DAVID LEARMOUNT / LONDON
Figures show 13-year bid to prevent pilots breaking through cleared heights yields results
A unique 13-year UK National Air Traffic Services (NATS) effort to curb "level busts" - incidents in which pilots break through their cleared height - may be starting to yield results, according to 2003 figures from the UK Civil Aviation Authority.
However, reported level busts for the UK still averaged more than 400 a year between 1998 and 2002, while consequent "loss of separation" incidents remained steady at about 75 events. The latter are defined as a reduction in separation below the normal limits of 1,000ft (300m) vertically and 3nm (5.5km) horizontally.
Eurocontrol has been monitoring the unprecedented NATS study and, having assembled a European industry level-bust focus group, is creating a "toolkit" to help airlines and controllers reduce the incidence of these events. By mid-July it hopes to release a CD-ROM containing a video and briefing material largely based on NATS findings, but with added data now emerging from other European states. The Flight Safety Foundation, meanwhile, has joined the effort, and American Airlines is fronting similar moves across the Atlantic.
The UK air traffic services provider began its study of level busts in 1990 and at the same time launched a flightcrew awareness programme and a reporting system for pilots and controllers. Although industry knew that level-busts were happening, no data on the phenomenon had been gathered or analysed in the UK or elsewhere.
Reported incidents showed an upward trend from low levels for the first three years, dipped in 1993 and resumed their climb when NATS relaunched the reporting scheme in 1994 (see bar chart), so the apparent increase in events is probably more a reflection of a rise in controller and pilot confidence in the reporting system than an actual climb in level bust frequency, says NATS general manager London Terminal Control Centre Ian Linton. He admits, however, that data shared between a few co-operating airlines and NATS suggests that the actual number of level busts may be up to three times the reported figure.
Between 1998 and 2002 reported incidents for UK airspace have averaged a slight downward trend, and in November 2002 NATS started giving presentations to airlines, controllers and pilot groups using analysis from its compiled data. A part of the presentation was a staged re-enactment of the control room scene during some real level-bust incidents that had led to airprox (near-collision) reports, involving the triggering of the airborne collision avoidance system (ACAS).
NATS believes its approach has worked. Although the total UK airspace level-bust figures for last year are not yet available, Linton has determined that the 2003 figures for the London Terminal Control Area (LTCA) have halved since 2002, and numbers of the associated loss-of-separation events are down by more than 33%. The LTCA is the busiest and most complex airspace in the UK and normally accounts for 45% of the total UK level-bust incidents. Loss of separation, Linton points out, does not necessarily mean an airprox event.
NATS' study, conducted in association with the CAA Safety Regulation Group (SRG), has revealed the most common causal factor in the chain of events leading to level bust is a breach of standard operating procedures. This can be in anything from cockpit drills to incorrect radio messages or readbacks. Linton says NATS plans a more intensive industry awareness campaign in the next few months.
Level-bust causal factors
NATS' analysis showed that the major causal factors in level busts included (some events had more than one factor):
1 Not following standard operating procedures (45% of cases);
2 Pilot misreads standard instrument departure chart (37%);
3 Autopilot fails to capture level (20%);
4 Callsign confusion (17%);
5 Late altimeter setting (14%);
6 Input error to flight management system (13%);
=7 Workload during operational difficulties (11%);
Climbed or descended to a specific heading (11%);
9 Cleared to FL110 but went to FL100 or vice versa (10%);
10 Failure to resolve confusion with air traffic control (8%).
Source: Flight International
