Inside an Air Traffic Control Center
I have to confess to you right now that when I first purchased a scanner, my sole purpose was to monitor airplanes. At the time I was a student pilot, and I loved the radio in the instrument panel even more than I loved lifting that Cessna 152 off the runway in a climb-out over the California coastline. I used the scanner as sustenance during the interminable times between flights or when the fine West Coast weather went south, tuning the tower or approach control, keeping me flying even when I was on the ground. Eventually I learned that scanning is as boundless as the high blue sky, but no matter what I tune to, I'll still always come back to the aviation band as an old friend. If you have never explored aviation communications, or if you are bewildered by what you're hearing, sit in on Aviation Monitoring 101 and I'll give you the basics. There are two sections of the radio spectrum specifically devoted to aviation: 108-137 MHz is the stomping ground of civilian air and 225-400 MHz belongs to military air operations. We'll leave military air for another column and focus our attention on civilian air communications. By the way, this is a hands-on article, folks, so pull up a scanner as you read. The civilian aviation band is further divided into several sections: 108-118 MHz is where you'll hear the beeps of navaids, otherwise known as aeronautical aids to navigation. They aren't much fun to listen to, but they provide the invisible electronic airways that aircraft follow across the skies. 118-137 MHz is the meat of the aero band, the place where you will find all the air-to-ground and ground-to-air chatter that goes into controlling aircraft. Backing up a bit, you may want to remember that the 128.825-132 MHz slice of spectrum belongs to commercial aviation company communications, where you can hear airliners and commuter aircraft calling their company headquarters to report routine and not-so-routine information. That's the civilian aviation band, in a nutshell. Finding it is easy, understanding what is being said there is the hard part. But, like any other specialty, once you learn the lingo, you'll have no problem. Take a look at my Crash Course in Aviation Terminology table to decipher some of what you're hearing. Aviation, contrary to appearances, is a very ordered society. Though it is still possible to take off and cut lazy 8's through the sky, for the most part, aircraft are forced to follow the directions of air traffic controllers to get from Point A to Point B. For the majority of the flights you will hear on a scanner, this control begins before engine start, on a frequency called Clearance Delivery. C/D (also abbreviated Clnc Del on aviation charts and in frequency books) is operational at most major airports, especially those that serve commercial airliners. Though the exact frequency differs depending on the airport, the content of the communications does not. Clearance Delivery is the place where aircraft receive their instructions for flight, otherwise known as an Instrument Flight Rules (IFR) clearance. The clearance is simply a recitation of assigned heading to fly after departure, altitude to climb to, route of flight to follow, departure frequency to tune to, and electronic code (squawk) to enter so that air traffic control can pick up the flight on radar. The clearance is pre-filed long before departure and goes through the system, picking up any amendments necessary to keep this flight from occupying the same airspace as any other flight. The clearance is issued by the C/D controller and read back by the pilot. After engine start, the next frequency to come into play is Ground Control. On most airports, nothing moves on the ground unless the Ground Controller approves it. Again, the specific frequency varies with the airport. Depending on airport size, there may also be more than one Ground Control frequency. Once in line for take off, the next radio destination is Tower. The Tower controller is lord and master of the runways and everything flying within about five miles of the airport. Pilots receive departure and landing clearance on the Tower frequency. At large, multi-runway airports, there may be more than one Tower frequency in use. Tower is the best frequency to monitor at an airport, as you'll hear the most action here. After lift-off, a departing flight contacts Departure Control. D/C is a radar controller that guides departing flights out of the airport area and safely on their way. Depending on the direction that the flight departs, there may be more than one Departure Control frequency.
Enroute and cruising, aircraft contact the Air Route Traffic Control Center
(ARTCC) or Center controller for enroute handling. There are twenty ARTCC's
throughout the United States, each divided into geographical sectors
containing their own frequencies.
Approaching their destination, aircraft use frequencies in the reverse of the
departure sequence. The only difference is Approach Control, which handles
aircraft descending to and entering an airport area.
A spin through the aviation band may reveal silence at first, but if you give
it time you're sure to find some communications going on. Remember that
aviation transmissions are terse and quick, to provide clarity and save
valuable time. Also remember that because of the height of airplanes, you
may hear pilots and not hear the controller on the same frequency. If you're
close enough to the transmitting antenna, without any hills, mountains or
other obstructions in the way, you will likely hear the controller as well.
Besides the major frequencies of Clearance Delivery, Ground, Tower, Departure,
Center and Approach, there are numerous other frequencies to be found, such
as unicom (uncontrolled airport advisory frequency), air to air, flight test,
flight service stations and more. Of course, as I mentioned before, finding
frequencies is only the first step, understanding what is being said is the
next.
Most aircraft are identified by their registration number, also known in the
U.S. as "N-numbers," a series of numbers and letters displayed on the
fuselage. Call signs are derived from these and usually begin with the type
aircraft: i.e. Cessna 3564U, Learjet 656RE. Letters are pronounced
phonetically, so "U" would become Uniform and "RE" would become Romeo Echo.
After initial contact with a controller, call signs are shortened to the last
three numbers/letters. Commercial airliners and commuter aircraft use
specially assigned call signs representing their flight number, i.e.
United 212, American 601. Numbers are the heart of aviation and you'll hear
quite a few of them over the air. For instance, all altitudes above 18,000
feet are referred to as flight levels. Flight level 190 is 19,000 feet,
flight level 370 is 37,000 feet, etcetera. Direction is given in the form of
magnetic headings, with 360 degrees being due north, 180 south, etc.
Aviation monitoring is both challenging and fascinating. Don't be put off by
the rapid fire communications or the onslaught of numbers; with time you will
learn what it's all about. For an indepth look at aviation listening, pick up
a copy of my book Air-Waves: The Aviation Monitor's Handbook, available from
Tiare Publications (1-800-420-0579.) And, of course, you can always e-mail
me with your questions at lauraq@funtv.com.
A CRASH COURSE IN AERO TERMINOLOGY
ABEAM - 90 degrees across from a fix, point or object. "Abeam the runway."
ADF - Automatic Direction Finder, a navigational aid.
ATC - Air Traffic Control.
ATIS - Automatic Terminal Information Service, a continous weather broadcast
on a designated frequency assigned to each airport.
ALTIMETER - Device used to display barometric pressure reading adjusted for
atmospheric pressure.
DIRECT - A straight line clearance between two navigational fixes.
DME - Distance Measuring Equipment.
FIX - A geographical position.
FLY HEADING ### - A controller instruction for an aircraft to fly the
indicated magnetic direction.
GO AROUND - Break off landing approach and circle for another try.
GPS - Global Positioning System.
IFR - Instrument Flight Rules.
IDENT - Instruction for a pilot to activate the aircraft transponder to assist in identifying him on radar.
ILS - Instrument Landing System.
LOCALIZER - The course guidance part of an ILS approach.
RESTRICTED AREA - Military operating area off-limits to civilian flight.
SQUAWK #### - Activate radar transponder on discrete code.
TCAS - Traffic Collision Avoidance System.
VECTOR - Controller assigned aircraft heading.
VFR - Visual Flight Rules.
VOR - Very High Omnidirectional Receiver, a navigation aid.
VORTAC - VOR/TACAN radio navigation equipment.
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