Test duration in choice of helicopter maintenance policy

In the course of a review of the maintenance policy for a certain type of helicopter, it was found necessary to determine whether the frequency of one of the scheduled tests could be reduced without a shorter mean time between failures (MTBF). A sequential-testing procedure was worked out for an experiment in which hypotheses on the ratio of the respective MTBFs under the new and present maintenance policies are to be verified. In it, two groups of helicopters, subjected to the respective maintenance systems, are observed (Manipulation of two groups in parallel neutralizes the effect of uncontrolled factors, such as variation of the helicoptersʹ operational load and the maintenance-quality level). It is assumed that the failure rates are constant. A decision-making algorithm, and a matlab program, were constructed. The program yielded the decision parameters, densities and mean test duration up to the positive/negative decision in dependence on the actual MTBF ratio, the permissible level of the latter, and the given error probability levels. Prediction of the test duration, together with the operating characteristic, permits more rational planning of the experiment, hence of the maintenance policy.