| Why All The Fuss
About Level Busts?
by
Graeme Stagg
It should not have escaped your attention,
especially if you have been reading any UK CAA/Industry Flight Safety
publications recently that there is a lot of attention being paid to
the subject of Level Busts. A ‘Level Bust’ awareness campaign,
spearheaded by the CAA, is underway and gathering momentum with an
increasing amount of industry publicity being given to the topic
together with encouragement for Aircrew and ATC to report these type
of incidents. Despite this campaign there are, I suspect, many
sceptics, especially on the flight deck, who consider they have never
been involved in a level bust, or that if they have and no conflict
resulted, then there was no danger. Such people may be wondering, ‘why
all this fuss?’
It is my aim, by the end of this short article,
to convince any sceptics of why level busts are a serious flight
safety issue and are of concern to all pilots and controllers.
In 1993, it became noticeable that reported
incidents of aircraft in UK airspace, deviating significantly (more
than 300 feet) from their vertical clearances was increasing. This
increase had become a significant safety issue because it was causing
a rise in the recorded rates of loss of ATC standard separation. Early
studies into the problem, as it affected UK airspace, lead to the
publication of two CAA “Data Plus” safety leaflets regarding level
busts ,in 1994 and 1996, and NATS held a series of workshops and
presentations with airlines because it was recognised that this was a
serious safety problem. A third Data Plus leaflet has just been
published highlighting level busts recorded in UK airspace since 1994.
The most noticeable change over this three years is the sharp increase
from 1996 to 1997 in the number of incidents involving loss of ATC
separation. Clearly the level bust problem is not going to reduce by
itself.
On 12 November 1996 there were two particularly
significant level busts; one occurred in Indian airspace near Delhi
 and the other a few hours later in the Lambourne Holding Pattern in
the London TMA. The major difference between the two occurrences was
that the level bust near Delhi resulted in a tragic mid-air collision
whilst in the one over Lambourne, good fortune meant the only injury
was to the pride of a B737 crew and considerable anxiety for the
controllers who had to resolve the situation. Industry safety
publicity about the Delhi accident is scarce (to date no formal
accident report has been published by the Indian authorities).
(Ed note: The report has now been published)
Whilst
it seems that, little has been learned from the Indian accident to
prevent a recurrence elsewhere, the UK CAA has taken the matter
seriously. Soon after it was realised that the cause of the Delhi
accident was linked to a level bust, the CAA embarked on a more
vigorous level bust safety initiative which is still on-going.
Pilots and air traffic controllers are all
acutely aware that life in recent years has become busier as civil air
transport movements continue to grow year on year and as a consequence
airspace is becoming more congested. Whilst ATC systems have steadily
improved and many facilities now incorporate safety nets in the form
of conflict alert systems, the design and performance of modern
aircraft have improved quite dramatically in recent years.
Improvements, particularly in aircraft navigation and the advent of
the computerised ‘glass cockpit’ flight management systems now mean
that the aircraft are flying with great accuracy. When combined with
the advanced auto flight abilities and impressive climb and descent
qualities of these aircraft, it means that when positional errors are
made by aircrew or controllers, regarding their safe airspace
management, any conflict with other traffic in the same piece of
airspace will result in much closer proximity. This can increase
significantly the threat of a mid-air collision.
Within the London Terminal Control Area (LTMA),
the ATC task is to integrate safely the flight paths of aircraft
arriving at and departing from the major London Airports with those of
over-flying aircraft and those wishing to join the airways system in
the London area. The complex nature of the operation is eased by the
use of Standard Terminal Arrival Routes (STARs) and Standard
Instrument Departures (SIDs) which specify predetermined tracks and
levels to be flown by arriving and departing traffic. Normal ATC
co-ordination procedures are augmented by the use of ‘Standing
Agreements’ (SAs). SAs allow aircraft to enter the airspace of an
adjacent sector without individual co-ordination of their transfer
arrangements as long as certain conditions regarding levels and
routings are met. Therefore, STARS, SIDs and SAs are now fundamental
to the operation of a busy ATC units since they facilitate the safe
and efficient flow of traffic during the departure and arrival phases
of flight. Moreover, these procedures are heavily reliant upon
accurate and reliable level keeping by the pilots flying their
aircraft.
To put this aspect of operations into some sort
of perspective; in 1997 the London Area and Terminal Control Centre (LATCC)
provided an ATC service to over one and a half million commercial air
transport flights and of these nearly one million operated in the
London TMA. Considering each of these flights would have made,
perhaps, 10 level changes inside this very congested airspace to
maintain safe operations, the need for accurate vertical navigation by
pilots, and clear, unambiguous communication of level change
instructions between aircrew and air traffic controllers becomes self
evident.
ATC radar conflict alert systems and the
increasing number of commercial aircraft fitted with TCAS have helped
to reduce the threat of a level bust resulting in a mid-air collision.
The advent of the mandatory carriage of TCAS II for aircraft with 30
seats or more, from 1 January 2000, and improvements in airborne
collision avoidance and the ATC conflict alert systems should help to
further reduce the risk in UK airspace. However effective, such safety
nets cannot be relied upon in isolation.
Level busts are an ever-present safety menace in
today’s increasingly crowded and complex airspace environment. Both
pilots and controllers must be fastidious in checking and monitoring
all vertical clearances, pilots must be constantly aware of the
hazards in deviating from assigned levels and do their utmost to
prevent them whilst controllers and pilots must remain vigilant at all
times to the mid-air collision threat they pose - which is really what
all the fuss is about.
Note: Graeme Stagg is the ATC Flight Safety
Analyst in the CAA’s Safety Data Department, the JAAP Executive and
report book editor. He is also a member of the CAA/Industry Level Bust
Working Group (LBWG). However, this article has stemmed from his work
in all 3 areas and has been written primarily for the LBWG’s current
awareness campaign. Nevertheless, the views he has expressed in this
article are personal ones and do not necessarily reflect the views of
the CAA, the JAAP or the LBWG
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