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unavailable.
Provision of
instructions or
information which
may result in a critical
near mid-air collision
or a critical near
collision with the
ground.
The ATC separation service
provided to aircraft that are
airborne or are inside a
runway protected area in one
or more sectors is suddenly,
and for a significant period of
time, severely degraded or
compromised (e.g.
contingency measures
required, or controller
workload significantly
increased such that the
probability of human error is
increased).
The ATC separation service
provided to aircraft on the
ground outside a runway
protected area is suddenly,
and for a significant period of
time, completely unavailable.
Provision of instructions or
information which may result
in the separation between
aircraft or aircraft and the
ground being reduced below
normal standards.
No ATS action possible to
support aircraft emergency.
The ATC separation
service provided to
aircraft that are
airborne or are inside
a runway protected
area in one or more
sectors is suddenly,
and for a significant
period of time,
impaired.
The ATC separation
service provided to
aircraft on the ground
outside a runway
protected area is
suddenly, and for a
significant period of
time, severely
degraded.
ATS emergency
support ability is
severely degraded.
No effect on ATC
separation service
provided to aircraft.
Minimal effect on
ATC separation
service provided to
aircraft on the
ground outside a
runway protected
area.
Minimal effect on
ATS emergency
support ability.
While the assessment of the severity of the consequences will always involve some degree of subjective
judgement, the use of structured group discussions, guided by a standard risk classification scheme, and
with participants who have extensive experience in their respective fields, should ensure that the outcome
will be an informed judgement.
Once the assessment of severity has been completed for all the identified hazards, the results, including the
rationale for the severity classification chosen, should be recorded in the hazard log.
STEP 4: ESTIMATION OF THE LIKELIHOOD OF A HAZARD OCCURRING
The estimation of the likelihood of a hazard occurring uses a similar approach to that adopted in Steps 2
and 3, i.e. by means of structured discussions using a standard classification scheme as a guide. Table 13-4
shows an example of a classification scheme for this purpose, based on JAR-25, taken from the U.K. CAA
CAP 670, Air Traffic Services Safety Requirements.
Table 13-4 specifies the likelihood as qualitative categories, but also includes numerical values for the
probabilities associated with each category. In some cases, data may be available that will allow the making
Chapter 13. Safety Assessments 13-9
Table 13-4. Probability classification scheme
Probability of Occurrence Definitions
Extremely
improbable
Extremely
remote Remote
Reasonably
probable Frequent
Qualitative
definition
Should virtually
never occur in
the whole fleet
life.
Unlikely to occur
when considering
several systems of
the same type, but
nevertheless has
to be considered
as being possible.
Unlikely to occur
during the total
operational life of
each system but
may occur several
times when
considering
several systems of
the same type.
May occur once
during total
operational life
of one system.
May occur once
or several times
during operational
life.
Quantitative
definition
< 10–9 per
flight hour
10–7 to 10–9 per
flight hour
10–5 to 10–7 per
flight hour
10–3 to 10–5 per
flight hour
1 to 10–3 per
flight hour
of direct numerical estimates of the likelihood of failure. For example, for the hardware elements of a
system, extensive data are often available on historical component failure rates.
The estimation of the likelihood of occurrence of hazards associated with human error will generally involve
a degree of subjective assessment (and it should be borne in mind that even when assessing hardware,
there is always the possibility of failures due to human error, for example, incorrect maintenance
procedures). However, as with the estimation of severity, the use of structured group discussions with
　
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