Introduction to Bowtie Analysis EASA & FAA
Introduction by Sofema Online
Introduction to Bowtie Risk Assessment Methodology
Bowtie is a risk evaluation method that can be used to analyze and demonstrate causal relationships in high-risk scenarios, in addition to enable an understanding of the control scenarios by identifying control measures.
The assessment of the bowtie elements facilitates the identification of the safety and risk priorities. Providing a visual interpretation of how improvements can be enabled and understood throughout the aviation system.
Bowtie is a visual tool that effectively depicts risk providing an opportunity to identify and assess the key safety barriers either in place or lacking between a safety event and an unsafe outcome.
Note - Bowtie models are a key component of Performance-Based Regulation (PBR)
Considerations related to the use of Bowtie
The definitions below originate from the United Kingdom Civil Aviation Authority (UK CAA), the European Aviation Safety Agency (EASA), and the Federal Aviation Administration (FAA).
This coincides with the terminology used by Shell, where the method was first applied.
Hazard: A hazard is defined as condition, object, or activity that can potentially cause harm or damage, including injuries to personnel, damage to equipment, properties, or environment, or reduced ability to perform an action as intended.
Top Event: The top event is the moment when the controlled state of a hazard is lost. It is yet to cause any damage or negative impact, but can lead to undesired outcomes if all prevention barriers fail. The term “top event” is derived from the fault tree analysis, where the critical event is on the top.
Threats: Threats describe initiating events that can potentially cause, through several pathways, the occurrence of the identified top event if all safety barriers fail. There can be one or multiple threats leading to the top event.
Consequences: Consequences are potential outcomes or a chain of outcomes resulting from the release of the top event, directly resulting in loss of control or damage if all mitigation barriers fail.
Barriers: Barriers, which are also called controls or layers of protection, are measures that prevent or mitigate undesired outcomes or reduce the likelihood of their occurrence, as well as maintaining the desired state.
Derived from the fault tree and event tree analysis, the barriers can be categorized based on their location in the Bowtie diagram and their function.
Prevention barriers are located on the left side of the Bowtie diagram, between the threat and top event. They eliminate the threat entirely or prevent the top event from occurring and, consequently, they prevent the hazard from being released.
When the top event is reached, mitigation barriers become effective and reduce the likelihood of the consequences to occur or limit the severity of the undesired consequences.
These barriers are located between the top event and the consequences.
Escalation Factors and Escalation Factor Barriers: Safety barriers are not 100% effective, as well as having inherent and temporary weaknesses.
In Bowtie, the conditions influencing the effectiveness are called:
a) escalation factors
b) degradation factors
c) or barrier decay mechanisms
These factors are depicted using branches from the main path barrier. Once the escalation factors are determined, the next step is to identify barriers that are in place to manage the escalation factors. Those barriers are called escalation factor barriers or degradation factor safeguards
In the Bowtie diagram, those barriers are placed between the escalation factor and the affected barrier on the main threat path.
Sofema Aviation Services and Sofema Online provide EASA Compliant Regulatory & Vocational Training including Quality & Safety Assurance and Root Cause Analysis Techniques.
Please see the websites or email: office@sassofia.com or online@sassofia.com
