Approach/Departure Control
As with the Air Route Traffic Control Centers, and Flight Service Stations, Approach/Departure control is provided by FAA facilities and personnel. It is normally based at the airport involved, but may eventually be placed with the ARTCC's.
What does Approach/Departure Actually Control? It controls aircraft in the Class B or Class C airspace surrounding the airport, and any areas outside that airspace that the surrounding Air Route Traffic Control Center agrees to give it.
You'll see Class B or C airspace on an aeronautical chart, but the zones that you see marked are where all aircraft (even those flying by visual rules) are required to be in contact with Approach Control. For instrument flights, Approach Control's zone of influence is actually much bigger. Where your chart shows Class C airspace to be 10 miles in diameter and 4000 feet high, you may have planes forty miles away and up to 10,000 feet actually in contact and under control. This is because each airport's Approach/Departure Control has its own agreement with the surrounding Air Route Traffic Control Center (ARTCC) for handing off aircraft. To really know what's going on, you'll have to do a lot of listening.
Also note that your airport's Approach/Departure control could also be providing radar service to uncontrolled airports nearby.
Where Do I Find the Frequencies? Your best source is an aeronautical chart or approach plate. Most airports with approach/departure control have more than one frequency, and it's difficult to determine by listening just how the frequencies are used. If, however, you are forced to search, search the same ranges as you would for the Control Tower.
Why So Many Frequencies? Because while only one plane takes off or lands at a time, there are many aircraft approaching or circling the airport. There are usually several controllers, and the labor has to be divided logically between them. This can be done in a number of ways. Frequency use can be divided among:
How they do it at your airport is hard to determine by listening. Look at the reference area on your aeronautical chart or in the Airport/Facility Directory. That will give you the best information availble.
What Do I Get from Monitoring Approach/Departure? Not much if you're just plane-watching. In fact, I usually don't even program the frequencies in. To me, it's chief function is for following individual planes across the sky. This is more a factor with military aircraft.
Other Approach/Departure Tips:
Simulcasting - As with all air traffic control frequencies, each controller simulcasts on both a VHF and UHF (military) frequency. In addition, one or more controller positions may be consolidated and simulcast. You could hear the ground station or 4 different frequencies, but you won't know which is correct until you hear the aircraft reply. Because of all this simulcasting I recommend that you NOT use your scan delay function when monitoring at an airport.
Airspace - To get the most out of Approach/Departure and ARTCC frequencies, you should learn as much as possible about the airspace system. I recommend Understanding Aeronautical Charts, published by Tab. The Airman's Information Manual (also published by Tab as AIM/FAR) is an indespensible help.
Distance Can Be Your Friend - Approach/Departure and ARTCC frequencies are often better to listen to if you are too far from the ground station to hear anyone but the aircraft. This eliminates much simulcasting confusion, and makes it easier to isolate military from civilian traffic. Since pilots always read back any instructions that they're given, you really don't miss much.
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