Air traffic control systems were temporarily hit after the incident on August 28, causing hundreds of delays or cancellations to flights
Air traffic control systems were temporarily hit after the incident on August 28, causing hundreds of delays or cancellations to flights AFP

The head of British air traffic control on Wednesday blamed a "one in 15 million" event that caused the country's worst systems failure in almost 10 years, stranding thousands of passengers.

Air traffic control systems were temporarily hit after the incident on August 28, causing hundreds of delays or cancellations to flights departing and arriving in the UK and costing carriers an estimated GBP100 million ($125 million).

Martin Rolfe, chief executive of National Air Traffic Services (NATS), blamed the outage on duplicate "markers" on a flight plan that the system could not process.

He told the BBC it was "a one in 15 million" chance, and meant engineers had not encountered the problem before.

In its initial report published on Wednesday, NATS said that it received details of a flight that was due to cross UK airspace on August 28.

Two markers along the planned route had the same name but were in different places, crashing the system and triggering it, and the backup system, to automatically shut down for safety reasons.

"With planes and crew out of position and most flights already booked up, many people found themselves stuck abroad on what is usually a big day for travel -- a bank holiday -- facing long waits to get home," said Rolfe.

The last Monday of August -- a public holiday in England, Wales and Northern Ireland -- traditionally sees large numbers of passengers returning from summer vacations.

The UK Civil Aviation Authority (CAA) regulator announced later Wednesday that it was to launch an independent review into "the wider issues around the system failure and how NATS responded to the incident".

"If there is evidence to suggest NATS may have breached its statutory and licensing obligations we will consider whether any further action is necessary," said Rob Bishton, joint interim chief executive at the CAA.

Transport Secretary Mark Harper said last week there would be an independent review into the worst incident of its kind for "nearly a decade".

London's main airports were the worst hit, with Heathrow and Gatwick cancelling dozens of flights Tuesday.

British Airways was the worst affected airline.

Airlines UK, the industry body representing UK-registered carriers, welcomed both the NATS report and the CAA investigation, and said airlines wanted compensation for the huge costs they had to bear due to the breakdown.

"Airlines are seeking clarity on what options exist for NATS to cover our costs under the current legislation and will continue to engage with Government on all options for redress," said Airlines UK chief executive Tim Alderslade.

Airlines were required to put up many stranded passengers and lay on additional flights to get people home.

"Airlines cannot be the insurer of last resort though and there must be accountability from NATS when things go wrong," said Alderslade.