Page 49 - RCM - A practical Guide_V1

RCM - A Practical Guide

A combination of tasks consists of two separate tasks from different task categories, the most likely
potentially being a hard-time task supported by an on-condition task at some fraction of the hard time
interval. This approach is often used to substantiate an initial hard-time life calculation. It must be
stressed that each task must fulfil its own applicability and effectiveness criteria and be able to control
the aspect of the failure mode required. This approach must not form part of a default strategy and
must be used with care so as not to unnecessarily increase maintenance, cost and complexity.
In some RCM standards combinations of tasks are not mentioned and it is suggested that in such cases
there are in fact two failure modes being considered. In these instances a revisit of the FMEA may be
required to ensure that all failures have been considered in order to establish separate failure modes
for each element.

Lubrication and/Servicing tasks
As with the combination of tasks some RCM standards highlight a requirement for lubrication and
servicing tasks but this is by no means universal across all standards.
Although many items are lubricated on assembly and the lubricant is designed to last the ‘life’ of the
component, some items operate in such a way as to require periodic purging and replenishment of oils
and greases. As such, a task to replace aging or consumed lubricant may be a required activity, if the
inherent reliability of the component is to be realised. This is often the first consideration during task
selection.
Similarly, other ‘consumables’ may require replenishment, in order for the system to function
optimally, for example:
Fuel systems.
Nitrogen in back-up accumulators.
Fluid levels in hydraulic systems.
Etc.
In the RCM standards which do not specify this type of task it could be argued that for each it is a
matter of failure consequences which drives the need for maintenance. For instance if an oil of
lubricant requires periodic purging and replacing, it could be argued that it is a hard time task to
discard a worn out lubricant and replace with a new one. Similarly, if a sample was taken and then,
after analysis of this sample, the lubricant is replaced based on its condition it could be construed that
this would be an on condition task. In the case of operational refuelling etc. then this could be seen as
not a preventive task at all and simply a requirement of operations. So, it is therefore, possible to
handle certain requirements differently depending upon the RCM standard applied.

RCM and Uncertainty
RCM can be pretty subjective. Behind the formulae and the physics of failure and the engineering
knowledge required to adequately perform the analysis are layers of dependency on data and the
common sense of those involved.
There will be times where an analysis will not know. Not for sure and it will have to default to known
behaviours.
These behaviours are pessimistic, safe and necessary to ensure that a RCM study progresses through
uncertainty.

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