Abstract The steady state unavailability of the k- out-of- n system is evaluated from special Markov models assuming constant transition rates, total repair from every system state with failed components and a common transition into the totally failed state due to common mode failures. The repair time distribution is modelled individually for each system state with failed components using an Erlang distribution of appropriate order. The general formulae of the solution are derived. An exact solution is also given for the limiting case of fixed repair times. Simple approximate formulae are given for application to important special cases.