Features August 2013 Issue
Well Structured ATC
Mysterious and complex as it may seem from the outside, an air traffic controllerís job relies on only three simple concepts: teamwork, organization, and communication.
In spring 2003, I was halfway through my flight training. My instructor and I were chatting about air traffic control. He’d just visited the local TRACON. From his description, I pictured a dark room filled with the intense chatter of men and women half-lit by radar scopes and blinking sci-fi lights.
A few short years later, I’d be a radar controller in one of those dark rooms. My imagination hadn’t been far off. In due course, I also became a tower controller.
In ’03, that all was rocket surgery to me. I’d only visited my local control tower. Their ability to manage a pattern full of Cessnas from their glass office seemed magical, but what most intrigued me were those mysterious voices answering to “Departure,” “Approach,” and “Center” who lived beyond our airport’s airspace. I wasn’t really sure how all of those facilities were interconnected.
There’s no denying the USA’s air traffic control system is layered, technical, and complex. But I’ve discovered every day I do my job, the ideas behind all that are actually pretty simple.
To understand any system, one must first understand its basic purpose. Engines produce thrust. Wings generate lift. Air traffic control’s intent is summed up by its FAA-issued mandate: “The safe, orderly, and expeditious flow of aircraft.”
That’s nicely phrased, but not exactly overflowing with details. Controllers work as a team to help aircraft get from their departure point to their destination safely. We sequence them with other traffic and separate them from threats to their safety like other aircraft, special use airspace, and obstacles. If they have a mission to accomplish along the way—be it military operations or civilian requests like IFR training or aerial photography—we coordinate amongst ourselves to get the pilots what they request. Additional services include emergency assistance and weather advisories.
This is a team effort. A typical airline flight from New York to Los Angeles may talk to several dozen controllers. A local IFR flight may only talk to a few. In either case, each controller watches out for potential conflicts.
There are approximately 15,000 FAA controllers in the USA, with thousands more working at military airfields and in privately-managed contract towers. Employment status doesn’t matter. They all apply the same rulebook—FAA Order 7110.65—and use the same standardized phraseology.
While the rules are universal, they’re broken down by the three main types of ATC facilities: tower, TRACON, and center. Each facility type has a very specific role to play.
Ask a random member of the non-flying public about air traffic control. The majority will describe that most visible of aviation icons: an airport control tower.
Your journey through the national airspace system may begin with a tower controller saying “Cleared for takeoff” or may end with “Cleared to land.” Before the controller can utter those phrases, he must exercise good judgment and ensure spacing exists. He is, after all, watching over your most critical phases of flight: takeoff and landing.
At times working in those glass offices can feel like an Olympic ping pong match, fast and furious, controllers making snappy decisions in tight quarters: How do I fit this jet in with my pattern traffic? Do my departures match my overlying radar controllers’ expectations?
When they’re watching airplanes through the windows, towers can run airplanes closer than a radar controller relying on sensors dozens or hundreds of miles away from their traffic. Required tower separation in VFR conditions is at times measured in feet, not miles. “Tower-provided visual separation” makes tight squeeze-plays possible.
Experience with specific aircraft performance characteristics is a huge plus. We use “miles per minute” a lot. If I have a Boeing 737 clocking 120 knots on two mile final—putting him over the threshold in sixty seconds—then I shouldn’t clear a Piper Cherokee for takeoff in front of him. Swap that Boeing on final for a 70 knot Cessna who will take almost two minutes to fly those two miles and I can launch the Cherokee.
Towers have to coordinate with overlying radar facilities for departures on IFR flight plans to maintain the required IFR separation in all weather conditions: a minimum of three miles horizontally, 1000 feet vertically, or 15 degrees of heading divergence. A tower controller can’t just clear an IFR departure without letting radar know he’s coming. Traffic conflicts with other IFR traffic can occur if radar isn’t protecting airspace for the departure. It’d be like throwing a football at someone without first yelling “Catch!”
Towers and radar therefore have letters of agreement (LOAs) that specify how departure coordination is handled. A tower controller may have told you,
“Hold short. Awaiting release.” He’s waiting on radar to grant him permission to “release” your departure. These releases may be done verbally or automated via computer. The bigger the airport, the more automated the system. The end result is organized, safe service.
The airspace within fifty miles of a major airport is as complicated as it gets. Jets and high performance aircraft are screaming out of the flight levels to land. Other fast-movers are rocketing off the runways, scrapping for higher. In between, lower level general aviation and military traffic goes about its business.
This crossing traffic requires a different facility: the Terminal Radar Approach Control, or TRACON. If tower controllers are ping pong players, then TRACONs are racquetball players, with a bigger court and planes that are moving fast and quickly changing altitude.
TRACON’s racquetball court is typically up to 10,000 feet within 50 miles. However, there’s no such thing as a “typical” layout. Each is designed to fit a specific traffic flow. Miami Approach owns up to 15,000 feet, for example. TRACON facilities themselves range in size from only a few radar sectors to massive consolidated facilities like Potomac TRACON whose dozens of scopes cover many airports.
Radar sectors are staffed according to traffic level. During slow periods, one controller may work a TRACON’s entire airspace. As traffic increases, the airspace is subdivided into sectors to divide workload. Each sector’s scope can be manned by two people: the radar controller who’s talking on the frequency, and a radar handoff. The handoff acts like a second pair of eyes, helping out the primary controller by pointing out traffic conflicts and coordinating with other sectors and facilities.
Every controller must have instant recall of his and his neighbors’ sectors and facilities, including all the VHF and UHF frequencies. When you’re busy, there’s no time to look it up. If you don’t know this stuff, you don’t get certified.
For departures, TRACONs have to adhere to LOAs with their overlying center and any adjacent TRACONs, assigning aircraft specific routes or altitudes as required. Arrivals are fed to the TRACON according to those same LOAs to keep them separated from the departures. Once the planes start heading in, controllers need to make some critical choices. Who’s number one to the airport? Number two, three? Once they make that call, they vector—using heading, altitude, and speed assignments—to organize the planes.
As big as some TRACONs are, they’re nothing compared to the twenty Air Route Traffic Control Centers (ARTCCs) that work all of the remaining airspace.
Miami Center alone is responsible for the airspace between Orlando, Florida down to the Dominican Republic. That’s 2.95 million cubic miles of airspace. Managing that demands a lot of coordination and staff. Centers employ hundreds of FAA controllers, divided into geographic jurisdictions called “areas.”
Each area is then subdivided into individual radar sectors which—like TRACONs—are combined and split based on traffic. One center sector alone can be thousands of cubic miles, overseeing multiple TRACONs. Center scopes are manned by R-Side controllers, the folks you hear on the frequency. If traffic complexity builds, a D-Side can plug in, who performs the same functions as a TRACON’s Radar Handoff.
Center traffic is the most varied, including jets streaking across the country in the flight levels and heavily traveled oceanic routes, down to traffic at tiny uncontrolled airports in the boonies. Their most complex work usually involves organizing traffic to and from the major airports beneath them. When you’re dealing with a hundred or more aircraft arriving at an airport per hour, that organization needs to start early.
Look at a major airport like Miami. Its international and domestic arrivals come en masse from all directions and altitudes. To ease controller and pilot workload, Standard Terminal Arrival (STAR) procedures simplify flow into these large airports. They require planes to cross certain fixes at certain altitudes, creating predictable descent paths that can begin over a hundred miles from the destination airport. As aircraft are sequenced to join a STAR, and if they follow the course, they should remain in sequence. These STARSs relieve workload, reduce frequency-cluttering vectors, and allow the controller to maintain better situational awareness.
Over the course of hundreds of miles, Miami Center controllers take this chaotic flow, and through STARs, vectoring, speed, and altitude control, carefully compress them into organized “streams” of traffic that are then fed to Miami Approach. Miami Approach takes these multiple streams, blends them further, and directs them to individual runways.
For departures, the same thing happens in reverse. Departures come off a runway in a single stream. Approach starts prying them apart, assisted in this process by Standard Instrument Departures (SIDs)— the departure equivalent of STARs. SIDs and STARs are designed to keep their respective departures and arrival streams from crossing.
Doin’ It the Hard Way
ATC tech is no different than any other aviation equipment: it has limitations, and it occasionally breaks. When computer automation isn’t up to the task, mere mortals take up the slack by actually talking to each other. All related facilities are joined by voice landlines.
Weather issues and system failures make those landlines buzz. TRACON may advise center that an aircraft is deviating around a thunderstorm. Center can call tower and advise them they’re rerouting certain outbound airplanes around weather. Computer glitches can force us to pass on flight plan information verbally. I’ve worked several long days when the complete Host interface failed and we had to verbally coordinate every single plane in and out of our airspace.
ATC’s aim is to keep this interconnected infrastructure transparent to the pilot while you stay busy flying the plane. It’s our job to make this complex network work smoothly so you get to your destination safely in an orderly and expeditious fashion.
Inside the Tower
Each tower is staffed by a team of controllers, divided into four key positions: Flight Data, Clearance Delivery, Ground Control, and Local Control. Fully certified controllers are capable of working each position interchangeably.
Flight Data doesn’t talk to aircraft directly, but records ATIS broadcasts and handles weather updates, emergency coordination, and NOTAMs. In some towers, like mine, controllers are certified to take weather readings both manually and via various sensors.
Clearance Delivery issues IFR clearances, VFR departure instructions, and advises pilots of flow control programs to their destinations. They also provide flight plan quality assurance, confirming correct altitude for direction of flight, reasonable routing, etc. They fix these on the ground, before they become problems in the air.
Ground Control manages the movement of aircraft to and from the runways and their parking areas. Departures need to be sequenced to the runway in proper order. For instance, if a Chicago-bound airliner has a release time of 1204Z, he shouldn’t be put behind a Charlotte-bound airliner with a 1209Z time.
Local Control—“Tower”—sequences and separates aircraft on the active runways and within the tower’s designated airspace. They’re constantly making decisions to build gaps for arriving and departing aircraft and keeping their pattern traffic appropriately spaced.
Major airports and even some busier GA airports have such high traffic volume that they’ll split off multiple GC or LC positions, each with their own frequency. During slower periods, all of these positions may be combined on one person. On my late-night closing or early morning shifts, I’ll just put on all of the hats and work everything on my own. —TK.
Hi-Tech the ATC Way
Each ARTCC’s computer manages flight plans and automates data transfer between facilities. The decades-old version, Host, is being replaced by ERAM—En Route Automation Modernization. ERAM provides additional capabilities for ADS-B and other NextGen products.
Each tower and TRACON is paired with its center’s computer. When you file an IFR flight plan, it winds up in the center’s computer that determines which facilities need a copy. If your departure airport’s tower has a Flight Data Input/Output printer, your flight plan will print for the controller. The TRACON, if any, gets a strip too.
All of that information enables automating basic ATC tasks. If I’m working TRACON and want to hand your flight off to Houston Center, I type an “H” and click on your target. Hundreds of miles away, a controller in Houston sees your target flashing on her scope because the computer knows you’ll pass through her airspace. She clicks your target and it stops flashing.
Back on my scope, your airplane flashes and its target symbol changes to an “H”. Now I know Houston accepted the handoff. I then tell you to contact Houston Center on the appropriate frequency. Done. Automation makes it easy. —TK
Tarrance Kramer never outgrew his inner child’s need to ask, “Why?” He strives to keep learning as he works traffic in the southern U.S.