Causal risk models of air transport - NLR-ATSI

Fatal accidents are rare events. When using accident data (i.e. the realization of
probabilities) to quantify aviation risk, this must be taken into account. Because fatal
accidents are so rare even large observed differences need not attain significance. If a coin
is tossed only four times, then no possible outcome - even four heads or four tails- would
provide statistically significant evidence that the coin is not fair [Czerwinski & Barnett
2004]. The number of aviation accidents with fatalities has become so low that it is
problematic to use those as the only indicator of air transport risk, there is too much
randomness in the data. Therefore non-fatal accidents and incidents should be included in
the considerations on the construction of a causal risk model. But the question then arises
which occurrences should be considered. Incidents and (fatal) accidents are not necessarily
causally related. Trips and falls for instance result in many injuries but are rarely causally
related to catastrophic accidents. Therefore only those occurrences, associated with the
operation of an aircraft, which affect or could affect the safety of operation 7 should be
included.
2.4. Risk criteria
Unfortunately, safety is not self-sustainable [SAE 2003]. Some sort of safety management
is required to improve or even maintain the current level of safety. Policies to control major
risks have been in development from the 1960s onwards. Many of these policies are based
on some sort of quantification of the risk that could be allowed to continue. This section
explains how causal risk models can be used in relation to such quantitative risk criteria and
risk control policies.
A Target Level of Safety (TLS) is the ‘amount’ of safety that is aimed for. The concept of a
TLS appears intuitively obvious [Joyce et al 2001]. The prime user of the TLS concept in
aviation has been ICAO. Over the years, ICAO has developed TLS concepts in various
safety critical areas of the industry, using international groups of experts. Such areas have
included the North Atlantic System Planning Group (1992), the All Weather Operations
Panel (1994), the Obstacle Clearance Panel (1980) and the Review of the General Concept
of Separation Panel (1995). The latter work panel has offered the following definition:
“A Target of Safety (TLS) specifies an acceptable value of risk which can be used as a
yardstick against which the risks associated with a system or procedures can be evaluated.
The concept of a TLS is particularly useful when planning changes in safety critical
operations such as air traffic control.”
[RGCSP 1995].
The definition for the TLS concept that is offered will vary in accordance with its particular
application and its intended use. For example, the UK Civil Aviation Authority (an
organisation which has also been instrumental in the development of TLS concepts),
defines the TLS concept for controlled airspace as:
“…a fundamental concept in any mathematical / statistical approach to systems planning
when questions of safety are involved…The target level of safety is the level of safety which
the system is designed to achieve. Put the other way round, the system is designed to an
assured level of safety. By this specification it is possible to define planning objectives
which fit in with the safety constraints, and also provide a safety yardstick against which
potential changes can be assessed and objectives pursued.” [Brooker & Ingham 1977].
A Target Level of Safety is a level of safety that either must be achieved in order to carry
out some activity (i.e. a mandatory target) or must be aimed for but need not necessarily be
7
This is ICAO’s definition of an ‘incident’.
17