DETERMINING MEAN TIME BETWEEN FAILURES. Twenty air-conditioning systems designed for use by astronauts in Russia’s Soyuz spacecraft were operated for 1,000 hours at a Russian test facility. Two of the systems failed during the test—one after 200 hours and the other after 600 hours.
APPROACH \blacktriangleright To determine the percent of failures [FR(%)], the number of failures per unit of time [FR(N)], and the mean time between failures (MTBF), we use Equations (17-2), (17-3), and (17-4), respectively.
FR(%) = \frac{Number of failures}{Number of units tested} \times 100\% (17-2)
FR(N) = \frac{Number of failures}{Number of unit-hours of operating time} (17-3)
MTBF = \frac{1}{FR(N)} (17-4)