver, airport code KAPA. Centennial is a very busy, very nice, “reliever” airport on the south side of Denver, near Aurora. It has two big, parallel north-south runways, plus a smaller runway at an east-west angle.

    Centennial’s big runways, and its relatively flat-land location, make it attractive for aircraft ranging from little piston airplanes like ours to sizable jets. But the airport works within a narrow band of constraints, in this sense:
    — Not far west is the front range of the Rockies, which rises up with impressive suddenness.
    — Immediately north is the military airspace of Buckley Air Force Base.
    — Immediately up is the “Class Bravo” airspace for Denver’s main international airport, which it’s better to stay out of.
    The Buckley AFB airspace goes up to an elevation of 7500 feet; the Denver Class B starts at 8000 feet. Thus the last part of our approach to Centennial was within that 500-foot buffer, at 7700 feet.
                                                                                                                               .
  • The momentary landscape. On the way up from Kansas (where we’d been on a reporting trip, and where we’ll return after this week’s Aspen Ideas Festival) we’d followed NEXRAD weather displays of a great big thunderstorm in the mountains west of Centennial. The in-cockpit weather display is updated about every six minutes. Cycle by cycle, we could see the storm staying very large, but moving slowly south and east. That took it away from Centennial airport, which left the airport clear, but it still occupied a whole lot of the sky just a few miles to the south.

    We were within about ten miles of the airport — just a minute or two before beginning to descend, going through the checklist for landing — and were glad that the storm was far enough away to seem no problem. (If it had been closer, we would have diverted to an airport in the plains east of Denver and waited things out.) The winds at the airport were from the south, so planes were landing from the north, on Centennial’s Runways 17 (left and right). This is the route shown more or less by the magenta arrows in the map above, and in more detail here:
    Landing from the north, the magenta line; from the south, in orange. All the inbound planes were planning to follow the magenta line, until they were switched around to the orange.
  • The big change. Just as we were getting oriented to land from the north, the Centennial control tower reported a sudden and significant change in the winds. For reasons probably related to the nearby storm, winds at the airport quickly reversed direction. Instead of coming from the south, they started blasting in from the north — and were so strong that it would be risky to land with them as tailwinds. (Simple explanation: the speed with which an airplane meets a runway is its airspeed minus the headwind. If you land at 80 knots airspeed with a 20 knot headwind, you’re meeting the runway at 60 knots. But if you land with a 20 knot tailwind, you’re meeting the runway at 100 knots, and you could well run out of asphalt before you could stop.)

That’s the setup for the air-traffic recording you can hear below. It captures the transmissions from the control tower of a very busy airport, working within a constrained physical and regulatory space, when it all of a sudden has to re-direct a flow of incoming aircraft. The challenge for the controller is to send each airplane somewhere that will keep it out of the others’ way, and also away from the thunderstorm (and the mountains), and then sequencing planes one by one for a landing in the opposite direction from what they were expecting.

The orchestration of this complex event is what you hear below. Starting at time 10:50, the controller announces the change in winds. Rather than landing from the north on runways 17 Right and 17 Left, planes are swung around to land the opposite way, on runways 35 Right and 35 Left.

You’ll hear a few pilots still on the ground deciding that the sudden strong tailwinds — “11 knots gusting 23 knots” — were within limits for taking off. Then an inbound pilot decides to break off his landing, and the controller directs him for a “go around.”  Lots of action happens in the dozen minutes starting around 10:50, with the controller coordinating planes into some orderly queue for getting on the ground.

If you listen late into the recording you’ll hear an airplane called “Cirrus 435 Sierra Romeo” get into the action. That’s us, with me speaking in a clipped way to minimize air time on a crowded frequency. Near the end of this recording, after I’ve landed and am being handed off to the ground-control frequency, I wind up my transmission with a a very quick “good job!” to the controller That was spontaneous by me and not really standard practice, but it was sincere because of the calm synchronization I’d just seen, and been part of.

Here is the clip, via LiveATC.net and the SoundCloud player. (And here is a link to the recording starting at time 10:50.)

***

People may imagine that Air Traffic Control plays a minute-by-minute life-and-death role in keeping planes from falling out of the sky, more or less like in the movie Airplane!. As William Langewiesche explained in this wonderful Atlantic piece from the 1990s, usually it’s not that way at all. In most phases of flight, planes cruise along perfectly well on their own. The ATC’s most important role for airliners is sequencing them efficiently and safely in and out of the highly congested space at major airports.

Update: For tech screw-up reasons on my end, a version of this piece was initially published last night, and then mysteriously un-published (when an unsaved draft over-wrote it). Thanks to the Atlantic’s tech team for retrieving the right version, and sorry for any confusion. Also, Deb may add a post with her account of what this looked like from the right-hand seat as we were being vectored all around.