Air Traffic Control - Highways In The Sky!

     You’ve likely always heard that planes follow instructions from the "control tower"... but in reality, that’s a bit of a generalization. Things don't work exactly like that.

Additionally, you’ve heard the term "air traffic"—but do you really have any idea just how "crowded" the sky actually is?

Let’s clear things up a bit...

Airspace schematization

Airspace

To begin with, you should know that air traffic is organized into an incredibly intricate network of "highways in the sky," called airways. Near airports, these merge into large circular areas where arriving and departing traffic is managed. All of this exists to make the flow of traffic more disciplined and predictable, as today’s skies are much busier than you might imagine.

Just to have an idea, take a look at these video clips:

Airspace is governed by many rather complicated rules, but without such an orderly structure, there would be chaos, and collisions would be a daily occurrence! The advantage of being in the sky, however, is that space can also be utilized vertically. This means that in a small area, you can have multiple planes at different altitudes, or two planes crossing in opposite directions on the same airway.

Two airplanes in the same airway at different heights 

Because of the sheer volume of traffic, every commercial flight—from the moment it pushes back from the gate until it shuts down its engines at its destination—must be "controlled" by specific entities on the ground.

Ground vs. Center

Depending on where the aircraft is located, it will be under the control of different agencies. The "path" a plane takes is established in the Flight Plan, which is distributed to all the agencies that will be responsible for the aircraft as it passes through their sector.

A chart of the airways over Europe (!)

During ground movements, takeoffs, and landings, the controllers are indeed located inside the famous Control Tower. However, from shortly after takeoff until the final approach for landing, there are Control Centers (ACC). These centers constantly receive information from every aircraft in flight, representing all traffic on screens that allow controllers to organize the flow and issue radio instructions to pilots.

An air traffic control center

These centers are all interconnected, making it easy to "hand off" control of an aircraft from one sector to the next. Each agency has specific radio frequencies that pilots tune into to communicate.


Remote Areas and Holdings

In remote parts of the planet where reliable radar coverage isn't possible (such as over the poles or oceans), traffic is regulated by "procedural" agreements. It is established that from Point A to Point B, the aircraft will follow a specific route at a specific altitude and speed, reporting over certain points at set times. Communications are handled via Long Range (HF) radio or satellite systems that allow for text messaging—just like your phone's Short Text Messages (see this link).

The areas with the highest concentration of traffic are, naturally, those near airports. When traffic becomes too heavy, there are predefined paths called "Holdings" (or holding patterns), where arriving aircraft "stack up" and circle at different altitudes while waiting for their turn to land.

Needless to say, to ensure everyone understands each other worldwide, a common language must be used: and that language is English.

Safety Systems: TCAS

You might think that funneling so many planes into predefined corridors could be dangerous, and that a controller’s oversight or a pilot’s misunderstanding could lead to a collision.

Fortunately, all airliners are equipped with a device called TCAS (Traffic Collision Avoidance System), designed specifically to prevent collisions. This device "talks" to the TCAS of all nearby aircraft, exchanging altitude data and vertical trends (climbing, descending, or level flight). This information is shown to pilots on a navigation screen.

Aircraft in the vicinity are shown on the navigation display as small diamonds, with a number indicating the altitude difference, and an arrow pointing up or down if they are climbing or descending

If a situation develops that could lead to a collision risk, the two TCAS units "agree" on the best maneuver for each aircraft well in advance. A voice alert (and not a very gentle one!) communicates this to the pilots, who must immediately follow the instructions to the letter.

What About Mountains?

While commercial flights spend most of their time much higher than the tallest peaks, every airway—as well as every approach or departure path—has Minimum Altitudes that guarantee adequate clearance from the terrain.

Outside of airways, the entire planet is divided into small rectangular sectors, each indicating a minimum safe altitude based on the highest obstacle in that area.

Onboard technology helps us here too. Systems like the GPWS (Ground Proximity Warning System) sound an alarm if we are too close to the ground when not in a landing configuration. Furthermore, the aircraft’s position and altitude are continuously compared against an "electronic map" that stores terrain data. If we are flying toward an obstacle higher than our current altitude, another convincing warning allows pilots to take action in time to avoid the worst.

DontWorryFlyHappy!