The Invisible Hand

Imagine a Piper J-3 Cub parked beside a mighty double-decker Airbus A380. The two airplanes couldn't be more different, yet both function on the same four principles: thrust, drag, lift, and weight. The 1.2 million pound Airbus just scales them up a thousand times over the 1,200-pound Piper.

Each aircraft is designed for its purpose. The A380 is perfect for carrying you and 600 friends around the world, but wouldn't do well on a short, rough grass strip where the Cub would shine.

Air Traffic Control facilities are also designed to their mission. ATC's control tower, the J-3 of the system, manages traffic within a few miles. Managing the flow around the larger airports are Terminal Radar "Approach" Control facilities. Above the TRACONs lie Air Route Traffic Control Centers with hundreds of controllers watching thousands of cubic miles of sky. Each facility has the same purpose—order and safety—on increasingly larger scales.

As large as Centers are, the "A380" is the Air Traffic Control System Command Center in Warrenton, Virginia. It is responsible for the entire National Airspace System. Unlike tower or radar controllers who micromanage up to 30 planes at a time, the ATCSCC oversees the five thousand or so aircraft at any given moment over the United States.

Pilots never talk to it. It has no frequencies or sectors. Like a conductor keeping time for a large orchestra, it plays no instrument, but instead keeps every note and musician working in unison. On a national scale, it enforces the same order and safety as your local tower through the use of information, coordination, and restrictions.

Task saturation plagues all aspects of aviation. The more complex a situation, the easier it is to fall behind in the cockpit or at the radar scope, and the more order and safety may be compromised.

The Command Center's job is to ensure a balanced flow of traffic, preventing any part of the system from getting overwhelmed by too many aircraft. Predicting and handling an overload demands accurate data and teamwork.

The Collaborative Decision Making organization, a joint effort by the ATCSCC, its subordinate ATC facilities, over two dozen airlines, and academic personnel, is tasked with balancing the traffic flow throughout the US.

The ATCSCC talks with all CDM members every two hours. The decision making starts with facts. Every airport has a limit to how many arrivals it can handle, known as the Airport Arrival Rate. The ARR is typically 60 airplanes per runway per hour. Also, by agreement with the CDM, the airlines share with the ATCSCC their proposed flight schedule. By tallying up all the flights into a particular airport, they determine how much traffic that airport—and any ATC facility responsible for it—will be facing.

The result is an operational plan for the day that accounts for those known facts as well as forecast problems, such as runway closures, equipment outages, and, of course, weather issues. Everything in this system is interconnected. What affects one airport will affect all of the Centers and TRACONs who feed it. It's a ripple effect on a national scale.

This collaborative system is designed to adapt and handle the unpredictable, random events that can interrupt the national flow of traffic.

Major Class B airports—such as New York's La Guardia and JFK, Atlanta's Hartsfield, and Charlotte's Douglas—face solid streams of moving metal inbound from all directions. That's thousands of aircraft per day, per airport. Even when the weather's clear and equipment's working normally, that amount of volume needs to be tightly managed.

If the Command Center determines that due to over scheduling or deteriorating weather the number of arrivals and departures scheduled for a given airport exceeds the field's actual capacity, then it's TMI time. Not "Too Much Information," but "Traffic Management Initiative," a solution to handle the excess traffic.

TMIs can start out small. Let's say a low overcast blankets Chicago O'Hare. Now relying on instrument instead of visual approaches, it must slow the arrival rate. The Approach controllers are already vectoring their butts off, but are facing a huge influx expected between 1500Z and 1600Z. The inbound aircraft are spaced only five or ten miles from one another.

Chicago Approach can coordinate a TMI with the ATCSCC and Chicago Centre, requiring that Center feed the airplanes to Approach spaced by thirty miles. The Center controllers have to work a bit harder, but it offers breathing room to the Approach guys sequencing planes to final.

Sometimes, though, a bigger problem needs a bigger hammer. If demand is forecast to exceed capacity for several hours, a more drastic TMI is needed.

Via CDM conference calls, the ATCSCC can propose a new solution—a Ground Delay Program (GDP)—and include it in their operational plan. However, it won't actually put it into effect until it's been coordinated with the affected airlines and ATC facilities. This is a team effort that affects hundreds of flights and tens of thousands of passengers, so the Command Center wants to make sure all parties have their say.

Once a revised, reduced Airport Arrival Rate is agreed upon, it's number crunching time. The original proposed arrival time for every flight scheduled to O'Hare is run through a series of programmed algorithms. Every flight is then given an EDCT: an Expect Departure Clearance Time. The plane cannot takeoff until it's within five minutes before or after that time, as described in AIM 5-2-6. This thins out the arrival stream into Chicago to match the new AAR.

The ATCSCC's software assigns those EDCTs on an equitable basis. Chicago, in this case, is one of United Airlines' major hubs. Therefore, UAL gets assigned a higher than average amount of the arrival slots per hour.

Airlines can also be flexible within the slots they're given. Before the GDP, a UAL 737 was scheduled to depart at 1205Z from Boston. Fifty of its passengers have international connections, but it gets slapped with an EDCT of 1345Z. In the meantime, a UAL Embraer 170 with only five connecting passengers gets a time of 1245Z. United can swap the times and allow the 737 to go first. The regional jet's passengers won't be too happy, but the airline's philosophy is that scheduling damage can't be eliminated, but it can be reduced.

EDCTs apply to general aviation too, not just air carriers. If you're departing from a towered airport to any airport facing a GDP, the Clearance Delivery controller should advise you if you've been affected. If you're departing an uncontrolled field for a major airport, you can check if you're subject to an EDCT via the ATC Command Center's EDCT web site: http://www.fly.faa.gov/edct.

What if, despite Call For Release (CFR) and EDCTs, a particular Center or TRACON still has planes stacked in holding patterns, waiting to land? The ATCSCC can chop the throttle to idle and shut down traffic flow at the source.

Enter the ground stop, the ultimate GDP. Airplanes intended for the beleaguered airport are now forbidden to takeoff, giving controllers a fighting chance to organize their traffic. It's a drastic measure that can disrupt airline and passenger schedules badly, in trade for more of that ol' order and safety.

These ground stops are the measure of last resort, often instituted when a critical situation occurs, such as an accident, unusually bad weather, or ATC equipment failure. While the ATCSCC is the focal point for all traffic management decisions, ground stops often originate at lower level facilities. Last year, I saw an entire Center ground stop all traffic in its jurisdiction because of a major weather system covering up half the state. Even towers can demand ground stops to handle emergency situations.

Fort Lauderdale-Hollywood International Airport (KFLL) has only two operational runways—9L and 13—and they intersect. What if an airliner suffers a nose gear collapse and grinds to a halt where both runways meet, shutting down the entire airport in one sparking, nerve wracking moment? The damage is done. Now it's time to act quickly and effectively to avoid a catastrophe.

The tower calls Miami Approach and tells them to hold all incoming traffic in the immediate area. Both ATC facilities then conference call with Miami Center, possibly neighboring Jacksonville Center, the CDM, and the ATCSCC to organize a proper national ground stop. Until everything gets sorted out on the ground at KFLL, the hundreds of inbound planes and planned departures must be held on the ground or taken to alternate airports. Affected parties will be kept updated via situation reports and the game plan will be adapted as needed.

With this situation, and all others faced by the ATCSCC, the focus is on accurate information and timely coordination. The restrictions that result may seem inconvenient, but without them, the National Airspace System could descend into chaos. Through sound leadership, teamwork, and rapid response times, the Air Traffic Control System Command Center ensures order—therefore enhancing safety—for the thousands of pilots, passengers, and controllers beneath its oversight.

Tarrance Kramer is an Air Traffic Controller in the southeast US and is a contributing editor to IFR magazine.

This article originally appeared in the March 2013 issue of IFR magazine.

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