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qualitatively and quantitatively. Some techniques require specialist expertise in their application. Typically,
the hazard analysis process involves:
a) development of fault schedules;
b) construction of fault trees; and
c) quantification of the likelihood of human error, equipment failure and operational factors.
Fault schedules
3.2 Fault schedules are used to record the results of the HAZid process for each hazardous scenario.
An example of a hazardous scenario might be a mid-air collision between an arriving and a departing aircraft
when the arriving aircraft fails to intercept the localizer.
3.3 The initiating event for this scenario would be that the arriving aircraft heads into the flight path of
the departing aircraft. The fault schedule would record possible causes for the initiating event, including
airborne or ground equipment faults, and human error by either the pilot or ATC (for example, call sign
confusion). Recovery factors include existing or missing defences designed to reduce the likelihood of the
initiating event becoming a hazardous scenario. Each recovery factor is examined as to why it failed to
prevent the situation from developing.
Fault trees
3.4 Information contained in the fault schedules may be used to construct a fault tree. The level of
analysis for the fault tree will depend on the situation. However, as a general guide, a simple pessimistic
model should be used initially to determine the likelihood of human error, equipment failure and operational
factors and thus the operational risk exposure. This risk exposure is then compared with the risk criteria for
17-APP 2-4 Safety Management Manual (SMM)
the target level of safety. If the pessimistic model produces a result that is lower than the target criteria, then
further resource allocation is not required as it would not alter the risk management decision.
Consequence analysis
3.5 The amount of loss for ATS-related risk assessments is normally measured as the number of
fatalities that would result from the most drastic possible outcome. For example, a simple analysis of mid-air
collisions and collisions with the ground assumes that all people on board the aircraft will die as the result of
a mid-air collision and most collisions with the ground.
4. RISK ASSESSMENT
4.1 As outlined in Chapter 6, a key phase of risk management involves the assessment of identified
risks. Formal risk assessments must be performed:
a) for significant changes to ATS procedures compared with current operations;
b) for significant changes to equipment used to execute ATS tasks compared with current operations;
and
c) when changing circumstances, such as increased traffic levels, and different aircraft performance,
indicate that existing procedures may not be appropriate.
4.2 Table 17-APP 2-1 offers several steps for assessing risks inherent in hazards found in ATS
procedures.
Risk analysis
4.3 Risk is calculated as the product of the likelihood of a hazardous event and the consequences of
the event happening. Risk analysis may be quantitative or qualitative depending on the risk information and
data readily available, the magnitude of the hazard, and other factors. Use of quantitative data helps clarify
most decisions and should be used where available; however, some of the most important factors in a
decision can be impractical to quantify. (For example, often when examining people and procedures in the
provision of a separation service, qualitative descriptions and comparison scales are all that are available.)
Care should be taken to consider these factors also.
Risk management
4.4 The principles and steps of risk management are outlined in Chapter 6. Management must decide
if:
a) the risk is so great that it must be refused altogether;
b) the risk is, or has been made, so small as to be insignificant (however, any actions that reduce risk
and require little effort or resources must be implemented); or
c) the risk falls between the two states in a) and b) and has been reduced to the lowest level
practicable, bearing in mind the benefits derived from its acceptance and taking into account the
costs of any further reduction.
Chapter 17. Air Traffic Services (ATS)
Appendix 2 17-APP 2-5
Table 17-APP 2-1. ATS risk assessment procedures
Step 1 Identify whether the change involves a change in control procedure, in equipment, or in both.
Step 2 Break down the procedures into manageable components. For example, control procedures
might be divided into:
a) transfer of control procedures;
b) coordination procedures;
c) radar procedures;
d) holding procedures;
e) speed control procedures; and
f) runway procedures.
Equipment user procedures might be divided into:
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Safety Management Manual (SMM) 安全管理手册(135)
