"So what real failure rate was the unexpected archilies heal in this
accident ? and what action should be taken to address it?"
lomapaseo, For argument's sake I'll offer a value of 0.3524163824. And I'm
not sure that the recommendations in the NTSB draft report go far enough in
addressing it.
There were two independent endplay measurement checks (each of which produced
a reported measurement based on several contributing measurements). The two
values were 0.040 and 0.033. If Ybar denotes the average of two independent
measurements of an endplay whose true but unknown value is mu, if S denotes
the sample standard deviation, then (Ybar - mu)/S/sqrt(2) is a Student-t
distribution with one degree of freedom. In this case Ybar = 0.0365, the
sample standard deviation equals 0.004949747468, and based on the two
measurements the probability that mu was greater than or equal to Douglas's
specification of 0.040 for the maximum limit was 0.3524163824. That's not a
failure rate value, just an indication that on the evidence the a/c should
not have been in the air.
Among the
16 NTSB recommendations - is that a record for one report?
- there is little mention of measurement error or procedures for dealing
with it. Recommendation 8 states "establish an end play check interval
that (1) accounts for the possibility of higher-than-expected wear rates
and measurement error in estimating acme nut thread wear."
Why limit the recommendation concerning measurement error to nut thread wear?
Why not specifically raise the issue of policy concerning measurements that
do not agree? The recommendations stress the need for recording wear
(recommendation 9) but such measurements may be of dubious value without other
actions. According to the ALPA submission that Belgique links to, the NTSB Systems
Group determined that "the worst example using an Alaska clone tool was a measured
endplay of .012 inches when the actual endplay was .023 inches." Given that
kind of inaccuracy an (acceptable) reading of 0.033 could easily have come from
actual endplay that exceeded not only Douglas's published maximum limit
specification but also the true maximum limit.