MIL-HDBK-470A
The maximum time to repair is given by:
M
Max c t
(Equation 12)
t' + z(t',. a ) a t ,
=antiln
where z(t' | . a ) = the value from the normal distribution function corresponding to the
percentage point (1-a) on the maintainability function for which M M a x
is defined. Most
commonly used values of z(t' i_ a ) are shown in Table XIV.
TABLE XIV: Values of z(t'
) Most Commonly
l
Used in Maintainability Analysis.
(1-a)
z(f 1-a )
0.80
0.85
0.90
0.95
0.99
0.8416
1.036
1.282
1.645
2.326
4.4.3.2 Normal Distribution. In maintainability, the normal distribution applies to relatively
straightforward maintenance tasks and repair actions (e.g., simple removal and replacement tasks)
which consistently require a fixed amount of time to complete. Maintenance task times of this
nature are usually normally distributed, producing a probability density function given by:
M
§(t=
ct) =
1
S
M
exp
c t
-(
^
M
c
t l
-
M
2 S
( Mct)
ct^
(Equation 13)
2
where:
M c t = repair time for an individual maintenance action
Z(Mct.)
—
N
M ct
= average repair time for N observations
X(Mcti
Sm
ct
- Mct)'
N-1
= standard deviation of the distribution of
repair times, based on N observations
N
=
number of observations
4-55