Introduction
Having any kind of route structure is moot if there isn't a way for everyone to use it equally. This is where the human interaction is most important. There are no traffic lights in the sky. There are also no road markings, nor traffic cops to keep things flowing. That's the job of the Air Traffic Controller. Often referred to as "controllers" or ATC, they have a very important role in our skies.
Types of Controllers
There are different types of controllers, and they all work different parts of the sky. Terminal controllers control airport airspace and airport traffic. Approach/departure controllers control aircraft leaving or arriving at airports, and enroute controllers control those aircraft cruising over the countryside.
Terminal Controllers
They are the most widely known controllers to non-aviation folk. They include the obvious, the control tower operators, but they also include ground control and clearance delivery. The pilots always first talk to either ground control or clearance delivery. Most larger airports have a seperate clearance delivery frequency. These controllers issue route clearances to aircraft about to depart. They use a type of shorthand to relay the clearance to the pilots. If the route the pilots filed for was accepted, then the clearance delivery controllers will tell the pilots "cleared as filed". If they have to make changes to accomodate high traffic areas, they will say "change route of flight to read..." They also issue instructions to fly specific departures or arrivals, and issue the pilots their transponder code and a valid departure control frequency for use after takeoff.
If the airport isn't large enough to have a seperate clearance delivery, pilots get their clearance from ground control. Ground control also controls all traffic taxiing on the ground. They essentially own all the territory up to the runways. They give instructions to pilots on where to taxi, or specific taxi routes to get from the runway to the ramp. At airports that aren't busy, they will often allow the pilot to taxi at his or her own risk and choose their own route.
The tower controllers control the runways and the airspace around the airport. Busy airports have airspace that can extend up to 20-30 miles from the airport and as high as 10,000 feet. Smaller airports with control towers may only control the airspace 5 miles from the airport, and 2,500 feet above it. The tower controllers coordinate via landline to the departure control for takeoff clearance. When the departure controller is ready for the new traffic, he will tell the tower controller to release the airplane for takeoff. Then the tower controller will issue a takeoff clearance, and any additional instructions. Once the airplane is airborne, the tower controller will tell the pilots to change frequencies to the departure controller.
Departure Control
The departure controller will take over traffic management from the tower controller once the pilots check in on his or her frequency. At this point, the job of the departure controller is to prevent mid-air collisions and aid in traffic flow of departing aircraft. They may tell a pilot to fly a departure procedure of some kind, or just tell the pilots to proceed on their planned route of flight.
Once the aircraft exceeds the departure controller's airspace, either vertically (usually above 10-15,000 feet) or laterally (25-50 miles or so), they tell the pilots to switch to an ARTCC (Air Route Traffic Control Center) controller.
Center Controllers
ARTCCs are designed to provide enroute control, and they are often called "Center" over the radio. For example, if you are talking to the Houston ARTCC, then you call them "Houston Center". They provide traffic management and flow for all enroute aircraft. If another aircraft is climbing or descending and may come too close to another aircraft (3 miles and 1,000 feet below FL290; 5 miles and 2,000 feet at or above FL290), they will provide routing to keep them at safe distances. Likewise, if a pilot wishes to climb or descend to find more favorable wind conditions, or to avoid turbulence, they put that request in with the Center controller who then calls ahead to other controllers and asks if the requested altitude would conflict with other traffic. If it does, then the controller will "put it on request", meaning that as soon as the conflict is resolved then they will approve the altitude change. Or if there will not be a conflict, they will clear the pilot to climb or descend as needed.
Center controllers are also working to help airline passengers have a better experience. They will often request "ride reports", and pass the information along to other airliners so the pilots can make a decision whether to stay at the current altitude or avoid the bumps and climb or descend.
Center controllers will also tell pilots to slow or speed up if the airspeed will affect traffic flow. If there is an airplane flying slow, say .65 mach, and all the airplanes sequenced behind it are flying .75 mach, then they will ask the first airplane to speed up. If the first airplane cannot speed up, they will then tell the following traffic to slow down. If one of them cannot slow down for whatever reason, they will assign the conflict aircraft a different altitude, or vector them off the route to allow the faster traffic to pass.
Approach Controllers
Approach controllers take over once the aircraft pass into the terminal airspace, usually below 18,000-15,000 feet. From here, the airplanes are sequenced into the airport by either using a Standard Terminal Arrival Procedure (STAR) or by vectoring the aircraft around, or both. The approach controllers will take approach requests, and pass along airport information to the crew. Finally, the approach controllers will eventually clear an aircraft for an approach, and tell the crew to contact the tower controller. At this point, the aircrew is back in the airport environment, lands and is controlled by the same type of people as listed at the top of this page. And so the cycle starts over.
Summary
Controllers are very important in the airspace system. But keep in mind, controllers cannot make a pilot do something that the pilot is either not allowed to do, or the aircraft can't physically do. Another myth is that pilots are assigned routes. Well, that's true in a sense, but in reality, the pilots will file for a route, and unless that route conflicts with other traffic or can't be entered into the system because of errors, the filed route is usually accepted. And no, the pilot/controller relationship isn't as formal as Hollywood usually puts on....there are no radio calls like "Ground Control to American two zero niner, taxi to runway five, over...". It's more like "American two zero niner taxi to runway five, contact tower one one nine point eight". And they certainly don't use trucker lingo ("ten-four", etc). And last but not least, it's quite common to hear simple courtesies up there...."Houston Center, good morning! Delta five six three checking in level three five zero"...."Delta five six three, roger, good morning to ya, how's your ride?".
Behind the Scenes Index
Enroute Navigation...Getting There
Arriving and Departing an Airport
Take-Off and Landing Data