Improving the Safety of
Communications
Kim O'Neil
Advanced
Aviation Technology Ltd.
Nigel Corrigan
March 1999
Abstract
There is a long history of fatal accidents and
serious incidents that are directly attributable to blocked transmissions. This
risk to flight safety is caused by a fundamental weakness in the current system
of VHF (double side band) radio communications. This paper describes a simple
and low cost solution to this problem, CONTRAN® - which drastically reduces
an entire class of communication failures, so greatly reducing the risk to
aircraft safety. Airborne and ground equipment have been developed and both
have been fully certified for operational use. Thus Airline and ATC
communications can now benefit from this safety enhancing solution. This paper
also provides summaries of many incidents involving blocked transmissions.
1. Improving Aviation Safety
Much attention in the media has focussed on
aviation safety. Air crashes are big news. In trying to understand the causes
of crashes and to prevent further occurrences, the industry has tended to focus
on more complex technical solutions - in an attempt to provide major
improvements to improve aviation safety. This focus has assumed that big
problems need big solutions. Unfortunately, this approach often means that
simpler solutions to longstanding key problems are overlooked or overshadowed.
Necessarily, "big bang" solutions are as
expensive as they are glamorous. More to the point, their benefits remain less
tangible as the rate of aircraft losses continues, with no sign of significant
improvement. This paper addresses one major contributor to aviation accidents
and incidents - that of "blocked" or "stepped on" radio transmissions. This is
a long-standing vulnerability of the current VHF Radio communications system,
which occurs far to often to be ignored. This common communications problem can
be addressed effectively and at low cost.
2. Problems with VHF Radio Communications
Aeronautical VHF radio communications have
experienced many difficulties over the years. The 'AM Double Side Band' VHF
radio system has fundamental weaknesses. These include: overcrowded spectrum,
poor quality, high susceptibility to interference and a high risk of blocked or
"stepped on" transmissions. The first is an institutional and regional planning
problem, whilst the second and third are features of the chosen technology
which can only be resolved by a move to digital radio. The fourth issue can be
effectively dealt with by installing anti-blocking devices such as
CONTRAN®.
Blocked transmissions are a significant and
recurring problem - and have been a key factor in several major accidents and a
large number of serious incidents (some UK examples are listed in the appendix
to this paper). Perhaps the most tragic of these accidents was the collision
between two 747's on the runway at Tenerife airport in March 1977, which lead
to the greatest loss of life ever experienced in aviation - over 583 crew and
passengers. This resulted in recommendations by many Aviation Authorities
including the FAA and the UK CAA in support of anti-blocking devices and the
publication of several Air Information Circulars (AICs) encouraging airlines to
fit anti-blocking equipment. Indeed, CONTRAN® was developed as a direct
result of this tragic aircraft crash. Some forward thinking and safety
conscious airlines such as Britannia have taken a pro-active stance and
equipped their fleet with CONTRAN®.
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| Figure 1: Britannia Airlines has
equipped its fleet with CONTRAN® |
3. Blocked or "Stepped on" Transmissions
The mechanics of blocked transmissions are
easily understood. In general, it may involve simultaneous transmission from
two aircraft or simultaneous transmission from an aircraft and ATC. These
transmissions interfere with each other, "blocking" both signals. There is a
widespread belief that such blocked transmissions would always be detected (i.e
heterodyne heard by the pilot and/or ATC). An equally widespread belief, held
by controllers, is that ATC transmissions are always heard. Neither is
necessarily true. However, even if detected, it is often too late to do
anything and aircrew and ATC are usually too busy managing events in
progress.
The risk to air safety is also easily
understood, although the circumstances of some accidents can be very complex.
For example, if ATC give an instruction to aircraft A, which is mistakenly
taken by taken by aircraft B (say, due to a clipped transmission) then the
result may cause either aircraft to come into conflict with the other or with
surrounding traffic. The consequence could lead to aircraft B descending
prematurely from a stack into the path of another aircraft. Such events are all
too common.
There are many circumstances, which might
initiate a blocked transmission. Pilots generally, 'wait their turn' on VHF
radio before transmitting. Usually this is easily done, as pilots listen in on
VHF and build up a mental picture of traffic around them (often of those in
front and those behind). When an expected break in VHF traffic occurs, they
'key their mikes' and make their transmission. This is fine when things are
going well and if nothing occurs to break the routine. However, there are
occasions when this routine is disrupted - and this is when the weaknesses in
VHF radio system show through. Sometimes it is the routine itself, which is the
cause e.g. premature anticipation of an expected message.
The general nature of blocking transmissions may
result in the total loss of both messages or each message "clipping" the other.
This, in turn, may lead to the loss of essential information - such as the
aircraft's call-sign. This is the commonest event and one that leads to pilots
anticipating messages from ATC that were actually destined for other aircraft.
A key fact to be born in mind is that two messages are lost or damaged and this
adds greatly to both the level of risk encountered in blocked transmissions.
4. Read-back Procedures
ATC Procedures, such as read-back, have been
introduced to minimise mis-communications between the pilot and ATC. However,
such ATC procedures do not lead to the elimination of the problem (as is
indicated by its persistent occurrence) but merely seeks to mitigate its effect
- meaning that a significant percentage of certain types of incidents will
continue to occur. Essentially, Read-back procedures attempt to recover from
incorrect communications, but are not always successful.
- Prevention is always better than a
cure.
Interestingly, in some of the most serious
incidents, it is read-back that has been blocked (sometimes from the
pilots of two separate aircraft reading back to the same ATC instruction!). In
essence, anti-blocking devices are complementary to ATC procedures such as
read-back and can significantly enhance its operation.
5. UK and US Reported Incidents
Two more detailed examples of near mid-air
collisions are given in the attachments to this paper, one extracted from a UK
CAA Airprox report and another taken from the US Aviation Safety Reporting
System (ASRS). In addition, brief summaries of a list of Occurrence Reports
involving blocked transmissions in the UK between January 1994 and June 1998
are also included. It is hoped that these summaries will give a clear
indication of the widespread nature of the problem and the need to do something
about it. Both the UK and the US operate comprehensive accident and incident
reporting schemes. To these lists must be added the many events that occur
routinely around the world every day (many of which go unreported). Some of
these incidents have tragic yet preventable consequences.
6. CONTRAN®
CONTRAN® protects commercial aircraft
operations against simultaneous, conflicting or unintentional VHF radio
transmissions between commercial aircraft and Air Traffic Control. It does this
by monitoring VHF channel activity and automatically inhibiting inadvertent
transmission into an already busy frequency. Such transmissions have been cited
as a major causal factor in several aviation accidents and in numerous
air-misses. A secondary function is to protect aircraft from unintentional
transmissions (e.g. 'stuck mike') which may block the VHF voice channel to all
users. The system also includes provisions for a full time receiver on the
International Distress Frequency, 121.5mHz.
As noted above, CONTRAN® senses a
transmission in progress, and inhibits any attempt to initiate a new
transmission on the same frequency. However, it also makes the pilot aware that
the frequency is in use, and that re-transmission will be necessary when the
preceeding transmission is complete.
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| Figure 2: CONTRAN® MCU
Enclosure |
7. Compliance with Technical Requirements
The Anti-Blocking System CONTRAN® has
received approval from the US Federal Aviation Administration against TSO C122
and TSO C128 for VHF Communication Equipment. It has also received similar
certification by the UK Civil Aviation Authority (CAA). CONTRAN® also meets
the requirements of the International Federation of Air Line Pilots'
Associations (IFALPA), Policy Manual Annex 10, Communications; and the
International Federation of Air Traffic Controllers' Association's (IFATCA)
Policy Manual.
The system is easily installed and meets or
exceeds RTCA DO-207: "Minimum Operational Performance Standards for Devices to
Prevent Blocked Channels in Two Way Radio Communications Due to Unintentional
Transmissions" and RTCA DO-209: "Minimum Operational Performance Standards for
Devices to Prevent Blocked Channels in Two Way Radio Communications Due to
Simultaneous Transmissions". This means that CONTRAN® can be installed in
any aircraft on the US or UK Civil Aviation Registers and also in other
countries normally accepting US and UK certification.
CONTRAN® is manufactured by British
Aerospace (Systems and Equipment) Limited under licence from its inventor Nigel
Corrigan. CONTRAN® has been delivered to its launch customer, Britannia
Airways. Britannia has equipped its entire fleet of Boeing 757 and 767
aircraft. Virgin Atlantic has also carried out trials on its Boeing 747-200
aircraft. A Ground based version of CONTRAN® for ATC has been developed and
Airports such as Bournemouth and East Midlands are among the first to
equip.
8. Summary
The fitting of anti-blocking devices such as
CONTRAN® is recommended but not currently mandatory. Air Information
Circulars (AICs) from the FAA and the UK CAA have recommended the fitting of
such devices - as have several agencies responsible for investigating Incidents
and Accidents. Some Agencies are now considering stronger action to alleviate a
persistent safety hazard affecting civil aircraft operations world-wide.
It is perhaps remarkable that airlines have been
prepared to fit more complex and certainly more expensive safety devices, yet
have apparently overlooked the potential safety benefits to be gained from such
an inexpensive solution to a serious and very long-standing communications
problem. As the skies get ever more crowded and as the pressure increases on
our runways, it is to be hoped that we will not have to wait for another
communication induced disaster before airlines are stirred into action.
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