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b)  The next step is to develop failure rate and survival curves versus time since overhaul (TSO). A failure rate curve shows the failure rate per 1 000 hours for each component in each 100-hour time bracket. A survival curve shows the number of units remaining at any given TSO. The shape of the survival and failure rate curves are valuable when determining the deterioration of reliability. The operating time which can be realized between consecutive overhauls is determined by the area which is under the survival curve and is bounded by the horizontal and vertical axes.
c)  Additional information is available from these data by developing a probability curve. This curve will show the probability of a component reaching a given TSO and the number of components expected to fail in a given time bracket. The number of components that would probably fail in a given time bracket is obtained by taking the difference of the ordinates at the beginning and end of a given time bracket. This would also be a reflection of the slope of the survival curve at that point. The percentage of components which survive to a given TSO is also the probability of a single component operating to that time without failing.
d)  A still better evaluation is possible by developing a conditional probability curve. This curve will show the probability of failure of a component within a given time interval. Data for a conditional probability is obtained by dividing the number (or percentage) of components entering an interval by the number (or percentage) of components removed during an interval. It is considered that this curve best depicts the relationship between reliability and overhaul time.
1.8.12.8  Some advantages of this type of analysis are as follows:
a)  a determination can be made as to whether failures are being prevented by the TSO specification;
b)  an indication is given statistically concerning the current TSO limit and whether or not it has reached an optimum point;
c)  an indication is provided as to what might occur to the overall premature removal rate if the TSO limit were changed;
d)  an indication will be provided of any unusual high rate of premature removals/failures that have occurred immediately after a check and repair or overhaul;
e)  in some cases, an indication may be given that scheduled interim maintenance would result in an improvement of the overall premature rate;
f)  other useful conclusions can be made concerning the relationship of the failure to the time in service, time intervals, engineering change accomplishment, etc; and
g)  this technique of in-service component reliability analysis readily lends itself to computer programming.
These advantages emphasize the value of such an analysis in determining a maintenance programme that is best for the component involved.
1.8.13  Control for adjusting time limitations
1.8.13.1  When considering the merits of a time extension, there are many different methods which may be used. The programme should identify these methods and the group responsible for the preparation of a substantiation report to justify the requested time extension. The programme should show that such action is approved by at least two separate organizational segments of the operator, one of which exercises inspection or quality control responsibility for the operator. The programme should also identify the other organizational segment responsible for the performance of the function. When evaluating a particular programme, consideration should be given to the following:
a)  Are the specific parameters used to determine time extensions spelled out (i.e. sampling, functional checks, unscheduled removal, etc.)?
b)  If sampling is used, does it explain the method, number of samples required, when they will be taken, and at what time interval?  Time on units or exhibits used as samples should be specified.
　
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