By Tom Symonds BBC transport correspondent

The Swanwick centre is one of the biggest air traffic control facilities in Europe - its spacious operations room is filled with the latest computer technology.

After six frustrating years of trying to get the centre open, managers might have been hoping that once it was, things would go smoothly.

They could not have been more wrong.

Five months later, they are grappling with two big computer problems that each day threaten to severely disrupt the flow of air traffic.

We are still trying to understand the software

Richard Everitt, Nats chief executive

Before Friday, it was a system at the old West Drayton centre, near Heathrow, that was causing all the problems.

It takes in data from various sources about flights in UK airspace, standardises it, and passes the information on to the Swanwick centre.

In March and April some of that data corrupted the system - the fault crippled Swanwick where controllers were forced to sit in front of their expensive computer screens keeping note of aircraft with pen and paper.

At the time National Air Traffic Services (Nats) stressed it was not Swanwick's own systems that were at fault, but a separate computer and data-link.

Knock-on effect

They also admitted this computer had itself been upgraded.

Now the Swanwick system itself has failed.

After a routine operation to enter data overnight, computer engineers found most of the controllers' workstations were not working properly.

Six out of 20 screens were faulty

The night-time update had been postponed a day because the skies were busy with football fans flying home after the European Champions' League football match between Real Madrid and Bayer Leverkusen.

Only six out of more than 20 screens were faulty, and Swanwick had enough spare capacity to deal with half the normal number of flights.

Even so, cutting the UK's daily aircraft movements by 50 % has had a devastating effect on passengers.

They have suffered delays of up to six hours.

As was the case in the previous breakdowns, engineers managed to restore full capacity quickly - but the knock-on effect of delays can last all day.

Privatisation

Nats chief executive Richard Everitt says: "We are still trying to understand the software. It was unacceptable".

Critics would ask why, after six years of problems with Swanwick, Nats still has not ironed out all its bugs.

There is also a another dimension to the problem.

Nats has been partially privatised - with seven UK airlines taking a stake.

Those airlines are now suffering the consequences of these days of disruption.

In the end, everyone is suffering, the company, its customers, and in turn, their customers.