National Air Traffic Services (Nats) cannot quickly replace the ageing computer system at the centre of last week's delays because it is too complicated.
Thousands of passengers were delayed on Wednesday when the Flight Data Processing System (FDPS) at the West Drayton centre failed.
The system prints the paper strips that controllers use to plot the movements of planes. When it fails the strips have to be written out manually. The same system failed two weeks ago for 20 minutes, causing similar delays.
Although the hardware for the FDPS was upgraded six months ago, the application still contains some code created in the 1970s. But Nats said the system, which is not due for replacement until 2007, cannot be replaced quickly because no off-the-shelf product would fit.
Ordering a new system will be complicated because of the extent to which it has been modified, explained a Nats spokesman.
"It's a bespoke system and we have upgraded it every so often," he said. "[To procure a new system] you write the specification which says what the system needs to do.
"Then you go to the companies that build them and say can you do this or something close to it. But it is such a specialist system that they aren't sitting out there waiting."
Nats stated that when a replacement is ordered, a more sophisticated backup facility than the existing reliance on pencil and paper would be put in place.
"We still have to decide what system we will use, but I don't doubt that the level of backup will be one of the considerations," said the spokesman.
The FDPS was one of the few systems left unchanged following the move of much of the air traffic control operations to a new base in Swanwick which opened in January five years late and £180m over budget.
"What we decided with the air traffic controllers was that the change to Swanwick was so major that it would not be appropriate to make more changes than necessary," Nats said.
But it defended the performance of the system. "The engineers have isolated the software area where [the problem] occurred," it said.
"It processes 6,000 flights a day. It failed on Wednesday and two weeks ago but only for 20 minutes and that's not a lot of time compared to when it is up and running."
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