SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Laurent Purrey
Date posted: Fri Nov 26 1:14:45 US/Eastern 1999
Who can tell me how we can calculate the MTBMA (Mean Time Between Action)for a system. What is its value when the components or system is "ON CONDITION".
NOTE: as assumption, MTBF, MTBUR are available.
Reply Posted by: Bruce Dudley
Organization: Reliability Analysis Center
Date Posted: Mon Nov 29 12:00:17 US/Eastern 1999
The term MTBMA stands for mean-time-between-maintenance actions. The general algorithm for this expression is:
MTBMA = Total Time / Total # of R&Rs + Fixes + CNDs
Where: total time is calendar time or active time or up-time as define in MIL-HDBK-338 Electronic Reliability Design Handbook,
R&Rs are items that have been removed and replaced with a different item to correct a malfunction (failure) or due to "time change requirements" (preventive maintenance),
Fixes are on-equipment repairs that do not require R&R of a repairable item, they include adjustment as well as corrective actions,
CNDs are reported malfunctions that cannot be duplicated via on-equipment testing.
This MTBMA calculation includes the on-conditioning monitoring that requires replacement of items before failure and / or adjustments. Replacement falls under the R&R term and adjustments fall under the Fix term.
The MTBF term that you have could help in identifying the time factor and total number of failures, however you still need to add the number of on-conditioning replacements and the number of preventive maintenance replacement and number of adjustments to the failure list. The term MTBUR (mean-time-between-unscheduled -removals) only identifies the R&R term for malfunctions as it excludes time change replacements by definition.