MTTR is a basic measure of maintainability. It is a measure of the mean time to perform a corrective maintenance action. All maintenance actions in the maintenance logs are corrective. This is an assumption of any FRACAS and is not particular to Windchill FRACAS.

The equation for calculating MTTR is:

For MTTR calculations, the repair time can be entered in the maintenance log in hours or minutes (default). The unit to use is specified by General > Repair Time units in the Project properties. For more information, see General Project Properties.

MTTR is typically calculated for a specific maintenance level. In the Master List Library file, the Maintenance Levels list has the three predefined choices: Depot, Intermediate, and Organizational.

To use maintenance levels to filter maintenance logs, a maintenance level must be assigned in the maintenance log for each repair. If a repair does not have a maintenance level assigned, it is not included in any MTTR calculations that are filtered by maintenance levels.

If maintenance levels are not being used in a particular System file, then the MTTR calculation does not need to be filtered by maintenance levels. On the FRACAS Calculations page in the Calculate window, you select --No Filter-- for the MTTR calculation so that all maintenance logs are used. On the MTTR Calculations page, you can also apply different filters to the MTTR calculation to compute as many as 10 different variations of MTTR. For more information, see FRACAS Calculations Page.

The value for Total Maintenance Time is used for calculating MTTR. The equation is:

Although other times such as start dates and end dates can be recorded, they are not used for MTTR calculations. However, these dates can be used to generate graphs that show the number of repairs completed per some selected time period. For more information, see FRACAS Graph Wizard How-Tos.