The national air traffic control centre at Swanwick is still beleaguered by a catalogue of problems almost a year after opening, according to a member of staff there.

But such allegations have been denied by the chief executive of National Air Traffic Control Services (Nats), Richard Everitt, who said the system was safe "in aviation terms".

Philip Butterworth Hayes, a journalist and author specialising in air traffic control, explains the centre's importance at the forefront of air travel safety.

How important are transmissions from aircraft to air traffic controllers?

In terms of safety, transmissions from aircraft are absolutely vital.

It's the controller and the pilot working together to make sure the aircraft can fly safely in controlled airspace.

It's very difficult to make general assertions about what happens if the controller and the pilot can't communicate

My understanding is that this comes from a year review of the new Swanwick system and that this has happened a few times.

But the transmission cut-off time was only 2 or 3 seconds and then the message is reread between the controller and the pilot.

How serious a problem is this?

It's difficult to say how much of a problem it is. And there's always room for error.

One of the issues is that voice communications have inherent problems, especially when you're dealing with something like High Frequency which is long range and affected by static and not at all clear. It's always an issue.

What impact does this have on safety?

It's very difficult to make general assertions about what happens if the controller and the pilot can't communicate.

But if there is a problem then the message is read back.

If, for whatever reason, the controller isn't happy about the situation then the aircraft is kept further apart than normal in radar-controlled airspace.

There are safety levels there. But until we know the detail of exactly the traffic situation at the time, the separation distances, it's very difficult to make any generalisation.

It varies between instances which are fairly regular between controllers and pilots who read messages back if they're not clear - that's an everyday occurrence - it happens hundreds of times a day - to a situation where, if there's very little separation distance, then, it could be critical. But it's not unheard of.

Controllers complain their screens are unclear. Is this a serious issue?

Controllers were part of the team that designed the new screens.

The colour variation is very clear and much clearer than the old screens they had at West Drayton.

I think problems are going to come in the coming years because of the fact it is such a bespoke system that it is unique only to the UK

So, this is unusual for the fact that they're not clear because there was controller input into their design.

The centre has been running for a year now. We know there were issues of colour early on.

Any time you change screens and add colour, there are potential problems. But I am surprised this is emerging now, when such a long time has passed.

What view are the authorities likely to take on such issues?

I think the annual review of how the centre is functioning is clearly a big issue.

These issues need to be addressed. But these problems raised are not major problems.

But there are some issues that will emerge in a few years time which could be more problematic, especially the situation with the flight data processing system.

My understanding is that it's a system individual to the UK and requires enormous updates every time new software is loaded. It is unlike any other European system.

It's a fine system as it is but I think problems are going to come in the coming years because of the fact it is such a bespoke system that it is unique only to the UK.

Nobody else has one like it. If you go to Germany or France they use a lot more common off-the-shelf components, meaning they are easier to adapt and easier to upgrade.

Do these problems undermine their credibility?

I don't think they undermine their credibility. They make life more difficult. What's happening is that in choosing the sort of system which they did, they have created a long-term problem. It's not in terms of safety but in terms of capacity.

Why is Swanwick now handing over work elsewhere?

Air space is being shared all the time. Controllers are resectorising and this is happening more and more and control is being sometimes being handed from one country to another, if it's easier and makes more sense to do that.

With the European Single Sky policy they are going to redraw all the sectors.

That will be a huge undertaking. So swapping bits of airspace with other centres is not necessarily an issue for Swanwick but an issue for airspace design.

If there's a traffic build up over the Dutch coast which could be more easily dealt with by Dutch authorities then it makes sense for them to deal with it.