a constant probability of failure at all ages. Pattern F starts
with high infant mortality and drops eventually to a constant or very slowly increasing failure probability.
Studies on civil aircraft showed that 4% of the items
conform to pattern A, 2% to B, 5% to C, 7% to D, 14% to
E and no fewer than 68% to pattern F. (The distribution of
these patterns in aircraft is not necessarily the same as in
industry, but as equipment grows more complex, more and
more items conform to patterns E and F.)
These findings contradict the belief that there is always a
connection between reliability and operating age – the belief
which led to the idea that the more often an item is over-
hauled, the less likely it is to fail. In practice, this is hardly
ever true. Unless there is a dominant age-related failure
mode, fixed interval overhauls or replacements do little or
nothing to improve the reliability of complex items.
Most maintenance professionals are aware of these findings, and coming to terms with the reality of randomness
after decades in the bathtub. However, the fact that the bathtub curve still features in so many texts on maintenance is
a constant probability of failure at all ages. Pattern F starts
with high infant mortality and drops eventually to a constant or very slowly increasing failure probability.
Studies on civil aircraft showed that 4% of the items
conform to pattern A, 2% to B, 5% to C, 7% to D, 14% to
E and no fewer than 68% to pattern F. (The distribution of
these patterns in aircraft is not necessarily the same as in
industry, but as equipment grows more complex, more and
more items conform to patterns E and F.)
These findings contradict the belief that there is always a
connection between reliability and operating age – the belief
which led to the idea that the more often an item is over-
hauled, the less likely it is to fail. In practice, this is hardly
ever true. Unless there is a dominant age-related failure
mode, fixed interval overhauls or replacements do little or
nothing to improve the reliability of complex items.
Most maintenance professionals are aware of these findings, and coming to terms with the reality of randomness
after decades in the bathtub. However, the fact that the bathtub curve still features in so many texts on maintenance is
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