Identifying the System Hazards

Once the set of accidents has been agreed upon, hazards can be derived from them. This process is part of what is called Preliminary Hazard Analysis (PHA) in System Safety. The hazard log is usually started as soon as the hazards to be considered are identified. While much of the information in the hazard log will be filled in later, some information is available at this time. 

There is no right or wrong list of hazards—only an agreement by all involved on what hazards will be considered. Some hazards that were considered during the design of TCAS are listed in chapter 7 and are repeated here for convenience  

Horizontal advisories were originally planned for later versions of TCAS but have not yet been implemented. 

TCAS causes or contributes to a near midair collision (NMAC), defined as a pair of controlled aircraft violating minimum separation standards. 

TCAS causes or contributes to a controlled maneuver into the ground. 

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Technical Authority (ITA) recommended in the report of the Columbia Accident Investigation Board. The risk analysis itself is described in the chapter on the new hazard analysis technique called STPA (chapter 8). But the first step in the safety or risk analysis is the same as for technical systems to identify the system hazards to be avoided, to generate a set of requirements for the new management structure, and to design the control structure. 

As in the previous examples, the first step in creating a STAMP analysis is to identify the system hazards, the system safety constraints, and the hierarchical control structure in place to enforce the constraints. 

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