Delays tend to last at least 24 hours following a computer failure

Civil aviation expert Philip Butterworth-Hayes looks at what procedures are in place when an air traffic control system breaks down.

How does the air traffic control system work?

There are three elements to control.

Firstly there is the Radar Scope which shows where the aircraft are, which aircraft is coming in and what height they are going.

Then you have the Flight Data Processing System next to it, which is what went down at West Drayton.

This has all the flight numbers on it and this information comes from a flight plan.

All the information is filed beforehand by the pilot.

The radar controller and the flight progress controller work on the progress of the flight together as it comes in.

Thirdly you have the Communication System which is a radio link between the air traffic controller and the pilot.

What generally triggers the computer to crash?

In this case I think it was new software.

But it all depends on the airport and the computers themselves.

These computers tend to carry much more information than a normal computer system.

They are built solely to deal with air traffic control and when the software is changed the whole process becomes very complicated.

Everything is purposely written in the UK in terms of software and you don't know what impact it will have until you press the button.

As a result there are usually a lot of teething problems whenever a new system is installed.

What back up is available?

Normally there are back up systems available for everything on the air traffic control system.

With the Flight Data Processing System there should be an electronic read out which gives basic information.

How common are these errors?

These type of errors should never occur. The system should be fail safe.

There is usually always a back up and there are usually other systems they can use.

What happens in a situation like this?

Air traffic control basically stops planes from taking off and landing. It also clears the air space to get every plane down as quickly as possible.

How long do delays tend to last?

It depends on how fast they fix the problem. Even if they are quick, it normally takes a day before everything is back to normal.

It doesn't just affect every airport in the UK. It affects every single airport in Europe and the North Atlantic.

Civil aviation security expert Chris Yates looks at how air traffic control computer glitches can be avoided.

How can these errors be avoided?

There is always an element of error with an air traffic control system because it is very complex. Computers create glitches all the time and the system will crash from time to time.

But there are always back up systems available.

The way we dealt with the problem at West Drayton was by using a manual data system.

The minute you start doing things manually it takes a lot more time and there are delays as a result of that.

What changes will be made in the future to avoid computer crashes?

I am not sure there are any changes that can be made.

The growth in the air industry is substantial. More and more people are travelling every year and this puts a strain on the system.

Whether the sort of glitch that we saw at West Drayton happens again remains to be seen.

But the air traffic control system is very robust.

How worrying are these glitches?

They can knock public confidence particularly for people who are nervous about flying.

Public confidence has generally been knocked over the last three years following events such as 9/11.

Glitches can put people off travelling sometimes but air travel is still the safest form of travel.

Why did so many problems occur at Swanwick (the new air traffic control centre which is gradually taking over from West Drayton) ?

Swanwick's air traffic control computer is a relatively new system which is one of the most advanced in the world.

With extremely complex systems like this you are bound to have teething problems when it is put into place for the first time.

This happens with the majority of new systems.

Philip Butterworth-Hayes is Civil Aviation Editor and Chris Yates is Aviation Security Editor of Jane's Information Group

LONDON AREA AND TERMINAL CONTROL SECTORS Most commercial flights operate in airspace divided into sectors Each sector is looked after by its own team of controllers Some sectors have been simplified for clarity