National Air Traffic Services (Nats) has issued new warnings to air traffic controllers that they cannot always trust information displayed on their computer screens at the £623m Swanwick air traffic centre.
Garbled radar data, especially for one part of a busy North Sea air space sector, has led to some of Swanwick's screens showing indications of non-existent aircraft.
To avoid indications of "ghost" aircraft, Nats has asked controllers to consider providing, in certain circumstances, an additional spacing of four miles laterally, or more, between aircraft.
In a separate problem, corrupted radar data can lead to aircraft being shown on screens at the wrong height. A false radar reading of 500 feet or more at variance with the aircraft's actual height is not recognised by Swanwick screens as incorrect.
An internal Nats notice to controllers, TOI113/02, dated 27 September, said the incorrect height reading appears on screen as "erroneous" but "credible" data.
Nats insists that safety is not compromised but controllers say new procedures to circumvent the problems are likely to increase their workload, at a time when they are already severely understaffed.
The latest problems are unrelated to the complaints of some controllers, highlighted by Computer Weekly, that they may misread height readings on their displays because numbers or letters are unclear or too small.
The warning notices issued by Nats at Swanwick in the past fortnight reveal that there are no immediate technical solutions, only procedures to mitigate the effects of these difficulties.
In one part of a North Sea airspace sector, "data garbling" can, in certain circumstances, lead to Swanwick's displays showing what one Nats notice refers to as ghost tracks, in which real planes may appear to be shadowed by one that does not exist.
The Nats' notices refer to a particular problem involving track data blocks - boxes of data displayed close to aircraft positions on controller screens The data blocks show the aircrafts' flight details and level.
A Nats notice says operational staff have observed these data blocks "swapping" between real and non-existent radar tracks.
Another problem with ghost indications is that Swanwick's systems may interpret the indications of ghost or non existent aircraft as planes that are flying too close to one another, and so set off a Short Term Conflict Alert, warning controllers of a loss of separation between planes. So Nats has asked controllers to consider providing four miles or more of extra lateral spacing between aircraft.
Controllers say that, because of understaffing, any spurious conflict warnings may add to their workload.
The separate problem of incorrect radar height readings is not confined to the North Sea sector.
As with the problem of ghost tracks, the system may not recognise false height data as anomalous and may set off a short-term conflict alert. Nats has told controllers to treat all alerts as real, which may mean issuing avoidance instructions to pilots.
A Nats spokesman said similar problems occur with air traffic control systems around the world. Controllers say, however, that the latest difficulties with ghost aircraft information did not occur at West Drayton, the centre superseded by Swanwick.
A Nats spokesman said "Safety has not been compromised on the occasions when the problem has arisen. Should such a problem occur Nats has in place procedures to ensure that no loss of separation [between aircraft] occurs."
Civil Aviation Authority on alert
The Civil Aviation Authority's (CAA) Safety Regulation Group, which approved Swanwick's "go-live", has said it is aware of the problem of garbled data on controllers' screens.
The CAAsaid in a statement that it had "ensured that Nats has put in place procedures to guarantee that Mode C data jumps [incorrect readings for aircraft height] and duplicate tracks [ghost aircraft tracks] do not affect high levels of safety, and [the CAA] will continue to monitor Nats' progress to a long-term solution."