6.3.3 ARINC Apportionment
Technique (Ref.
[6])
This method assumes series subsystems with constant failure
rates, such that any subsystem failure causes system failure and that
subsystem mission time is equal to system mission time. This apportionment
technique requires expression of reliability requirements in terms of failure
rate.
The following steps apply:
(1) The objective is to
choose _{} such that:
where:


is the failure rate allocated to subsystem “i” 




l* 
is the maximum allowable failure
rate 
(2) Determine the subsystem
failure rates (li) from past observation or
estimation
(3) Assign a weighting
factor (wi) to each subsystem according to the failure rates
determined in (2) above
(4) Allocate subsystem
failure rate requirements
Example 6:
To illustrate this method, consider a system composed of three
subsystems with predicted failure rates of l_{1}
= 0.003, l_{2}
= 0.001, and l_{3}
= 0.004 failures per hour, respectively.
The system has a mission time of 20 hours and 0.90 reliability is required.
Find the subsystem requirements.
The apportioned failure rates and
reliability goals are found as follows:
Solving (1) for l* gives
l* = 0.005 failures per hour
(5) The corresponding allocated
subsystem reliability requirements are