Flight disruption: Air traffic control glitch has been 'rectified'
A computer glitch that led to disruption across UK airports has been "rectified", the chief executive of the National Air Traffic Services has said.
Richard Deakin said a single line of code in one computer system at the UK's national air traffic control centre had caused the problems on Friday.
But he warned updating some of its "elderly" systems posed a "challenge".
Flights have been returning to normal on Saturday, although almost 40 were cancelled at Heathrow.
Passengers faced widespread flight disruption on Friday because of the technical fault in a flight data system at the Nats control centre at Swanwick.
'Lines of code'
The failure caused problems at airports around the country - including at Heathrow and Gatwick, where departing flights were grounded for a time.
Mr Deakin told the BBC that the software problem was "buried" among millions of lines of computer code.
"The challenge is that we have around 50 different systems at Swanwick and around four million lines of code. This particular glitch was buried in one of those four million lines of code."
"We haven't seen that particular issue before," he added.
He said Nats was spending an extra £575 million over the next five years to bring its systems "up to date", but warned making improvements was a challenge as they had to be made "while the engine was still running".
Transport Secretary Patrick McLoughlin is due to be questioned by MPs on Monday about the chaos caused.
On Friday, he said the situation was "unacceptable" and asked for a full explanation from Nats about what had gone wrong and what it would do to prevent such an incident happening again.
The BBC's political correspondent Robin Brant says Mr McLoughlin can expect to have a preliminary report from Nats on his desk by Monday.
Labour has called for ministers to "get a grip" and the Labour chairwoman of the transport select committee, Louise Ellman, said it was "vital that we establish what happened".
She said her committee, which will question Mr McLoughlin on Monday, would also call Nats and the Civil Aviation Authority to give evidence.
'Safety not compromised'
In a statement, Nats said the number of workstations "in use" at its control centre versus "in standby" fluctuated with the demands of the traffic being controlled.
"In this instance a transition between the two states caused a failure in the system which has not been seen before," it said.
"The failure meant that the controllers were unable to access all of the data regarding individual flight plans which significantly increases their workload."
Nats said it then had to "reduce the traffic into and out of the UK network".
"The controllers had a full radar picture and full communications with all aircraft at all times during the incident and at no time was safety compromised in any way," it added.
'Back to normal'
The glitch caused delays at Heathrow and Gatwick, while other UK airports reported knock-on effects.
Dozens of arrivals and departures at airports across southern England, and as far north as Aberdeen and Edinburgh, were delayed and cancelled.
On Saturday, Heathrow Airport said 38 flights due to arrive or take off before 09:30 GMT had been cancelled but the subsequent schedule was "back to normal".
A Heathrow spokesman said the cancelled flights could not be rescheduled because the airport ran at 98% capacity. He said passengers would be rebooked on other services.
British Airways said its Gatwick and London City flights were expected to operate as normal on Saturday and it was working to get Heathrow flights back on schedule.
Gatwick Airport said it hoped to operate a full service on Saturday although there would be "some backlog", while Stansted said all its flights were running on schedule.
Swanwick air traffic control centre
Swanwick controls the 200,000 square miles of airspace above England and Wales, cost £623m to build, and employs about 1,300 controllers.
But the facility, which handles more than 5,000 flights every 24 hours, has had a troubled history.
It opened in 2002, six years after its planned commissioning date - a delay which Nats said was due to problems with the software used to power its systems.
Almost a year after it opened, a senior air traffic controller raised concerns with the BBC about health and safety standards and complications with radio communications - which he said cut out erratically.
Technical problems and computer faults hit flights in 2008 and again last summer. And, in December 2013, problems with the internal telephone system then caused further delays.
Friday's problems came a year after a telephone failure at the Hampshire control room caused huge disruption - one of a number of technical hitches to hit the part-privatised Nats since the centre opened in 2002.
BBC correspondent Andy Moore said the issue only lasted for between 30 minutes and one hour but caused chaos because the UK's air traffic control system runs at 99% capacity.
Martin Clipp, a former senior operations manager at Nats, told BBC Radio 4's Today programme: "You can't build absolute resilience... in terms of the technology that backs up the systems.
"The system runs pretty much at full pelt. That means as soon as the slightest thing goes wrong the ripples go out enormously."
However, Mr Clipp said Nats was being required by the Civil Aviation Authority to cut costs, which had led to redundancies this year.
"There is no risk in safety but there is risk in service continuity," he said. "You get what you pay for."
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