## Utility optimal decision making when responding to No Fault Found events

No Fault Founds (NFFs) are an expensive problem faced by the airline industry. The underlying cause of NFFs are a major focus of research work in the field, but the dearth of consistent data is a roadblock faced by many decision makers. An important risk factor identified is the occurrence rate of NFFs.

This research work aims to help decision makers in the Airline Maintenance, Repair and Overhaul teams, when faced with recurring NFFs, to make a choice based on value derived from the system and risk preference of the decision maker under uncertainty. The value of the aircraft fleet is laid out using Net Present Value at every decision point along the system life cycle while accounting for the uncertainty in the failure rate information. Two extreme decisions are considered for the decision maker to choose between: rebooting the system every time a failure occurs and results in an NFF which allows for it to recur while reducing uncertainty of the failure rate; or eliminating the failure mode which assumes that the failure does not recur and therefore completely removes the uncertainty. Both decisions have their associated uncertain costs that affect the NPV calculated. We use a Monte Carlo approach to estimate the expected profit from deciding to eliminate the failure mode. We make use of Expected Utility Theory to account for the risk preference of a decision maker under uncertainty and build an Expected Utility Maximizing decision framework.

To conclude we give some guidance to interpret the results and understand what factors influence the optimal decision. We conclude that not accounting for uncertainty in estimating a failure rate for the future along with uncertainty in NFF costs can lead to an undesirable decision. If the decision maker waits too long to gather more information and reduce uncertainty, then rebooting the system for the remaining life could be more worthwhile than spending the large amount of money to Eliminate a failure mode. Finally, we conclude that, despite uncertainties in information of occurrence rates and costs of NFFs, an Expected Utility maximizing decision between the two options considered – Reboot and Eliminate – is possible given the available information.