Eurocontrol has just published the early results of studies of level busts, and next year intends to produce a "level bust toolkit" for airlines and controllers to use to reduce the frequency of such events. These are normally understood as aircraft passing by mistake through their cleared flight level, but Eurocontrol defines a level bust as "any unauthorised deviation of more than 300ft [90m] from an ATC flight clearance".

Military aircraft are statistically far more likely than other categories to bust their cleared levels, according to a UK National Air Traffic Services study quoted by Eurocontrol. Air transport aircraft constitute 92.3% of traffic but produce only 78.8% of the busts; private and executive aircraft make up 6.3% of traffic and produce more than twice that proportion (13.8%) of the busts; but military aircraft, only 1.4% of the traffic, produce 7.3% of the busts.

Eurocontrol concedes that UK NATS has carried out more level bust studies for longer than any other agency, and this is helped by voluntary reports by UK airlines of events occurring wherever they travel in the world. A British Airways geographical breakdown of its pilot reports show that, just because the UK records more level busts than anywhere else - 293 controller-reported events in 2002 - it is not the place they are necessarily most likely to happen. The BA sample of reports show that in the Americas there are 1.17 events per 1,000 departures, in Asia the rate is 1.15, Africa has a rate of 1.65, over the Atlantic the rate is 1.19, and in Europe it is 1.01.

The eventual "level bust toolkit" is likely to include: advice on the design of complex standard instrument departures and instrument arrival procedures at airports; advice to pilots and controllers to use standard phraseology and not include too much information in any single transmission, especially spoken numerical or digital detail; and changes in operating procedures like slowing climb or descent rates when nearing the cleared altitude.

Source: Flight International