Abstract: In order to utilize the resources available at present with maximum efficiency and enhance the reliability of a redundant flight control system to the utmost extent,it is of great value to study the change or the maintenance of a single stoppage unit in advance of a stoppage for a definite time t0 This paper investigates how the distribution of stoppageless work time of a single stoppage unit follows K（K=2）order Erlang distribution（in many practical problems Erlang distribution is of extensive adaptability）and then proposed an equation for the optimum advanced change time t0*,which minimizes both the continuous expection cost and the average expec-tion cost per unit time in a long period.This equation is simpler than the available ones and easy to make numerical calculation.By the aid of Erlang distribution table and exponential function table it is possible to make an approximate numerical calculation even by hand on the basis of the results in this paper.Finally,a numerical example is given for K=2.