

MILHDBK470A: Designing and Developing Maintainable Products and Systems 
 

B.3.2  Choosing a Demonstration Test Method
Table BI provides a summary of the test methods presented in this appendix. For each test method, the table also lists the maintainability index for which the plan is designed to demonstrate, a summary of assumptions, the required number of samples, the method by which samples are selected, and the demonstration specification parameters. Definitions of individual terms found in the "Specification Requirement" column of B1 can be found in the subsection where the method is presented.
In reviewing Table BI, it is evident that a number of factors influence the choice of a maintainability demonstration test method. These include the index to be demonstrated, any assumptions about the statistical nature of the index as related to the test method requirements, the means by which sample maintenance tasks are selected, the number of maintenance tasks that must be demonstrated to obtain a statistically significant number of data samples, and the individual producer's and consumer's risk for some of the tests. Guidance and discussions of some of these factors are provided in the sections that follow, to aid the user in making informed decisions when specifying and executing a maintainability demonstration.
Test Method
Matrix
Test Plan 
Test Index 
Assumptions 
Sample Size 
Sample Selection 
Specification Requirements 
1A 
Mean 
Lognormal dist. and prior knowledge of
variance 
See test method 
Natural occurring
failures or stratification^{1} 
H_{0},
H_{1}, , 
1B 
Mean 
No dist. assumption,
prior knowledge of variance 
See test method 
Natural occurring
failures or stratification 
H_{0},
H_{1}, , 
2 
Critical
Percentile 
Lognormal dist., prior
knowledge of variance 
See test method 
Natural occurring
failures or stratification 
H_{0},
H_{1}, , 
3 
Critical Main. Time or
Manhours 
None 
See test method 
Natural occurring
failures or stratification 
H_{0},
H_{1}, , 
4 
Median 
A specific Variance
Lognormal 
20 
Natural occurring
failures or stratification 
ERT 
5^{2} 
Chargeable maint.
Downtime/Flight 
None 
See test method 
Natural occurring
failures 
ORR or A, NCMDT/NOF
DDT/NOF, ,

6^{3} 
Manhour Rate 
None 
See test method 
Natural occurring
failures 
Manhour Rate MR 
7^{4} 
Manhour Rate 
None 
See test method 
Natural occurring
failures or statification 
µ_{R }, 
8 
Mean and
Percentile 
Lognormal 
See test
method 
Natural
occurring failures or Simple random sampling 
Mean,
M_{Max} 
Dual Percentile 
None 
Dual
Percentile 
9 
Mean (Corrective Task Time,
Prev. Maint. Time, Downtime) 
None 
30
minimum 
Natural occurring
failures or stratification 
µ_{c }, µ_{pm
}, µ_{p/c }, M_{Maxc} 
M_{Max} (90 or 95
percentile) 
10 
Median (Corrective task
time, Prev. maint. time), M_{Maxct} (95 percentile),
M_{Maxpm} (95 percentile) 
None 
50 minimum 
Natural occurring
failures or stratification 
_{ct} , _{pm} M_{Maxct} ,
M_{Maxpm} 
11 
Mean (preventive maint. task time) M_{Max}
(preventive maint. task time at any percentile 
None 
All possible
tasks 
All 
µ_{pm}M_{Maxpm} 
Notes for Table BI:
(1) See section 3.3 for stratification method for sample selection (2)
Test method 5 is an indirect method for demonstrating operational ready rate
(ORR) or Availability (A). (3) Test method 6 is intended for use with
aeronautical systems and subsystems. (4) Test method 7 is intended for
use with ground electronic systems where it may be necessary to simulate
faults.




 
 