Stacking ‘Em

ATC using altitude to separate aircraft is the perfect example of working smarter, not harder. And that makes it better for everyone.


Ever seen those videos of a retail store opening on Black Friday? The doors crack open. A tumbling mass of humanity spews in. Fights break out. People get trampled. Just good holiday fun…

There are some days as an air traffic controller where it seems every single IFR airplane is filed the same route at the same altitude, jockeying for that same piece of sky. My job is to prevent them from trampling each other.

What tools does ATC use to recognize when there’s a problem, and then how do we resolve it?

A Pound of Prevention

A good instrument scan is something you’re taught from the first hours of flight school. During my first cross-country training flight, my instructor noticed something I’d missed: Our Cessna 172’s oil temperature gauge was running unusually high. We landed at a nearby non-towered airport, let it cool off, noted the oil level was on the lower side of normal, and added some oil. We gave it another go, flying higher for cooler temps, and everything ran fine afterwards.

A good scan picks up the little issues that may or may not be the first clue to a larger issue developing. The earlier a problem is noticed, the earlier you can take action to remedy it. Those are concepts I’ve carried through into my air traffic control career.

I was working radar one afternoon. Center fed me a Cessna 310 at 7000, cruising along their filed route. I wasn’t too busy, but the controller in the sector next to me was getting his butt kicked. Controllers can easily view the traffic surrounding their sector, allowing us to be an extra pair of eyes.

My scan landed on an overflight he had, a Baron BE58, also at 7000. Its route would enter my airspace and intersect with my Cessna 310. Two aircraft at the same altitude and speed, at the same distance from the intersection of their routes? The conflict was clear.

How close would they get? Our radar scopes have a Minimum Separation tool. I pressed the “MinSep” key and clicked both targets. The computer ran the Distance = rate × time calculations and drew lines from each target to the closest point of expected crossing. We normally need a minimum of three miles between IFR aircraft. MinSep showed I’d have … 0.58 miles.

With three miles not happening, I’d need IFR altitude separation: at least 1000 feet. I also didn’t want to add any more work to my friend, especially since the cross-out would happen well into my airspace. I would adjust the Cessna 310, so he wouldn’t need to say anything to the Baron.

I asked the C310, “For crossing traffic, can you accept 5000 or 9000 as your cruising altitude?” He accepted 9000 and I cleared him to it. Just like that, the conflict was resolved. If he’d said “unable” or if they’d been closer, I certainly could have vectored him to build the required three miles horizontally. In this scenario though, with plenty of time to pre-plan, altitude separation proved the least painful option for all parties. Also, once they’d crossed out, I could’ve descended him back to 7000.

Altitude Altruism

What if I have multiple aircraft headed the same direction? A couple of days ago, I had a Beechcraft Baron, a Cirrus SR22, and a Cessna Skyhawk, all filed for 6000 feet, their routes all merging over the same fix and joining the same airway. With their speeds, they would be a tie at the fix.

Leaving the SR22 at 6000, I climbed the Baron to 8000 and dropped the Skyhawk to 4000. The Cirrus and Baron would outrun the Cessna. Altitude changes resolved all issues, with no vectors or routing amendments.

So, why did I choose 8000 for the Baron and 6000 for the SR22? Well, simply deconflicting the traffic isn’t enough. Controllers need to think about efficiency, and that means considering who’s landing first.

The SR22 and the Baron had similar cruising speeds, so they’d be stacked atop one another for a while. However, the SR22 was landing at an airport only 50 miles beyond my boundary. The Baron was landing 100 miles away. If I put the Baron underneath, some controller down the way would have had to vector one or both aircraft away from the other so the SR22 could descend through the Baron’s altitude.

The resolution was to leave the SR22 at 6000 and climb the Baron to 8000. This simplified everything for both controllers and pilots down the road. With the SR22 below, the Baron could just keep cruising along as the Cirrus veers off towards its destination. As I tell my trainees, it’s good to be a team player and set everyone up for success.

Rule in Your Favor

Just as we feed traffic to Center in a way that’s advantageous to them, they also feed us traffic in a way that makes sense. Our Letter of Agreement with our overlying Center requires them to feed us turboprop aircraft at 9000 feet or below, and jets at 10,000 feet.

1000 feet isn’t a big difference in the vertical, but it’s a huge gap when it comes to speed. Why? Because aircraft speed limits established in §91.117, namely: “(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots.”

Imagine that Center feeds me a Super King Air (BE30) at the prescribed 9000 feet. A Super King Air can cruise at 310 knots, but he’s already at 250 knots to comply with the regulation. Ten miles in trail, Center hands me a Falcon 900 (F900) showing 400 knots over the ground. Both aircraft are going to the same airport, on the same arrival. The BE30 has 70 miles until it’s at a midfield downwind, the F900 80 miles.

Now, could I tell the F900, “Descend and maintain 9000”—effectively assigning 250 knots as his speed— and make him follow the King Air? If the King Air was the tail-end of an established sequence of other airplanes inbound to their destination, then, yes, absolutely, I would do so.

Here, they’re the only two arrivals. Slowing the Falcon would be inefficient. Why? Let’s do some simplified math. The King Air at a constant 250 knots will take 16.8 minutes to fly the 70 miles. If I slow the Falcon to follow him, the 80 miles at a constant 250 knots will take 19.2 minutes. Hmm, only a difference of 2.4 minutes. But what if I use the benefits of altitude to my advantage?

Instead, I leave the F900 at 10,000 for seven minutes. At 400 knots, he’ll fly 46.7 nautical miles while the King Air will fly only 29.2—a difference of 17.5. Remember, the F900 started out 10 miles behind the King Air. Subtract that additional ten miles, and we find the Falcon’s now 7.5 miles ahead of the BE30. I only need three miles separation, so I can start descending the F900 through the King Air’s altitude.

Now let’s look at the time involved. The King Air, again, will take 16.8 minutes. The F900 flew 46.66 miles in 7 minutes at 400 knots, then flew the remaining 33.34 miles at 250 knots in 8 minutes for a total of 15 minutes. So, he only beats him by 1.8 minutes? Not so fast. Remember: if I’d made the F900 follow the King Air, he would’ve been 2.4 minutes in trail. So, total time saved for the Falcon was actually 4.2 minutes.

Four minutes and change doesn’t sound like much, but it makes a difference. Obviously, this saves the aircraft time and gets the passengers and/or cargo where they need to be. This is especially true for critical Medevac flights. This also benefits ATC—the sooner an aircraft gets to where it’s going safely, the sooner controllers can free up mental bandwidth to scan and focus on other aircraft.

Know Your Players

Even amongst jets, aircraft type matters. Controllers joke that Cirrus Aircraft’s SF50 jet is called the “Vision” because it only imagines itself a jet. Don’t get me wrong: it’s certainly a neat airplane, with its V-tail looks and ballistic recovery system. I’d certainly love one in my hangar! However, its speed and climb performance, compared to many other jets is … leisurely.

Nevertheless, the Vision is a jet and eligible to fly our jet-only Standard Instrument Departures. On its own that’s no problem. However, when you start mixing it with higher performance jets, you run into compatibility issues.

One afternoon, our tower launched an SF50 on a jet SID. I climbed him to 10,000 and handed him off to Center, who would climb him to the flight levels. Shortly thereafter, Tower departed a Cessna Citation X (C750) on the same SID. They climb like homesick angels, accelerate rapidly to 250 knots, and really start screaming when they hit 10,000 feet. If I did nothing, he was going to catch up to the Vision jet.

I had two options. First, vector the SF50 off the SID on a heading that diverged from the Citation X, call Center, and request approval to feed them the Vision on that heading. I’ve used this technique before, and would use it here if there were a string of fast departures eating up the Cirrus. However, while we use inter-facility coordination all the time, we try to limit it to situations where we can’t fix things ourselves.

Instead, with only two players, I altitude-separated them. I told the SF50, “Amend altitude. Maintain 6000.” That stopped his climb, while I paused the handoff to Center. To the Citation, I said, “Climb and maintain 10,000. Expedite climb through 7000.”

The Citation X hurried through 7000, topping the Vision. Then I switched him to Center for his climb. Once I saw the Citation well above and ahead of the SF50, I climbed the Vision to 10,000 and handed him off to the Center. No coordination necessary and the Vision was only slightly delayed.

Altitude separation might seem as simple as getting 1000 feet between airplanes. Effectively achieving it, though, requires foresight, knowledgeable application of the rules, occasional coordination, and awareness of aircraft performance. The end goal is the safe and sensible flow of air traffic.

Tarrance Kramer does his best to keep ‘em appropriately and efficiently stacked so everyone—Skyhawks, King Airs, Vision Jets, Falcons and other controllers—gets to proceed as painlessly as possible.


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