Level Busts have probably been with us ever
since somebody fitted an altimeter to an aeroplane and tried to stop
it (the aeroplane that is) going up and down too much. Nowadays, with
high performance aeroplanes carrying large numbers of goods and people
over ever more densely populated towns and cities, in an ever busier
air traffic environment, the necessity of understanding and adhering
to ATC vertical clearances has never been more important.
Much has been written about level busts, but
most of the articles I have seen mainly focus on cockpit procedures,
autopilot malfunctions and suchlike. This article, however,
concentrates on the Air Traffic System, explains how ATC separation is
achieved in a radar environment, points out some of the pitfalls of
the system and shows why level busts deserve high priority treatment.
The Air Traffic Controller’s sifting process
which results in the resolution of conflictions between aircraft
firstly relies upon the “reading” of a flight progress board to
initially determine if there might be a confliction. Secondly radar is
used to refine results and identify which flights require separation
by controller intervention using radar techniques. Radar, I am sure,
needs no explanation, but a ‘Flight Progress Board’ probably does.
Controllers have a note of traffic details printed on flight progress
strips which are displayed in moveable holders mounted on the flight
progress boards. These boards allow for the strips to be arranged so
that aircraft which follow the same, or crossing, routes can be
assessed by the controllers, to see whether or not they are separated.
This sifting of the available data goes through a number of stages.
The first level of sifting is based upon the
idea that flights which are displayed on the flight progress board as
flying at vertically separated levels are not in confliction.
It follows, therefore that those flights which are at the same level
may be in confliction. This first level of sifting results in
flights being categorised as "not in confliction" and "might
be in confliction".
The second stage is to decide, again using the
flight progress board, which of those flights now categorised as "might
be in confliction" are not in confliction. The separation
used for this is based on time, supplemented by the controller’s
experience and understanding of the particular airspace and traffic
situation geometry. This second level of sifting results in more
flights being categorised as "not in confliction" and a lesser
number being pigeon-holed as "might be in confliction".
The third stage moves the focus away from the
flight progress board to the radar display. Those flights which remain
in the category "might be in confliction" are assessed to
decide whether tactical intervention may be required. This stage sorts
the flights into "not in confliction" and "in confliction".
This time, though, the decision is based on ‘eye-balling’ the
situation to decide whether the tracks being flown by flights at the
same level are such that intervention by the controller is needed or
not.
The fourth stage of the process is tactical
intervention to resolve the confliction. This takes the form of the
controller instructing the pilot to change level, direction or speed,
or any combination of the three.
Looking over this process, it becomes clear that
there could be problems with the flights categorised as "not in
confliction". This is because the controller's (or controllers')
attention will rightly focus upon those flights categorised as "might
be in confliction" or "in confliction", and will be less
taken up with monitoring those flights which are understood by the
controller to be "not in confliction". To put it plainly, the
controller will believe that these flights are not problematic so they
may, to some extent, be ignored. It is worth pointing out that this
sifting process is not a formal one, rather it goes on inside the
controller’s head and the situation is constantly reviewed and
up-dated along with the ever changing traffic patterns.
So, what does this mean in terms of Level Busts?
First of all, when a controller assesses that a flight is not in
confliction with any other, less time will be spent in monitoring its
progress. That is not to say that the flight will not be watched, it
will, but to a lesser extent than those flights requiring
intervention. It follows that the greater the number of flights
assessed as “in confliction”, then the less time is spent in
monitoring the rest. This means that the controller is relying, even
more than normal, on the pilot flying the aircraft in accordance with
the vertical clearance. Additionally, controllers are less likely to
spot a level bust early since their concentration will be elsewhere.
Simple arithmetic tells us that with 1,000 ft
separation being used, a modern business jet climbing at 6,000 ft/ min
will take only ten seconds to change a situation from one where
separation is assured, to one where a collision is happening. Even
with lower rates of climb and descent the amount of time available for
the controller to notice, take in the situation and react is minimal.
When you further consider that the controller’s radar display is being
updated at, typically, once every 6 seconds the seriousness of the
situation can be seen even more clearly.
However, the danger of a collision between the
level busting aeroplane and another is only part of the picture. In a
busy traffic environment the controller(s) will have prioritised tasks
and be constantly reviewing these. When a level bust happens, it means
that these priorities have to be instantly changed, in exactly the
same way as they would in the event of a full blown emergency
situation. The extent of this re-appraisal will depend on the general
situation at the time, and whether or not the level bust has caused a
separation loss. Even if no separation loss has occurred, there will
be some increase in workload and, possibly, some dis-orientation of
the controller(s) involved. This could well be a contributory factor
in an another incident.
The problems associated with this process are an
integral part of our ATC system, and are likely to remain so as long
as the ATC system stays predicated on a level allocation foundation,
with people, Air Traffic Controllers, playing such a central role. For
all the computerisation and technological advances of recent years,
their main impact on the Air Traffic Control System has been to
provide controllers with better quality information, and some late
stage warnings of critical separation loss (short term conflict alert
systems), and to similarly provide pilots with late-stage warning of
impending collision (TCAS). Ultimately reliance is placed upon the
personal skill of the controller to understand the traffic situation,
sift out the real conflictions and solve them.
It is important to realise that this sifting
process works the vast majority of the time. The rub is that if it
fails the results could be catastrophic. So where do we go from here?
In the longer term systems will undoubtedly be developed which reduce
the reliance on people, and so reduce the likelihood of human error.
In the short term, however, the only viable way forward is through
greater understanding of the ATC system by pilots and controllers
alike, so that the people involved at the sharp end of aviation are
better able to minimise the risks inherent to their jobs.
Steve Sharp,
Inspector of ATC,
ATSSD,
Safety Regulation Group
UK CAA
ATS Standards Dept. Safety Regulation Group Civil Aviation Authority
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