Availability in the military parlance comes in various flavors. The intrinsic availability Asub0 is the MTBF/(MTBF+MTTR), which is the up time divided by the sum of the up time and the repair time. This number assumes that the repair parts are immediately available.
A more realistic, but extremely more difficult availability includes the MLDT, mean logistical delay time, in the denominator. MLDT takes into account the availability of repair parts and the possibility of needing to send for repair parts from the logisitics chain or depot. A typically used MLDT is 72 hrs, which is pretty absurd, but we use it. A correctly calculated MLDT should have individual MLDT's for each replaceable component weighted by its failure rate.
TTFN