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· If a system’s risk falls into the intolerable category, then action must be taken to
redress this. If this is not possible, the system should be halted or not implemented.
· If a system’s risk falls into the tolerable category, it must be proven to be as low as
reasonably practicable within that region for the system to be considered acceptable.
Showing a system is ALARP means demonstrating that any further risk reduction in
the tolerable zone is either impracticable or grossly out-weighed by the costs.
· If a system’s risk falls into the negligible category, no action is required other than
monitoring to ensure that the negligible risk is maintained.
COLLISION MODELING DEFINITIONS AND RISK METRICS
3-7
With the ALARP approach, the boundary lines between the risk categories negligible,
tolerable, and intolerable need to be specified; they are not automatically set. Also, it is
important that the risks from a given activity are shown to be within the negligible or
tolerable regions from both an individual and societal point of view. It is well known that
the public is willing to accept much higher risks in many other activities than in aviation.
‘ Z e r o ’ R i s k
R e d r e s s s i t u a t i o n o r
h a l t s y s t e m
M u s t p r o v e s y s t e m
i s A L A R P
M u s t s h o w f u r t h e r r i s k
r e d u c t i o n i s n o t c o s t - b e n e f i c i a l
N e e d o n l y m o n i t o r t o e n s u r e
r i s k i s m a i n t a i n e d a t t h i s l e v e l
U n a c c e p t a b l e
R e g i o n
T o l e r a b l e , i f A L A R P
R e g i o n
N e g l i g i b l e
R e g i o n
I n c r e a s i n g R i s k
T h e A L A R P A p p r o a c h
Figure 3-1. ALARP Approach
It could be argued that the risk of midair collision should also be examined in such way.
That is, the risk should be expressed both as a metric that can easily be related to the
individual risk (e.g., fatal accident per flight hour) and as a metric which can be easily
related to the societal risk (e.g., risk of collision per year or MTBA). In using this
approach, it is necessary to take into account that collision risk is only a small part of the
overall risk from flying.
Reference
The following reference includes a thorough study of the available literature on safety
analysis, metrics, and validation techniques. An appendix also includes descriptions of
most available tools for evaluating safety, capacity, and human factors.
“GENOVA Work Package 3 Report, Version 1, Methods and Techniques,” European
Union DG VII, Contract No. AT-95-SC.112, NLR, 1997. [GENOVA = Generic Overall
Validation for ATM.]
SEPARATION SAFETY MODELING
3-8
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MODELING CRITERIA
4-1
4.0 MODELING CRITERIA
This section presents criteria that have been suggested as a means of bounding the scope
of this modeling effort. Among those criteria is one which suggests that before modeling
the safety effects of some change in current air traffic systems, one should perform an
analysis to determine if such a change would result in economic benefit. Section 4.2
presents a discussion of some of the factors that must be considered in analyzing what
benefits might be derivable from reduced separation minima (standards) and suggests an
approach to estimating how much increased capacity might be derived from reduced
separation minima.
4.1 LIST OF CRITERIA
There is a need for criteria to define and bound the scope of the task. The criteria
currently adhered to are as follows:
a) An economic analysis should be performed to determine if any real benefit will gained
from a potential change. Realistically, will there be a real economic benefit from
reducing horizontal or vertical separation? If the economic benefit is slight or
nonexistent, there is no reason to reduce separation minima or engage in a major
collision risk modeling effort. (This is discussed in Section 4.2.)
b) Concentrate on en route at this, first stage, with terminal airspace to follow.
c) Consider separation between aircraft (aircraft-to-aircraft separation maintenance as
opposed to, for example, separation between jet ways).
d) Assume that some surveillance infrastructure will continue to be required, that is,
separation will not be guaranteed solely by aircraft navigational performance or
aircraft-to-aircraft means. This may or may not involve ground infrastructure. In
particular, it is assumed that in the foreseeable future, controllers will be responsible
for detecting and alerting pilots to a pending loss of IFR separation, and issuing them
guidance on how to maintain separation/avoid a collision.
　
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