Corrective Maintenance Time (CMT)

Additional information regarding CMT is available from multiple sources:

• Per DoD Handbook(HDBK)-791, Military Handbook on Maintainability Design Techniques Metric, CMT begins with the observance of a malfunction of an item and ends when the item is restored to a satisfactory operating condition. CMT may be subdivided into active maintenance time and non-active maintenance time. It does not necessarily contribute to equipment or system downtime in cases of alternate modes of operation or redundancy.
• In accordance with Logistics Principles and Applications, Second Edition by John Langford, Mean CMT (MCMT) is the average time required to accomplish repair actions within the defined steps of CM which occurs at random intervals. CM relates to the repair and restoration of a system required as a result of a random, unplanned failure or a disabling malfunction of a system, and is synonymous with repair.
• According to Logistics Engineering and Management, Sixth Edition by Benjamin S. Blanchard, CM consists of unscheduled actions initiated as a result of a failure (or a perceived failure), that are necessary to restore a system to its required level of performance. These activities could include troubleshooting, disassembly, repair, removal and replacement, reassembly, alignment and adjustment, checkout, etc. CM also includes all software maintenance that is not initially planned, such as adaptive maintenance and perfective maintenance. 
  • With regard to MCMT, completion of these aforementioned steps for a given failure constitutes a CM cycle. The mean corrective maintenance time, or the mean time to repair (MTTR), is a composite value representing the arithmetic average of these individual maintenance cycle times.
• In the Supportability Engineering Handbook by James V. Jones, as the design of the system matures, more detailed information about maintenance requirements becomes available. The actual time to perform CM allows a more complete description of the CM cycle for each item. Eventually, the MTTR is replaced with the MCMT. The MCMT considers other factors such as access time and preparation for maintenance that may have been either estimated or not initially considered when the MTTR was developed. This leads to a more realistic portrayal of the actual requirement for maintenance when the system is delivered to the user. The various people who perform maintenance typically have varying levels of proficiency. The conditions under which maintenance can be performed may vary depending on location, weather, and other factors. In-service maintenance data collection over a period of time will reveal the number of times that a specific CM cycle occurred and the actual time required for each iteration cycle. These times can be averaged to derive the MCMT.