An independent inquiry into a computer failure that forced thousands of flights over southern UK airspace to be disrupted in December 2014 has found a single line of computer code from the 1990s was to blame.

There was chaos above the UK following a failure of some United Kingdom air traffic control (ATC) systems on December 12, 2014. It caused severe delays and cancellations at London’s main airports, Heathrow, Gatwick, Stansted, Luton and City, as well as aircraft transiting UK airspace.

The Incident started with the failure at 14:44 GMT of a computer system used to provide data to air traffic controllers to assist in their decision-making when managing the traffic flying high above England and Wales.

NATS (formerly National Air Traffic Services) chief executive Richard Deakin said at the time:

"The problem was when we had additional terminals coming into use, we had a software problem that we haven't seen before, which resulted in the computer that looks after flight plans effectively going offline. The good news is that everything came back 45 minutes later."

The chaos, however, continued well beyond the 45 minutes Deakin referred to. At 14:55 all departures were stopped from London Airports and at 15:00 all departures were stopped from European airports that were planned to route through affected UK airspace.

By 19:00, the technical staff believed they had understood the cause of failure and full redundancy of the computer systems was restored at 20:10. Traffic restrictions were gradually lifted from 15:55 as confidence increased, and the final restriction was lifted at 20:30. The disruption caused by the restrictions affected airlines, airports and passengers into the following day.

Single Line of Code Dating Back 20 Years

According to the independent report for the Civil Aviation Authority and NATS, the failure occurred in the System Flight Server (SFS) at Swanwick Area Control. The SFS has two channels so that one channel will normally continue to operate and provide service, if the other fails.

The disruption on December 12, 2014 arose because (for the first time in the history of the SFS) both channels failed at the same time. A single line of code, Deakin said, was to blame. The report said the software glitch “has probably been present since the software was written in the 1990s”.

UK air traffic control failure-12/12/14: Interim Report. Interesting. Looks like a few factors, including s/w design. http://t.co/WQpxg7hO5Q — Tom Hartley (@TomsTechTweets) February 10, 2015

Airlines and airports will be seeking compensation for the 24 hours of major disruption and NATS has confirmed to Sputnik “that there will be a financial consequence for the company from the delay caused by the technical problem at Swanwick on December 12, 2014".

"Under the company’s regulatory performance regime, customers will receive a rebate on charges in the future. The amount is being calculated and will be notified to customers in due course.”

The Final Report into the incident, due in May 2015, will address the “wider issues” on the root causes lying behind the Incident – not least why the software was so old and one single line could cause such massive failure.