## 3.6 DIAAT Phase 4: Assess Risk 

A 

## 3.6.1 Overview 

In this phase, identify each hazard’s associated initial risk and plot each hazard on a risk matrix. 

ASSESS RISK 

 Assign risk level for each hazard based on severity and likelihood 

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. 

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. 

## 3.6.2 Risk Levels and Definitions 

Record all hazards and their associated risk levels. Hazards are assigned one of three risk levels: 

## 3.6.2.1 High Risk 

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. 

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. 

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. 

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. 

