Some controllers said their workloads have reached a point where they feel safety is being compromised. National Air Traffic Services (Nats) has been gradually lifting restrictions on the amount of air traffic handled by Swanwick, which it imposed when the systems were introduced on 27 January.
Documents entitled "Swanwick Weekly Safety Report" show that between January and 26 May 2002 there were 30 formal overload reports filed by air traffic controllers. This compares with 12 in the same period last year, when controllers used ageing equipment at West Drayton near Heathrow.
The Manual of Air Traffic Services, which controllers must comply with as part of their licence conditions, says overload reports shall be completed when controllers consider "they were involved in an air traffic control situation during which they experienced excessive workload to the point where safety was compromised".
Despite the number of overload reports, senior managers at the partly-privatised Nats argued that Swanick has performed well. One said the main indicator of safety is the number of "airprox" reports, which are filed when there is a serious loss of separation between aircraft.
Since the Swanwick centre went live there have been none of the most serious category A and B airprox reports and only one of the least serious category C reports, compared with seven in the same five-month period in 2001.
However, air traffic controllers remain worried. They have asked Nats to retain restrictions on the "target sector flow" rates - the number of aircraft allowed into each airspace sector - pending the resolution of problems with some unclear display screens. The request was denied, they said.
Colin Chisholm, chief operating officer at Nats, insisted that restrictions on the volume of air traffic were released only when it was safe to do so. He said that an initial 30% restriction on air traffic, imposed to allow controllers to become used to the new system, has been lifted by about 5% per week.
Swanwick is now handling at least as many aircraft as West Drayton, and slightly more in some airspace sectors, said Chisholm. Nats managers want Swanwick to handle 30% more air traffic than the West Drayton control centre to help reduce delays for passengers.
"We have not spent all that money down there [at Swanwick] only to have a system that has the same capacity as [West Drayton]," said Chisholm.
Even at its busiest, the Swanwick centre operates at about 20% below its theoretical capacity, to allow large margins for safety, he added.