9.1 Introduction

Availability is a measure of system performance and measures the combined effect of reliability, maintenance and logistic support on the operational effectiveness of the system. A system which is in a state of failure is not beneficial to its user; in fact, it is probably costing the user money. If an aircraft breaks down, it cannot be used until it has been declared airworthy. This is likely to cause inconvenience to the customers who may then decide to switch to an alternative airline in future. It may disrupt the timetables and cause problems for several days. The availability can be easily converted in to a Sigma level quality. Once again, we define the defects as the additional number of systems that are not available due to not meeting the target availability. For example, if the target availability for a system is 0.95 and the observed availability is 0.90, then one at the fleet level can expect 5% additional systems unavailable due to not meeting the target availability. That is, out of 100 units, 5 additional units will not be available. The DPU (defect per unit) equivalent of this is 0.05. The corresponding Sigma level quality is 3.14.

Availability is probably the most important measure of system performance for many users. For example, most large airlines have a very high utilization rate with the only down time being to do...