Availability modeling for the Federal Aviation Administration

Mathematical models developed to meet the FAA special availability requirements are described. These models must account for frequency of failure, sharing of backup resources, service restoration time, and repair time. An application of one of the models to a particular FAA service is provided, and these results are compared to the results obtained from applying two typical classical models. The comparison shows two things. First, the three models definitely do not agree, even within some tolerance for error. This alone does not indicate a `correct´ model, it merely underscores such important differences between the models that one or another must be preferred and the others abandoned. Second, there is a need for increased fidelity as the computed unavailability approached zero, due to the heightened sensitivity to small periods of outage. Finally, in view of the fact that the greatest source of FAA service failure is due to telephone-line problems over which the FAA has little control, the application of the model shows that supplying critical services with functional backup and fast automated switching is the most realistic method to reach the unavailability goal of 10^-5