When latent failures are possible, exposure times can be greater than flight duration. Similarly, when an item is used only in certain phases of a flight, exposure time can be less than the flight duration. In SAE ARP4761, Appendix L indicates that engineers might want to analyze each flight by explicitly specifying exposure times in some cases. This type of analysis is generally used to calculate worst case flight probability. To support this, you enter a value for Exposure Time on the Calculation Data page in the event’s Properties window. If you do not enter an exposure time, the default value used in the calculation is the value you enter for Flight time. This property is on the FTA > General page in the Calculate window.

As discussed earlier, the exposure time for an item can depend on several factors, which can vary for each flight. In the Properties window for an event, you can specify the factors that affect exposure time on the Latency page. For more information, see Latency Page (SAE Events Only).

The properties on the Latency page are available only when Failure rate/MTBF is selected as the model on the Calculation Data page. Therefore, to benefit from these features, it is recommended that you use this model whenever applicable.

If you do not supply latency data for an event, it is assumed that the item is inspected and repaired at the beginning of each flight. Therefore, the module also assumes that the item is operational and that there are no additional factors that determine latent failures for the item. These assumptions are particularly useful when exposure times are explicitly specified for worst case analysis. When explicit modeling is used for the worst case analysis, mission probability calculations are not applicable unless mission duration is equivalent to one flight hour.

To specify factors that affect the exposure time, you select the Latent failures checkbox. The Check Interval value specifies the number of flight hours between the maintenance checks. If the check interval is equal to the flight duration (or less than the fight duration), then there are no latent failures. Generally, the Check Interval value is determined by multiplying the flight duration by the number of flights between check intervals.

If the ratio of Check Interval to Flight time is not an integer, then the Check Interval value is adjusted to match the next integer value. For example, assume that Check Interval is 123 hours and that Flight time is 5 hours. The check interval is then adjusted to 125 hours so that maintenances are performed for every 25 flights.

The exposure time corresponding to the event that the item is failed before a specific flight increases with the flight number until the check interval. For the previous example, the exposure time increases up to 25 flights, and then it resets to zero.