National Air Traffic Services has changed the way controllers operate systems after a mid-air incident in which a jumbo jet pilot took sudden action to avoid another aircraft, causing an injury to a passenger.
It was the first "airprox" or near miss in which new systems at the £623m New En Route Centre at Swanwick, in Hampshire, were involved in compromising safety.
Computer Weekly has learned that the incident was caused by a combination of human error and a weakness in the design of systems. The near miss occurred on 15 November 2002, but details have not been disclosed publicly until now.
Two passenger aircraft, a Virgin Atlantic 747-400 and a Delta Airlines 767, came so close to one another over Lyneham, Wiltshire, that the collision avoidance systems in the cockpits sounded a warning.
A sudden avoiding manoeuvre by the pilot of the Virgin jumbo from New York to Heathrow caused a female passenger to suffer a broken leg. Her legal claim for damages is being resolved amicably by the airline's insurers and its customer relations staff, said a spokesman for Virgin Airlines.
Nats, which runs air traffic control systems and centres in the UK, has confirmed that the incident is currently the subject of both legal proceedings and an investigation by the independent UK Airprox Board.
Ironically, the incident occurred after a controller made use of a systems feature that gave staff the flexibility to manipulate data on their screens to improve clarity and safety.
There are symbols for each aircraft on controllers' screens. Next to each symbol is a small box of text that identifies the aircraft. Although the screens at Swanwick's predecessor site at West Drayton were more primitive, they allowed staff to distinguish between text blocks even where two overlapped. But on Swanwick's pixel-based screens, if two text blocks overlap, one can obscure the other, although controllers can use a mouse to move the text blocks around the screen.
In the incident in November, a controller is said to have moved the text block from its default position but may have forgotten that he had done so, with the result that the identities of the two aircraft became confused.
To avoid the two aircraft coming too close together, the controller is said to have given instructions to increase the separation between them. But in the confusion the two aircraft came even closer together, setting off a warning in their cockpits from the on-board collision avoidance system, which tells pilots in each aircraft what steps to take to avoid a collision.
Nats has since issued a "supplementary instruction" which tells controllers that "following completion of a recent investigation", controllers must, when moving text blocks from the default position, ensure they attach a "strut" between the text and its aircraft symbol. This line should ensure that a controller knows which text block is linked to which aircraft symbol.
But the change to procedures, nearly one year after Swanwick went live, raises questions about testing and whether the design of the system in this case encouraged or prevented human error.
Nats said, "Safety is Nats' first priority and the company launched an immediate investigation following an incident involving a Virgin Atlantic 774 and a Delta Airlines 767 on 15 November 2002. Since then, Nats has issued revised operating instructions to controllers to ensure that such an incident cannot recur."
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