What Is Fault Tree Analysis? (Examples, Uses & Formula)

Maintenance managers, manufacturing teams and other business leaders in industries where safety, reliability and risk management are critical to operations use fault tree analysis (FTA) to identify and understand the causes of a potential issue before equipment breakdowns occur. This top-down analysis method uses a graphical model to evaluate the possible causes of a potential failure mode within a complex system.

Bell Laboratories created FTA in the early 1960s to help the United States Air Force uncover potential flaws in its Minuteman missile system. It has since been used globally in industries from aerospace to nuclear power. Let’s break down how FTA works and how the methodology can be applied effectively to your business. 

FTA vs. FMEA 

Fault Tree Analysis (FTA) and Failure Mode and Effects Analysis (FMEA) are both used to identify risk assessment, but their similarity ends there. FTA is typically more suited to system-level analysis, while FMEA is the standard for component-level scrutiny. 

FTA takes a top-down approach to answering questions — it starts with a specific failure event and maps out its possible causes through branching pathways. This visual fault tree diagram gives teams a clear understanding of how various factors contribute to system failures.

FMEA comes to a conclusion on an undesired event using a tabular format. It’s a broader, bottom-up approach that helps teams investigate an asset and determine all the potential failure rates for a given asset or system as well as the effects they could have on an operation. 

When to Use Fault Tree Analysis 

While FTA can be used any time, there are some key points where implementing it can be more beneficial, such as: 

• When creating or installing a new system: Teams can catch potential issues well before they happen during the design or installation phase. Businesses can avoid risks early by doing a system analysis to see how each component might fail, preventing dangerous or costly problems in the long run.
• When improving an existing system: Even when a system is up and running, potential issues can be caught before they lead to catastrophic failure. The FTA diagram identifies existing vulnerabilities that may have been overlooked during the design and installation phase and is valuable when modifying or upgrading the existing system. It gives teams an in-depth understanding of the current failure probabilities, allowing for targeted improvements to a system’s safety and reliability without sacrificing operations.
• Understanding system safety/reliability: How safe and reliable an existing system is can be better understood through the use of FTA, as it shows how and where different failure points can occur. By doing a safety analysis to determine a system’s most vulnerable points, teams can better support these areas, reducing breakdowns and maintaining operational standards.
• Assessing regulatory compliance: Many industries have their own safety and reliability requirements. Teams use FTA to show that they understand failure modes and have adequate controls in place to mitigate risks.
• Optimizing maintenance costs: By identifying and prioritizing the most critical failure points and their most common causes, teams can focus their resources more efficiently, which leads to lower maintenance costs, reduced downtime and a longer lifespan for a system and its components. 

7-Step Fault Tree Analysis Process & Formula

Here are the key steps involved when utilizing FTA to identify and alleviate potential failures in a system: 

1. Determine the undesired issue/event: Before taking any other steps, the system failure or undesired event needs to be defined. This issue forms the basis of the fault tree and serves as the focus for identifying the problem.
2. Identify the event causes and contributing factors: After identifying the main issue, the team identifies all its potential causes and contributing factors. They could be mechanical, electrical, environmental or human error.
3. Create the fault tree using gate and event symbols: FTA uses an event tree analysis with the failure at the top and all the potential causes beneath it. These symbols are uniform across industries and illustrate how the interaction of multiple failures can cause the main undesirable event to occur. This visual representation makes it easier for teams to see how these factors can influence each other and where weaknesses may lie in a system.  
4. Gather the failure data: Teams collect data related to system failures to discern the likelihood and impact of modes where failure is possible in a system. This information can come from historical data, testing, or simulations.
5. Perform the analysis: The fault tree is analyzed to calculate the probability of the top-level event occurring. 
6. Interpret the results: After analysis, a team interprets the results to identify which failure modes show the greatest risk. 
7. Adjust for improvements and monitor: With this information, the next step involves making changes to reduce the likelihood of failure. 

Decoding Fault Tree Analysis Symbols 

FTA has universal symbols that are used in every industry to analyze system failures. There are two basic types of symbols — “events” and “gates.” Events represent faults or conditions leading to the system’s overall failure, while gate symbols represent the logical relationship between conditional events and other elements in the FTA. 

Some event symbols include:

• Top event (TE): The TE symbol is located at the top of the fault tree and represents the overall system failure, which is the reason for the analysis. As the starting point of the investigation, it is the undesired outcome that the analysis aims to understand and prevent.
• Intermediate events (IE): The IE symbol represents the preceding events that contributed to the failure. It can link to failures at different stages of the fault tree and may be caused by another event or cause failures further down the tree.
• Basic events (BE): The BE symbol shows the root causes directly contributing to the TE. They sit at the bottom of the fault tree and do not otherwise contribute to the overall failure. They represent the fundamental reasons for a system’s failure.

Some gate symbols include: 

• AND gate: This type of gate connects events that take place at the same time. 
• Priority AND gate: Similar to the AND gate, this gate occurs if all the input events happen in a specific order. It is used to model failure sequences where the order of events is important.
• OR gate: This type of gate may have one or more inputs, and an output event will occur if one or more of the input events happen. It is often used to show that multiple independent events can cause a failure.

Benefits of Fault Tree Analysis 

FTA benefits teams, systems and businesses in multiple ways, including: 

• Visualizing and narrowing down the causes of a potential system failure: When a system breaks down, it may be easier for teams to create a visual representation of the relationships between potential failure causes, particularly if the system and its components are extremely complex. Through a graphical interpretation, the problem’s root cause analysis can be narrowed down, allowing teams to focus on key problems and address them quickly. 
• Helping with calculations regarding the probability of a failure event occurring: A complex system has challenging calculations based on a variety of factors. FTA helps simplify probability calculations, giving teams greater insight into risk levels and prioritizing fixes.  
• Accounting for human error: By counting human error as a contributor to system failures, FTA calculations allow teams to create and implement guidance improvements for system operators. 
• Helping prioritize potential failures: FTA helps teams prioritize failure modes. Resources are then directed toward those with the largest risks, allowing for more effective and targeted preventive measures.

Disadvantages of Fault Tree Analysis 

While FTA is a powerful tool, there are limitations as to how effective it is. For example: 

• It relies heavily on the expertise of the analysts: FTA is only as good as the people implementing it. Its accuracy relies heavily on the knowledge of those conducting the quantitative analysis. Its accuracy and depth depends on their ability to identify potential failure modes and construct the fault tree properly. If the analysts lack knowledge of the system or its components, they may still miss critical issues, leading to incomplete or flawed results.
• It examines only one event at a time: Because FTA focuses on one top event or system failure at a time, it can be time-consuming in more complex systems with multiple failure modes and result in missing interactions between different failure events. 
• It depends on availability and quality of data: If detailed data is limited, incomplete, outdated or inaccurate, calculations can be inaccurate, reducing a team’s effectiveness when attempting to identify and mitigate risks.
• More complex systems require larger, more complex fault trees that can be time-consuming: The more complex the system, the larger the fault tree. Extensive fault trees require more time and effort to develop, analyze and maintain. As this is labor intensive, it can slow down actual risk identification and mitigation. 

Fault Tree Analysis Example 

Let’s look at an example of FTA as it applies to aviation. One of the worst failure events that could happen is having an engine fail during a flight, so this would be the specific event that an FTA is meant to address. The failure event’s possible immediate causes are broken down into three categories for teams to analyze — mechanical failure, fuel system failure and electrical failure — each of which has two main sub-events that could have led to the engine’s breakdown. The three categories could be analyzed as such: 

• Mechanical failure: This may have occurred due to blade fracture, which may be the result of material fatigue (i.e., wear over time) or foreign object damage (such as debris striking the engine). Bearing failure is also possible, as it can affect the engine’s ability to rotate properly, leading to a shutdown.
• Fuel system failure: For this to occur, fuel contamination as the result of improper maintenance (like failure to properly filter fuel) or storage issues (in which conditions lead to contamination) may be culprits. Fuel pump failure, which prevents the proper flow of fuel to the engine, may also be a possible cause of failure.
• Electrical failure: Of course, electrical failure may also have caused the engine to break down. Battery failure, through overcharging or age-related degradation, or wiring issues, which may cause interruptions in power, may also be sources of the issue. 

How a CMMS Can Help With FTA

Of course, system reliability is only as good as the work you put into managing a specific asset. You never want to allow potential failures to go undetected that leave your most critical assets non-operational, leading to a serious revenue loss. 

FTAs can provide a crucial reliability analysis to help you strategize about how to protect your assets and catch issues before they occur. A computerized maintenance management system (CMMS) like Coast helps maintenance teams not only manage their assets but also schedule preventive maintenance tasks and assign work orders with ease. A CMMS software also helps in collecting tangible data about failures across your business systems to ensure that the dependency on your equipment remains solid. 

See how Coast can help streamline your maintenance process by signing up for a free trial today!