If you're instrument-rated, you've already done business with those three-letter entities that (in the United States, at least) have abbreviations starting with the letter Z. If you're just getting started, here's a top-down look at how our airspace is put together. Actually, there are some things that even pilots with an instrument rating may not know. So let's begin, literally, with a big picture.
A depiction of the National Airspace System (often referred to in the ATC community by the three-letter acronym NAS) overlaid with ARTCC boundaries looks like a jigsaw puzzle. The typical center's airspace covers an area equivalent to several states. Even the very smallest of them, Chicago (ZAU), Indianapolis (ZID), and Cleveland (ZOB), are somewhere in the neighborhood of 70,000 square miles in area. You'd think that the biggest would be Anchorage, Alaska, which is about 10 times that size, but actually, it's not. And you'd think "little" New York Center (ZNY), which looks even smaller, would be the very smallest (still, about 17,000 square miles), but in fact that's not true either. New York Center is unique in that there's a separate oceanic component extending over halfway to northwest Africa and covering some three and one-quarter million square miles.
I chose the word overlay earlier because that is almost literally a center's relation to the lower, smaller, and - to the beginning pilot, anyway - more familiar tracon (terminal radar approach control) and the tower-controlled volumes of alphabet airspace. While ARTCC boundaries are distinct, they are nonetheless integrated smoothly with the approach controls that direct traffic within a few dozen miles of major airports, and also the towers that manage still smaller areas. (Some tracons, by special arrangement with adjoining centers, control considerably larger areas of low-altitude traffic.) ARTCCs are sizeable pieces of real estate, and that is only fitting, because our NAS is the world's largest; almost half the world's air travel takes place just in North America. There are approximately 17,000 controllers working in almost 200 tracon facilities, more than 500 air traffic control towers, and nearly two dozen ARTCCs. Depending on whom you ask and what criteria are applied, there are between 21 and 24 in the United States.
Each ARTCC in turn is divided into between 20 and 80 sectors. Sectors themselves can be 50 to more than 200 miles wide. Their size, shape, and altitudes are determined by traffic flow, airway structure, and workload. Areas in which traffic is light may cover several thousand square miles, while congested areas of heavy traffic can be fairly small.
Sectors have a vertical distribution, as well, being separated over at least two levels. The lowest generally cover the Victor airways (though in some centers they go right down to the ground). Lower sectors may go up to only 10,000 feet, or as high as FL 230 (for example, in Fort Worth Center). Higher-altitude sectors typically extend to around FL 350, and there are some "super high" sectors going right up to FL 600. Each sector is a polygon constructed in turn from one or more even smaller polygons called fix posting areas. (Interestingly, sectors don't always have flat tops or bottoms. Portions of a sector may extend to different altitudes.)
Typically, one or two controllers "work" a sector on a discrete frequency. The controllers in adjacent sectors "hand off" traffic among themselves, with tracons and towers adjoining their airspace, and with neighboring centers. Center boundaries can vary depending on altitude, by the way; Salt Lake Center would look slightly different if the horizontal "slice" were at a different (higher) altitude.
So what do these centers do? They do the same sorts of things that towers or tracons do: sequencing and separation of aircraft within their airspace, as well as into underlying tracons and other local airports. In addition, they provide weather and other safety-related advisories, navigation assistance, and more. The center is the primary en route facility responsible for vertical, lateral, and longitudinal separation of aircraft in controlled airspace. Think of centers as the bridge between departure and approach control. In most parts of the country, you talk to center controllers once you're up a few thousand feet and are on your way to wherever you're going. Airline flights are typically handled mostly by ARTCCs. (That's primarily under instrument flight rules, but centers can also provide advisory services to aircraft flying under visual flight rules on a workload-permitting basis.)
When a pilot files an IFR flight plan, it goes directly into his or her local center's Host computer, which then compares what was filed with existing preferred routes. Once your flight plan is entered into the Host, it starts looking for any traffic conflicts, known delays, or airspace restrictions. After about 30 minutes, a clearance is made available to whatever facility you'll contact on departure - whether it's a clearance delivery frequency at a towered airport, or a cell phone call from the boondocks to Flight Service.
You may wonder how one facility can exercise such far-reaching control. They do it with long-range radar coverage from remote sites, the data for which is sent over leased phone lines, microwave links, or satellite downlinks. Because radar data at a Center is usually a mosaic compiled from many different radar installations, it is not as precise as radar data from a discrete local facility, and this is one reason that separation standards are higher in Center airspace: five miles laterally and 1,000 feet vertically below 29,000 feet, with 2,000 feet of vertical separation at FL 290 and above.
As helpful as a big geographical picture can be toward understanding why we have ARTCCs, there is also the historical context from which our present airspace design has emerged. Since the earliest days of flight, as more aircraft were aloft at any given time and each was unaware of the others, the need arose for a third party to provide a little choreography. In 1926 the Air Commerce Act created the Bureau of Air Commerce, which designated the first airways and made the earliest air traffic rules. The very first air traffic controller - his name was Archie League - began working at St. Louis Lambert Municipal Airport in 1929. In Cleveland, Ohio, and other busy airports in the early 1930s, there were a few rudimentary radio-equipped air traffic control towers.
By the mid-1930s, due to a thriving airmail business, the airlines had grown and traffic was straining our nascent airway system. This was because nobody was controlling en-route traffic. The first real compre- hensive "ATC" had its beginnings on December 1, 1935, when TWA, Eastern, American, and United Airlines formed the first Airway Traffic Control Unit (ATCU) at Newark, New Jersey. In those days before radar blips, controllers used markers moved around by hand atop table maps. Chicago, Cleveland, Pittsburgh, and four other cities followed within the first year, and a couple of years later, the Department of Commerce acquired these ATCUs, which in 1941 were renamed Airway Traffic Control Centers under the Civil Aeronautics Authority (later Administration) created in 1938. By 1942 there were about two dozen ATCCs.
Back then, controllers didn't talk directly to pilots; they relayed messages via flight service stations or airline company radio (which is the origin of the phrase "ATC clears"). Direct radio communication didn't arrive until the late 1940s. In 1956 the first air route surveillance radar went into service, as did the first computer, in Indianapolis. (It wasn't much more than a programmable accounting machine, making simple predictions of arrival times over a fix and printing flight progress strips.) The early 1970s saw the installation of IBM 9020s, which automated many of the clerical activities controllers had to perform to "work" each aircraft. A few years later, the radar data processing system brought automation into the radar realm, replacing markers with data blocks and allowing hand-offs via computer.
These computers have been replaced by more advanced models, but the next advancements will arrive in the form of workstation applications for controller use. One is URET (User Request Evaluation Tool), which projects ahead for future conflicts. Another tool is TMA (Traffic Management Advisor), which optimizes the flow from Center airspace into tracon airspace. A third, the Final Approach Spacing Tool (FAST), will assist tracon controllers with runway and landing sequence information. These are all being developed and deployed throughout the NAS. Despite a gradual move toward the concept of free flight, in which aircraft will shun established airways and fly directly to their destinations, the air route traffic control centers will be with us for a long time to come.
Jeff Pardo is an aviation writer in Maryland with a commercial pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,100 hours in 12 years of flying. An AirLifeLine mission pilot, Pardo also has flown for the Civil Air Patrol.
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