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What is this 'Center' Stuff, Anyway?

'Z' What? IFR pilots already know about those three letter entities that start with a 'Z' (in the US at least).'Z' What? IFR pilots already know about those three letter entities that start with a 'Z' (in the US at least). But there are also some other things they may not know about. Have you ever wondered just what Centers, or 'ARTCCs' (Air Route Traffic Control Centers) are all about? Here's a little background...

The Big Picture: The National Airspace System is the world's largest; in North America alone, covering about 45% of the world's air travel, employing about 17,000 controllers in almost 200 Terminal Radar Approach Control facilities (TRACONs), almost 500 Airport Traffic Control Towers and almost two dozen Centers. A 'Center' is the primary en route facility operated by the FAA for the vertical, lateral, and longitudinal separation of aircraft in controlled airspace. (That's primarily IFR, but it can also include VFR aircraft on a workload-permitting basis.) A Center also provides weather and other safety-related advisories, navigation assistance, and other services.

How It Works: In most parts of the country, you would be talking to Center controllers once you're up a few thousand feet and are on your way to wherever you're going. When a pilot files an IFR flight plan, it goes right into the originating 'host' (Center) computer, which compares what you want with existing preferred routes. The computer looks for any traffic conflicts, known delays, or airspace restrictions, and generally after about 30 minutes, your clearance is made available to whatever departure facility you're at -- whether it's a clearance delivery or a cell phone call from the 'boonies to Flight Service.

Breaking It Down: There are 22 ARTCCs in the United States (see Figure 1) and each are divided into between 20 and 80 sectors -- based on traffic flows, airway structure, and workload -- and these sectors are distributed over at least two levels. The low level (altitude) generally cover the Victor airways. In some Centers, they go right down to the ground and up to 10,000 ft -- though they may reach as high as FL 230 (as in ZFW). The higher level covering up to around FL 350, with some 'super high' sectors going right up to FL 600. (Sectors don't always have flat tops or bottoms, by the way.) Controllers in each sector 'hand off' traffic among themselves and, likewise, each Center coordinates with adjacent Centers. Keep in mind that Center boundaries can vary depending on altitude. Figure 1 would look slightly different if the 'slice' was at a different (lower) altitude!

figure 1
Figure 1: North American ARTCC Boundaries (high altitude): Also showing are some sector boundaries within Salt Lake Center, and TRACON boundaries within Kansas City Center. (reproduced with permission from Flight Explorer Professional 3.10, courtesy Flight Dimensions International, Inc.)

Things You Probably Didn't Know: In 1926, the Air Commerce Act created the Bureau of Air Commerce, which established the first airways and made the earliest air traffic rules. The first air traffic controller (Archie League) was on the job at St. Louis Lambert Municipal Airport back in 1929. Beginning with Cleveland, Ohio, and other busy airports, rudimentary radio-equipped air traffic control towers were in operation in the early 1930s. However, by the mid-1930s the airlines had grown from the thriving airmail business, and traffic was straining the rudimentary airway system -- largely because nobody was controlling en route traffic!

The first real comprehensive 'ATC' actually dates back to December 1, 1935, when TWA, Eastern, American, and United formed the first airway traffic control unit at Newark, NJ, using 'shrimp boats' and 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 became Airway Traffic Control Centers under the Civil Aeronautics Authority (later Administration) created in 1938.

By 1942 there were about two dozen ATCUs. Back then, controllers didn't talk directly to pilots (direct radio communication didn't arrive until the late 1940s); they would relay messages via Flight Service Stations or airline company radio -- thus the preface 'ATC clears.'

In 1956 the first air route surveillance radar went into service, as did the first computer, in Indianapolis. (However 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 shrimp boats with data blocks and allowing hand-offs via computer.

The future: Center or 'host' computers have been replaced by more advanced models and the next change, this time in workstation applications and prototype systems used by controllers, is underway right now. Tools with names like URET (User Request Evaluation Tool), which projects ahead for future conflicts; TMA (Traffic management Advisor), which optimizes the flow from Center airspace into TRACON airspace; and FAST (Final Approach Spacing Tool), which will assist TRACON controllers with runway and landing sequence information, are currently being developed and deployed throughout the NAS. Until they can pour a few thousand miles more runway concrete, or airlines are willing to get creative and spread those hub-and-spoke push times out to other less popular times of day, these and other 'free flight' tools could stave off total airway gridlock for a few more years. That's the idea, anyway.

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