AVAILABILITY:

A measure of the degree to which an item is in an operable and committable state at the start of a mission when the mission is called for at an unknown (random) time.

MTTF = ΣTTF/n (Mean time to fail)

MTTR = ΣTTR/n (Mean time to repair)

MTBF = MTTF + MTTR (Mean time between failures)

Av = MTTF/(MTTF + MTTR) = MTTF/MTBF