How we avoid mid-air collisions

Another question I'm often asked is to do with air traffic...

Namely, that people are used to the concept of roads and the like on the ground so is there any kind of similar system in the air to keep planes separated?

There is!

Obviously, we don't have physical roads to follow in the air. But we use a range of systems for separation.

These systems can include pre-planned departure and arrival routes, tracking from waypoint to waypoint near busy airports, can include "airways" between certain markers and other means.

To the untrained observer, planes appear to go everywhere in a random pattern.

But in the air, we simply navigate in a different way to that on the ground. Also, bear in mind too, in the air, aircraft can be separated by altitude. A plane at one altitude simply cannot hit an aircraft at another altitude - no matter how they travel. This principle is used a huge amount in regular to-from patterns; more on that soon.

Arrival/departure paths:

Each busy airport has a set of what they call standard instrument departures and standard terminal arrival routes (SID/STAR). These are step-by-step guides, basically, that set out on a chart to pilots, where they are to go and how they are to plan their approach - literally, step by step.

Each runway at the airport has it's own set of SIDs and STARs and they are aligned for planes arriving from each direction. The charts are published by the government aviation authority (for instance, Civil Aviation Safety Authority in Australia (CASA) or the Federal Aviation Authority (FAA) in the US.) - and every airline pilot will have their own set.

So it's really quite simple in concept. Air traffic control advise an arriving aircraft to refer to a STAR. This will tell the pilot to fly to (or "track" to) a waypoint - which is basically an imaginary point in the sky, defined by GPS. His GPS will tell him where to go. At that waypoint, they follow each step on the chart. It may say to now turn to a new direction (in degrees, just like a compass) and descend to a certain height or slow to a speed. They'll follow that to the next waypoint. And so on until lined up with the runway.

This means too that other aircraft arriving after this will follow the same pattern - and it becomes like an invisible motorway in the sky - each aircraft following the one before.

Air traffic control's workload is significantly decreased - and most importantly, everyone is working on the same level so there is no confusion. All ATC have to now do is watch each plane on radar to make sure they are complying with the procedure - and if one is closing on the other, they will give directions such as to change speed (one speed up, the other slow).

Ditto for departures. In between the arriving planes, the departing ones will be told which SID to use.

The pilot then looks at his chart and knows then that after takeoff to head to a waypoint, climb to a certain altitude, and so on. The SIDs and STARs have been very carefully designed to ensure that in no way can one clash with another - so this now means that flights heading out are off on one "road" and ones coming in are on "another road".

It also explains why you may fly quite a long way from an airport in one direction, then turn around and head back. For instance, you may depart London for New York but head EAST to start with and turn around. This is to follow the SID out, climb to a safe height well clear of the airport - then turn around and pass back.

We've basically covered now how planes are kept separated on arrival and departure. But what about on the way?

Enroute:

As a passenger, you will never know but air traffic control are watching your flight on radar from literally start up to shut down. This means even if the pilot is in cloud and can't see...you are still being watched and kept clear of anyone else. Or at the same time, if the pilot makes a mistake and turns the wrong way or heads to the wrong altitude - air traffic control pick it up quickly and correct it.

Believe me...they are quick on it.

But enroute, the best way to keep the skies safe are to get planes moving as directly as possible from A - B. To ensure the less time wasted with diversions and circles - and less planes in the sky.

This is where radio beacons and altitudes come into play.

Radio beacons of different kinds and strengths are located worldwide. Some are very low frequency and can be picked up by a plane just about on the other side of the world...others are quite short range.

All combine to create a network.

Each country publishes it's own maps for radio beacons. These maps mark out where each beacon is - and give a 'name' to the path from beacon to beacon. The names are quite dull - such as "A-382" or "Y-117" but the point is, with such simple names, there is never confusion. These tracks are like roads. The pilot will, if using beacons, simply tune a special navigation radio onto the frequency of the beacon (similar to an FM radio station frequency). The radio picks up the signal and - keeping this simple - directs the pilot towards it. He simply has to fly the plane over that beacon - and he's followed the path.

But...what if someone is flying the other way on the same path?? Won't they collide?

Nope!

That's how altitude comes into use.

Remember I said earlier how two planes flying at different heights can pass each other but physically never collide? We use exactly that rule.

That is...above 10,000 feet, the LAW says, and this is universal, world-wide, that planes must be 1000 feet above and below one another.

So...countries have sat down and made a world agreement. Planes travelling WEST will travel on an even-numbered altitude (such as, 28000 ft, 32000 ft, 34000 ft) and planes travelling EAST will go on the opposite - odd numbered altitude (such as 27000 ft, 31000 ft).

West and east are used very broadly! If you look at a compass, it is 360 degrees, right?

Well...for flying, we call degrees 1 - 180 as "east" and 181 - 360 are called "west". So whichever of those degrees you are travelling, you are called east or west and will, by law, be flying at your odd or even height.

So if I am say, flying from London - New York, I'm headed west. I will fly at 34000 ft for example.

There is a plane coming the opposite way - he will fly at 33000 ft or 35000 ft.

Either way...he will pass right over or right under me and there is a 1000 ft gap.

This is why now we say that although the world's skies are the busiest they have ever been, they are the safest. Separation of planes is a very technical subject in depth, so what I have explained is just the basics.

But that shows how it works - and trust me, every single pilot, even just the private pilot flying on the weekend, has to understand separation or s/he is not allowed to fly.

Separation has been, sadly, learnt from the blood of the past. Accidents happened in the early days of jets and after long investigations, problems were found - we used those problems to develop our system today that, when properly used, makes mid-air collision virtually impossible.

For a mid-air collision to occur...remember, many things all have to go wrong at the same time. If the pilot makes a mistake, air traffic control will tell him.

If air traffic control make a mistake, the pilot will realise and tell them. In all jets today, there is a final backup also called "TCAS"...or the "traffic collision avoidance system". TCAS works, again, simply by it's own radar. If both air traffic control and the pilot have made a mistake and not realised - TCAS will scan the skies and tell the pilot through alarms and screens that they are too close to another flight. TCAS even "talks" to the other plane's radar - and between the two planes, the radar computers "agree" to turn separate ways to go apart.

Then TCAS will tell the pilot "turn left". If he doesn't do it in a matter of seconds, the computer does it for him.

So...we can now see...that it would take three major mistakes before there was even a risk of a collision.

Questions or comments - fire away!