The Fault Tree Analysis (FTA) is a bottom-up deductive analysis technique. The FTA and DFMEA complement each other in that they approach the problem from opposite directions. The fault tree analysis starts with the development of the top-level events. The selection of the top-level events for doing the system level (aka vehicle level) FTA is best considered from the safety goals, operational goals, and system requirements. These simple benign statements when inverted form a good basis for the top-level events.
For example, Pi has developed systems with a safety goal of avoiding unintended acceleration. The top-level event would be the inverse of this, stating that “unintended acceleration occurs”. The ensuing FTA will consider all the elements that need to fail in isolation or in concert with each other to achieve this top-level event.
The primary value in a qualitative FTA at this stage is to identify the minimum cut sets in the FTA. The minimum cut set order gives the engineer an insight into the level of redundancy in the system. Higher orders are preferred over first order. A first order minimum cut set is stating that a single point failure can occur in the system to reach the top-level event. In many cases this is an unacceptable design and changes must be made to either detect or inhibit this top-level event from occurring.