National Air Traffic Services (Nats) is asking some aircraft to avoid the £623m Swanwick air traffic centre by flying at lower levels to come under the control of the ageing West Drayton centre near Heathrow.
The Swanwick New En Route Centre near Fareham, Hampshire, went live in January at a cost of £623m - more than half of which was for new computer systems - so that it could take over from the West Drayton area control centre and bring an immediate 40% increase in capacity.
But a shortage of controllers at Swanwick has caused record delays for airlines this year, despite traffic levels being down after the 11 September terror attacks.
To help to overcome the shortage, which is expected to last through this Christmas and into next summer's busy period, Nats is transferring "area control" of some flights over 20,000 feet, that would normally be handled by Swanwick, to lower altitudes where they can be handled by "terminal control" at West Drayton.
But a leaked internal Nats notice alludes to a possible safety risk if aircraft are "height-capped" because too many faster jets could be transferred into a lower airspace normally occupied by slower-moving business and propeller aircraft.
"The impact of height-capping jet flights at [20,000 feet] or below can dramatically affect the complexity of a sector," warns the notice.
The height-capping reduces flight delays because terminal control at West Drayton is able to handle some of Swanwick's traffic. But it raises doubts about whether Nats can justify the £337m cost of Swanwick's systems when some of the new centre's work is being systematically transferred to West Drayton and other control centres.
An extension to earlier trial height-capping procedures by Nats is described in an internal Temporary Operating Instruction dated 23 November.
"These height-capping procedures are intended to move specific demand from the upper to lower airspace as a result of excess demand," says the notice.
It adds that managers "are to consider" height-capping flights in certain circumstances where delays of 20 minutes or more are expected.
As some airlines are "likely" in advance to lodge flight plans that specifically avoid Swanwick, the notice warns that height capping may have to be regulated, or delays could be shifted from Swanwick to other air traffic control sites.
Height-capping will incur a fuel penalty because jets use more fuel in the denser air of lower altitudes, and it could also have adverse environmental effects because jet engines are more efficient at higher altitudes.
Nats said this week that height-capping was an old technique that was applied routinely before Swanwick went live.
Staff, however, say that height-capping was used at West Drayton only in specific, tactical circumstances but is now being applied much more widely to exclude as much air traffic as possible from Swanwick.
A Nats spokesman insisted that height-capping was not solely in response to a shortage of controllers. "The [height-capping] service has proved to be beneficial to airlines not only in avoiding air traffic control delays but in assisting them to maintain schedules that have been disrupted for other reasons such as operational delays, poor weather, etc".
Nats added that pilots are height-capped more than in the summer. Its staff point out that air traffic is currently at generally lower levels than the summer.
Pilots' voice messages get cut off
A leaked internal Nats notice dated 12 December says that some pilots are having their voice messages to controllers cut off after two seconds, which is regarded as a possible safety-critical issue. "To date investigations have not been able to replicate the fault or locate its precise cause. The issue has a high priority and further investigation is being carried out to locate the source of the problem," says the notice.
A Nats spokesman would not comment on claims that pilots may continue talking without realising at first that they have been cut off.
The problem of "truncated transmissions" had happened on only a "handful of occasions in 1.5 million flights handled this year. Safety had not been compromised", said the spokesman.