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their associated risks are referred to. Figures 7-1 through 7-4 provide examples of previous accident
scenarios that have occurred. Note that many things had to go wrong for a particular accident to occur.
Each of these accident scenarios has their associated risk. It should be noted that every contributory event
has to be considered, as well as its event likelihood, when determining a specific risk. Consider that a risk
is made up of a number of hazards and that each hazard has its own likelihood of occurrence. Further note
that the potential worst case harm, which may be aircraft damage, injury or other property damage
represents the consequence, or the severity of the accident scenario. Likelihood is determined based on an
estimate of a potential accident occurring. That accident has a specific credible worst case severity. If the
hypothesized accident’s outcome changes, the scenario changes, and as a result, a different risk must be
considered. The steps in a risk assessment are:
· Hypothesize the scenario.
· Identify the associated hazards.
· Estimate the credible worst case harm that can occur.
· Estimate the likelihood of the hypothesized scenario occurring at the level of
harm (severity).
Figure 7-1 shows the sequence of events that could cause an accident from a fuel tank rupture on board an
aircraft. There are a number of contributory hazards associated with this event: fuel vapor present, ignition
spark, ignition and tank overpressurization, tank rupture and fragments projected. The contributors
associated with this potential accident involve exposed conductors within the fuel tank due to wire
insulation degradation, and the adequate ignition energy present. The outcome could be any combination of
aircraft damage, and/ or injury, and/or property damage.
Figure 7-2 shows the sequence of events that could cause an accident due to a hydraulic brake failure and
aircraft runway run-off. Note in this case there are again, many contributors to this event: failure of the
primary hydraulic brake system, inappropriate attempt to activate emergency brake system, loss of aircraft
braking capability, aircraft runs off end of runway and contacts obstructions. The outcomes could also
vary from aircraft damage to injury and/or property damage. Note that the initiating events relate to the
failure of the primary hydraulic brake system. This failure in and of itself is the outcome of many other
contributors that caused the hydraulic brake system to fail. Further note that the improper operation of the
emergency brake system is also considered an initiating event.
Figure 7-3 indicates the sequences of events that could cause an accident due to an unsecured cabin door
and the aircraft captain suffers Hypoxia. Note that this event is not necessarily due to a particular failure.
FAA System Safety Handbook, Chapter 7: Integrated System Hazard Analysis
December 30, 2000
7 - 4
As previously indicated, there are many contributors: the aircraft is airborne without proper cabin pressure
indication, and the captain enters the unpressurized cabin without the proper personal protective equipment.
The initiators in this scenario involve the cabin door not being properly secured, inadequate preflight
checks, and less than adequate indication of cabin pressure loss in the cockpit. The outcome of this
accident is that the captain suffers Hypoxia. Note that if both crew members investigated the anomaly, it
would be possible that both pilots could have experienced Hypoxia and loss of aircraft could have
occurred.
The safeguards that would either eliminate the specific hazards or control the risk to an acceptable level
have also been indicated in the figures. Keep in mind that if a safeguard does not function, that in itself is a
hazard. In summary, it is not easy to identify the single hazard that is the most important within the
scenario sequence. As discussed, the initiating hazards, the contributory hazards, and the primary hazard
must all be considered in determining the risk. The analyst must understand the differences between
hazards, the potential for harm and their associated risks. As stated, a risk is comprised of the hazards
within the logical sequence. In some cases, analysts may interchange terminology and refer to a hazard as
a risk, or vice versa. Caution must be exercised in the use of these terms. When conducting risk
assessment, the analyst must consider all possible combinations of hazards that may constitute one
particular risk, which is the severity and likelihood of a potential accident.
FAA System Safety Handbook, Chapter 7: Integrated System Hazard Analysis
December 30, 2000
7 - 5
Figure 7-1: Engine Covers Scenario
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