The Safety Analysis and Risk Mitigation Process  Section 3  5 1  3.6_SMSM_201607  Originally published July 2016  Uncontrolled copy when downloaded A  common cause failure  is a single fault resulting in the corresponding failure of multiple  components. An example of a common cause failure is found in a system with redundant  computers running on the same software, which is susceptible to the same software bugs.  When a system has a  single point failure , there is a failure of one independent element of the  system that causes or could cause the whole system to fail. The system does not have a  back-up, redundancy, or alternative procedure to compensate for the failed component. An  example of a single point failure is found in a system with redundant hardware, in which both  pieces of hardware rely on the same battery for power. In this case, if the battery fails, the  entire system will fail. Hazards with catastrophic effects that are caused by single point events or failures, common  cause events or failures, or undetectable latent events in combination with single point or  common cause events are considered high risk, even if the possibility of occurrence is  extremely improbable. This is unacceptable risk, and the NAS change cannot be implemented unless the hazard’s  associated risk is mitigated to medium or low. Existing high-risk hazards also must be reduced  to medium- or low-risk hazards. The predicted residual risk must be monitored and tracked in  relation to the safety performance targets. The predicted residual risk must be confirmed with  objective evidence suggesting an impact to the hazard’s causes or effects.  When assessing National Airspace System (NAS) equipment or existing hazards, the initial risk  may be equated to the  current risk , which is defined as the assessed severity and frequency of  a hazard’s effects in the present state.  Record all hazards and their associated risk levels. Hazards are assigned one of three risk  levels:  When assessing and mitigating safety risk, first determine the risk level prior to the  implementation of any safety requirements (see  Section 3.7.3). Initial risk  describes the  composite of the severity and likelihood of a hazard, considering only controls and documented  assumptions for a given system state. It describes the risk before any of the safety  requirements are implemented.  In this phase, identify each hazard’s associated initial risk and plot each hazard on a risk matrix.  3.6.1  Overview  3.6.2  Risk Levels and Definitions  3.6.2.1  High Risk  3.6  DIAAT Phase 4: Assess Risk   Assign risk level for each hazard based on severity and likelihood ASSESS  RISK A