MTTR stands for Mean Time To Restoration , where restoration is achieved by either Repair or Replace.
An additional standard term is:
MRT: Mean Repair Time (not including undetected fault time and delays)
MDT: Mean Down Time – often MDT and MTTR are identical
Why calculate MTTR?
MTTR is a basic part of system availability and maintainability analysis.
System availability is affected by the downtime that each failure incurs.
Thus, a system that is harder to restore (due to difficulty in detection and isolation, hard to reach items, and/or delicate alignment) will incur higher downtime and reduce availability.
In addition to availability, MTTR also affects the expected maintenance man-hours and expected annual maintenance cost.
How to calculate MTTR?
If field data is available, MTTR is calculated as:
MTTR=Sum over fault times / total number of faults
If failure data is not available, MTTR can be predicted by assessing and summing the following time intervals :
- Detection, Isolation and malfunction verification
- Preparation (e.g. warm up, power shut down)
- Replace / Repair time
- Final Testing
Some standard task times for item replacement can be found in MIL HDBK 470A  and MIL HDBK 472 .
Example for “Nuts or Bolts”: remove time is 0.34 minutes, and replace time: 0.44 minutes.
 IEC Electropedia (http://www.electropedia.org/), IEC 60050
 MIL HDBK 472
 MIL HDBK 470A
 EN 61703 and EN 50126