TABLE OF CONTENTS Predicted Reliability is the chance that a system or product will perform satisfactorily for the user or customer within a given time frame and when used under specific operating conditions.
The bathtub curve shown in Figure 21.1 graphically illustrates the reliability life cycle of a typical system or component. It consists of three phases: Infant mortality that accounts for early failures, a (theoretical) period of constant failure rate when inherent failures occur, and the wearout phase at the end of an item's useful life. However, not every system or component will exhibit the characteristics of this perfect model. The additional reliability curves also shown in Figure 21.1 were produced by a sample of aircraft avionics systems.
When more than one system is involved, the following formulas may predict the total reliability. (Figure 21.2 graphically illustrates the formula below.) This formula assumes the reliability function is exponential and the failure rate is constant.
Note: e is the natural logarithm base (e = 2.71828)
Example:
| Sample Data | |
|---|---|
| Mean Time Between Failures | = 2,500 Hours |
| System Operating Time | = 50 Hours |
If a system operates for 50 hours and has an MTBF of 2,500 hours, the probability that it will operate for the entire 50 hours is:
The probability that two identical systems will last for the same amount of time is:
The following formula will calculate MTBF or MTTF from the reliability value:
Note:
