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Reducing the Chance of a Mid-Air
Collision in the North Atlantic
by
Steve Kirby, Senior Research Analyst,
National Air Traffic Services
published in
the August 2006 edition of "Focus", the Official Publication of the
United Kingdom Flight Safety Committee
Brief Introduction to the North Atlantic
North Atlantic airspace is delegated to various states that manage
the airspace on behalf of ICAO. The UK and Ireland share a region of
the North Atlantic called Shanwick (derived from ‘Shannon’ and
‘Prestwick’), which extends out to the middle of the North Atlantic
Ocean. HF radio communications operators are based at Shannon,
Ireland, whereas the controllers are based at NATS’ oceanic centre
at Prestwick, in Scotland.
The
structure of airspace in the North Atlantic is very different from,
and much simpler than, UK domestic airspace, for several reasons.
Firstly, due to passenger demands, time zone differences and
restrictions on night time flying, there are two distinct flows
across the North Atlantic. Most westbound aircraft leave Europe in
the morning or early afternoon arriving in the Americas in the late
afternoon and early evening. Eastbound aircraft leave the Americas
in the evening and arrive in Europe in the morning.
The
jet stream, which changes daily, dictates the minimum time track
between city pairs. A series of tracks are published twice daily
which coincide with the minimum time track for the most popular city
pairs. Typically five or so tracks are published, and are usually
parallel to one another. This scheme is called the Organised Track
Structure. About half of all North Atlantic traffic flies on this
structure. This is an efficient way of managing such traffic.
Perhaps the biggest difference from domestic airspace is that
control is procedural, due to the fact that little radar coverage is
available. Pilots report their positions every 10° of longitude or
so and the time when at these positions. NATS’ controllers
scrutinise these periodic reports to ensure that aircraft remain
safely separated.
Improved Navigation and Lateral Overlap
The
introduction of very accurate aircraft navigation systems such as
global navigation satellite system (GNSS) along with sophisticated
flight management systems has enabled aircraft to navigate to such a
high level of accuracy that aircraft on the same track but at
different levels are increasingly likely to be directly above or
below one another. This improved navigational performance increases
the chance of mid-air collision if an aircraft deviates from its
cleared level, for whatever reason. The chance of getting lateral
overlap has increased eighteen-fold since 1977, and is set to
increase further.
The
chance of such a lateral overlap combined with increasing traffic
levels and density, and a relatively high number of large height
deviations at present, prompted the North Atlantic Systems Planning
Group (NAT SPG) to introduce the Strategic Lateral Offset Procedure
(SLOP) for North Atlantic region airspace on 10th June 2004.
How
Strategic Lateral Offsets Work
The
Strategic Lateral Offset Procedure was introduced specifically to
reduce the chance of mid-air collision by spreading out aircraft
laterally (see Figure 1). It reduces the chance of collision for
non-normal events such as operational altitude deviation errors and
turbulence induced altitude deviations. In essence, the procedure
demands that aircraft in North Atlantic airspace fly track
centreline or one or two nautical mile offsets to the right of
centreline only. However, the choice is left up to the pilot.
Figure 1. The SLOP intends to spread aircraft out laterally with the
use of two offsets to the right.
The
Strategic Lateral Offset Procedure recommends that pilots use all
available means to select the most appropriate position to fly,
including visual acquisition, collision avoidance systems and
communications with other aircraft. It allows pilots to change
position more than once in the flight, if appropriate. For example,
for overtaking manoeuvres, for avoiding wake vortex turbulence, or
to avoid flying directly above or below other aircraft.
An
alternative way that pilots may implement the procedure is to choose
by random a position to fly (i.e., track centreline or one or two
nautical miles to the right) and remain in that position for the
duration of the transit through North Atlantic airspace. This less
flexible approach provides less benefit than the tactical approach
described above, but nevertheless spreads aircraft out laterally,
thus reducing risk of collision for all aircraft, but particularly
for the offsetting aircraft.
The
Current Take-Up of the Procedure
For
maximum risk reduction, all flights would be distributed evenly
between the three lateral positions. However, recent studies by
NATS’ Operational Analysis department have indicated that few
flights in the North Atlantic airspace region routinely adopt
strategic lateral offsets. Analysis suggests that less than 10% of
flights are adopting strategic lateral offsets at present. This
take-up is somewhat disappointing considering the substantial safety
benefit, and that the procedure has been in place for more than two
years.
NAT
SPG and NATS are trying to redress this via an on-going education
campaign. Recent initiatives have included the production of a
training/educational DVD aimed at pilots, dispatchers and others
concerned with operations in the North Atlantic (available free on
application to customerhelp@nats.co.uk), and producing various
publications for the pilot community.
Because more than 90% of flights remain on the track centreline,
crews that routinely fly 1 Nm or 2 Nm offsets in accordance with the
procedure are benefiting from a very large reduction in mid-air
collision risk. Crews that fly track centreline are currently much
more likely to be involved in a mid-air collision.
Summary
In
summary, the Strategic Lateral Offset Procedure is designed to
spread out aircraft laterally in North Atlantic airspace in order to
bring a significant reduction in the chance of a mid-air collision.
The procedure will be especially effective when crews take local
traffic into consideration, using all means available to monitor
other aircraft. The procedure reduces the risk of collision for
non-normal events such as operational altitude deviation errors and
turbulence induced altitude deviations.
A
full description of the procedure can be found at
http://www.nat-pco.org.
This site also provides free access to the latest version of the
North Atlantic Operations Manual which details this and other
procedures specific to the North Atlantic region.
NATS is committed to supporting and promoting collision risk
reduction measures in the North Atlantic region.
Steve Kirby
Senior Research Analyst
National Air Traffic Services
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