You have probably heard that pilots follow "control tower" instructions when airborne... actually, this is a generic way of saying... Things are not exactly this way.
Also, you've probably heard about "air traffic"...
but... do you really have a clear idea of how much traffic there is up there?
Let's try to shed some light...
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| Airspace schematization |
First of all, let me tell you that all air traffic is organized in a very complex network of "sky highways", called "airways". They all eventually flow, close to the airports, into big circular areas, where taking-off and landing traffic is managed.
All this is done with the purpose of regulating the traffic flow and making it more predictable, as nowadays it is a lot heavier than you could imagine.
Just to have an idea, take a look at these video clips:
The so called "air space" is governed by tons of rather complicated rules, but now it's easy for you to understand that without such an organized structure, there would be chaos, and collisions would be very frequent!
There is an advantage of being up in the sky though... the space can be used in all three dimensions, thus in a small area it is possible to stack more aircrafts at different altitudes, or it's possible to have two airplanes cross paths in opposite direction in the same airway, at different levels.
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| Two airplanes in the same airway at different heights |
Depending on the geographical area the airplane is flying into, it will be under the control of a specific agency.
The route that a plane will fly is assigned through a "flight plan" which, after being approved by the authorities, is distributed to all the agencies that will be involved (each one for its own geographical area of responsibility) in controlling the flight along its journey.
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| A chart of the airways over Europe (!) |
For all the movements on the ground, the controlling agent is... yes! The control tower...
Since right after takeoff, and all the way to the final approach phase for landing, there are "control centers" that constantly receive information from every airplane in flight (by using the radar and other equipment), and where all the air traffic is shown on screens so that the air traffic controllers can organize the traffic flow and can radio out instructions to pilots.
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| An air traffic control center |
In remote areas of the planet, where it is not possible to get a reliable radio-radar coverage (such as over the ocean or in polar areas), air traffic is regulated by a sort of "agreement": pilots commit themselves to fly, from point A to B, along a specific route, at a specific altitude, at a specific speed, and over specific points at specific times.
Communications are guaranteed through the HF radio, which has a very wide range, or preferably through a satellite based system which allows the exchange of text messages, just like SMSs on your cellphone (see across the oceans).
The areas with the busiest concentration of traffic are of course the ones in proximity of airports, where all the incoming traffic is funneled and all the departing one is sorted.
When traffic becomes excessive, some specific racetrack-shaped routes, called "holding patterns", are activated. In such patterns, arriving flights are "stacked" at different altitudes, waiting for their turn for starting the final approach.
Needless to say, in order to make communication possible between pilots and controllers in every part of the world, it is necessary to use a common language, and that is English.
You could argue at this point that, even with such a mega-organized air traffic structure, "stuffing" so many airplanes in confined spaces in the sky could be dangerous, and a controller or a pilot error or distraction could lead to a collision.
Luckily, every commercial plane is equipped with a device called "TCAS", which helps preventing and avoiding collisions.
TCAS "talks" to all other TCASes of every airplane in the vicinity, sharing information about position and height, and also whether each aircraft is climbing, descending or flying at a steady altitude. Such data are shown to pilots on a navigation display.
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| Aircrafts 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 |
And... what about mountains?
Commercial flights usually take place at very high altitudes, well away from ground obstacles. Nevertheless, every airway, as well as every route designed to get to or away from an airport, are "built" with a minimum usable altitude, which guarantees adequate clearance from the ground.
Out of the airways, all the planet is divided into rectangular sectors, and for every sector there is a minimum altitude, based on the highest obstacle in that sector, at which it is legal (and safe) to fly.
Again, a specific device on board the aircraft will help pilots avoiding the risk of a collision with the ground. This is called GPWS (or EGPWS, in its "enhanced" version), which stands for Ground Proximity Warning System. Through a series of sensors, it will give a (again, very convincing!) warning when the airplane is too close to the ground and it is not in landing or takeoff configuration, or it's descending right after takeoff, or there is high terrain approaching along our flight path.
Pilots are obviously trained to promptly react to such warnings with specific escape maneuvers.
