Practical Reliability Engineering, 5th Edition

16

Maintainability, Maintenance and Availability

16.1 Introduction

Most engineered systems are maintained, that is they are repaired when they fail, and work is performed on them to keep them operating. The ease with which repairs and other maintenance work can be carried out determines a system's maintainability.

Maintained systems may be subject to corrective and preventive maintenance (CM and PM). Corrective maintenance includes all action to return a system from a failed to an operating or available state. The amount of corrective maintenance is therefore determined by reliability. Corrective maintenance action usually cannot be planned; we must repair failures when they occur, though sometimes repairs can be deferred.

Corrective maintenance can be quantified as the mean time to repair (MTTR). The time to repair, however, includes several activities, usually divided into three groups:

1. Preparation time: finding the person for the job, travel, obtaining tools and test equipment, and so on.
2. Active maintenance time: actually doing the job.
3. Delay time (logistics time): waiting for spares, and so on, once the job has been started.

Active maintenance time includes time for studying repair charts, and so on, before the actual repair is started, and time spent in verifying that the repair is satisfactory. It might also include time for post-repair documentation when this must be completed before the equipment can be made available, for example on aircraft. Corrective maintenance is ...