ABSTRACT
For systems with reparable failed components, the definition o f the overall system reliability includes, besides the mean time between failures (MTBF) , also the time required for components repair, i.e. the mean-time to repair ( M T T R ) . The components repair effectively prolongs the total system life, i.e. it makes the sys tem longer available. Thus, for systems wi th repair the system availability is de fined as:
A = MTBF MTBF+MTTR
It is a fundamental indicating factor integrating the system reliability and system maintainability, showing the portion o f time, wi thin the total time o f sys tem use, during which the system was available for execution o f operations it was designed for. Computer-based systems, used for direct digital control, have to guarantee at least 99.95 percent o f availability over the full year o f their noninterrupted use. This figure is achieved through the selection o f high-quality com ponents, use o f expedient maintenance and repair services, and, where applicable, duplication o f facilities.
