What is PM?

PM is a general term that encompasses all forms of pro-active maintenance including:

1. scheduled overhaul
2. scheduled inspections (CBM)

CBM is the prominent form of maintenance today. Although all scheduled activities are called “PM” (for example, in the CMMS or EAM), they are predominantly CBM activities (inspections) performed, often, at the same time as scheduled service and minor adjustments. It is important to recognize that these inspection activities are indeed “CBM” in that they often provide recorded observations which may contain  predictive “intelligence”. (See Unconventional CBM Variables.)

Even a repair (misleadingly labelled “corrective maintenance” in the CMMS) work order, following a failure often turns out to be mostly preventive (PM). The failed part is indeed replaced but the opportunity is taken also to inspect (a kind of unscheduled CBM activity) other parts of the system/component. Some of them will be renewed preventively (suspensions) even though they may not yet have failed. These unplanned suspensions and failures, if recorded, constitute an opportunistic source of  valuable data for RA.

What is PM? It is simply “maintenance”. That is to say, all maintenance activities are preventive. The term “corrective maintenance” has been used in CMMS software to mean a “repair” following a failure. Rather than “corrective maintenance” it is better to call this activity simply “repair”. The reasoning is that maintenance in general (even preventive maintenance) applies “corrective” (i.e. restorative) action. Why is such pedantry necessary in our routine speech? We should use precise language in order to avoid biased (reliability) analysis. “Repair” should imply, unambiguously, that something has failed or potentially failed. When we use the word “Corrective” it could mean that an unfailed item was renewed or replaced. That item may have been far from an unfailed condition. Extraction of a sample of work orders based on the failure code “corrective” would therefore yield a mixture of failed and unfailed instances of the failure mode of concern. Analysis of that sample could generate an erroneous conclusion and decision policy^[1].  A more precise “analytic” word to use, for the renewal an unfailed item, is “suspended”.

1. ^[1]For the consequences on CBM performance of mislabeling suspensions as failures see https://www.livingreliability.com/en/posts/defeating-cbm/↩