Example 1 - Simple MTBF Determination
[Download RGA7 Example File (*.rga7)]
Background
A new helicopter system is under development. System failure data are collected on five helicopters during the final test phase. The total number of flight hours is 500, accrued over a period of 12 weeks. The 500 flight hours are partitioned into six intervals, each covering a two-week period. Every two weeks, each helicopter undergoes a thorough inspection to uncover any failures that may have occurred since the last inspection. Therefore, although the actual failure times are unknown, the analysts have the cumulative total number of flight hours and the cumulative total number of failures for the five helicopters for each two-week period.
| Interval | Failures in Interval |
Interval Length (flight hours) |
| 1 | 12 | 0 – 62 |
| 2 | 6 | 63 - 100 |
| 3 | 15 | 101 - 187 |
| 4 | 3 | 188 - 210 |
| 5 | 18 | 211 - 350 |
| 6 | 16 | 351 - 500 |
Analysis and Discussion
A Standard Folio Data Sheet using the Grouped Failure Times data type is created by selecting the Developmental and Grouped Failure Times options in the New Data Sheet Setup window Expert view, as shown next.
The data set is then entered and analyzed using the Crow-AMSAA (NHPP) model, as shown next.
The results show that the Demonstrated MTBF value at the end of the 500 hours of testing is 8.7702, as can be seen in the "DMTBF" line of the Results area.
This can also be seen by using the Quick Calculation Pad (QCP) to determine the Instantaneous MTBF at 500 hours, with no confidence bounds, as shown next.
Notice that the instantaneous MTBF of 8.7792 matches that shown in the Results area.
A plot of the MTBF vs. Time results is shown next.
The plot shows the mean time between failures plotted against time. The points represent actual failures in the data set.
Notice that both the instantaneous MTBF vs. Time and cumulative MTBF vs. Time display. To show the instantaneous MTBF line only, right-click the plot and choose Show/Hide Items. The Show/Hide Items window displays, as shown next.
Clear the Cumulative MTBF check box, then click OK. The plot now looks like this.
