The definition of a ‘level bust’ used by the UK’s National Air Traffic Services (NATS) is a deviation of 300 feet or more from the assigned level. The Eurocontrol definition of a level bust is an unauthorised vertical deviation of more than 300 feet from an ATC flight clearance (within RVSM airspace, this limit is reduced to 200 feet) – other states and service providers use their own definitions.

The use of different criteria makes a direct comparison between the numbers of reported events in each country difficult; it is also difficult to make a direct comparison with operator data for the same reason. The NATS view is that the rate of occurrence in the UK is too high.

Level bust reports are investigated by the NATS ATC Investigations staff; the majority of these investigations will also involve contact with the operator. Based on the report, feedback from the operator and the subsequent investigation, a causal factor will be assigned to the event based on the NATS Event Factor Description scheme.

The top causal factors for level busts since the beginning of 2003 to date have remained broadly similar as has the proportion of events involving each causal factor.

One of the top causal factors of level busts, from January 2003 to April 2005, was ‘correct pilot readback followed by incorrect action,’ formerly described as CRM problems. Some events in this category will involve occasions where crews have received a clearance to a level which is known to cause confusion, such as FL100/FL110 or FL200/220.

The UK has introduced non-standard R/T phraseology to overcome this difficulty but the problem is still with us; in 2004 there were eight recorded occasions where a crew correctly acknowledged a descent clearance to FL110 but then descended to FL100. Other events will involve a breakdown in cockpit procedures; we don’t fully understand why this type of event occurs but it is possible that high R/T loading, high cockpit workload and communication issues are all contributory factors.

Mishear errors, failure to follow cleared SID, incorrect pilot readback by correct aircraft and poor manual handling also account for a significant number of level busts. Two of the most serious level bust incidents in 2004 involved poor manual handling by military pilots. Technical problems and altimeter setting errors have also contributed to level busts.

This short article is intended to raise awareness of the increase in the number of level busts in the UK. We are working in conjunction with operators, regulators and others to tackle the problem and we welcome the opportunity to address operators and safety organisations in more depth about the problem.

Richard Schofield, Deputy Watch Manager, National Air Traffic Services

This article forms part of the presentation given by NATS at BGAD05. As the issue of level busts has steadily gained increasing interest, EBAN will endeavour to follow up this article with additional information and commentaries in subsequent issues - Ed.