Likelihood is also divided into five levels, which run from, ‘Never heard of in ….. industry’ to ‘Happens several times per year in a location.’ The likelihood is estimated on the basis of historical evidence or experience. In other words: ‘Has the potential consequence actually resulted from a similar incident within the aviation industry, the company or at the location?’ Actual consequences have, by definition, occurred and hence fall on Likelihood C, D, or E on the risk matrix for the actual consequence level.
Note: this should not be confused with the likelihood that the hazard is released – we are concerned with the likelihood of the potential consequences resulting from the incident in question.
Example:
A helicopter rollover may be assessed as having a potential consequence of fatalities (severity level 4 or 5). The likelihood used for the risk assessment is that of a fatality resulting from the rollover, not the rollover itself. In other words, the key question is, ‘how often are people killed or major damages caused by helicopter rollovers?’ Not, ‘could people be killed in a helicopter rollover?’ Nor, ‘how often do helicopters roll over?’
There is no specific time limit for when accidents occurred. The guiding rule is to count all acci-dents that have occurred using current barriers or controls; i.e., current technology and processes. Accidents that occurred before a significant change in technology or process may be ignored, if the current technology and process would likely have prevented them.
Risk management
The industry consensus is that risks assessed as ‘High’ on this matrix demand actions to reduce the risk. Many companies use a structured approach to ensure that the actions taken to reduce risks to a tolerable level are indeed adequate. The use of a ‘bow-tie’ in such a structured approach is an industry best practice. The figure below gives a graphic depiction of the ‘bow-tie’ concept.
Bow ties should be developed for all High Risk hazards. In aviation, two generic hazardous events that should normally be addressed with a bow tie are release of an unairworthy aircraft and devia-tion from intended safe flight path. A bow tie should concisely document the barriers and controls in place to prevent the release of a hazard, and the recovery measures in place to minimize the con-sequences should the hazard be released. Cross references should be provided to link the reader to any other documents that define processes and procedures used to ensure the effectiveness of these barriers, controls, and recovery measures. The figure below suggests how many independent and effective measures should be in place for High, Medium, and Low Risk hazards.
Risk acceptance criteria
Control/Barrier High Risk (Intolerable) Medium Risk (Incorporate Risk Reduction Measures) Low Risk (Manage for Continuous Improvement)
Threat Minimum of 3 independent, effective controls/barriers to be in place for each threat Minimum of 2 independent effective controls/barriers for each threat Minimum of 1 effective control/ barrier for each threat
Consequence Recovery Minimum of 3 independent, effective recovery measures for each consequence Minimum of 2 independent, effective recovery measures for each consequence Minimum of 1 effective recovery measure for each consequence
Escalation Minimum of 2 independent effective controls/barriers for each escalation factor Minimum of 1 effective control/ barrier for each escalation factor Minimum of 1 effective control/ barrier for each escalation factor
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