Figure 1.20 Primary and Secondary Functions
Figure 1.21 Total and Partial Failures
Step 3: Failure Modes
A Failure Mode is what causes a Functional Failure. During Step 3 of the RCM process, Failure Modes that cause each Functional Failure are identified. It is often wrongly believed that all Failure Modes associated with the system being analyzed must be recorded. On the contrary, RCM provides specific guidelines for determining what Failure Modes to include in an analysis. Only Failure Modes that are reasonably likely to occur in the operating context should be included. If the answer to one or more of the following questions is “yes,” the Failure Mode should be included in the analysis:
•Has the Failure Mode happened before?
•If the Failure Mode has not happened, is it a real possibility?
•Is the Failure Mode unlikely to occur but the consequences are severe?
•Is the Failure Mode currently managed via proactive maintenance?
Failure Modes included in most analyses consist of typical causes such as those due to wear, erosion, corrosion, etc. However, it is very important to include Failure Modes that cover issues such as human error, incorrect technical manuals, inadequate equipment design, and lack of emergency procedures. Such Failure Modes allow issues to be analyzed as part of the RCM process so that solutions in addition to proactive maintenance can be developed.
Step 4: Failure Effects
During Step 4, a Failure Effect is written for each Failure Mode. A Failure Effect is a brief description of what would happen if nothing were done to predict or prevent the Failure Mode. Failure Effects should be written in enough detail so that the next step in the RCM process, Failure Consequences, can be identified. Failure Effects should include:
•Description of the failure process from the occurrence of the Failure Mode to the Functional Failure
•Physical evidence that the failure has occurred
•How the occurrence of the Failure Mode adversely affects safety and/or the environment
•How the occurrence of the Failure Mode affects operational capability/mission
•Specific operating restrictions as a result of the Failure Mode
•Secondary damage
•What repair is required and how long it is expected to take
Information Worksheet
Steps 1 through 4 of the RCM process are recorded in the Information Worksheet, as depicted in Figure 1.22. The Information Worksheet includes Functions, Functional Failures, Failure Modes, and Failure Effects.
Figure 1.22 The Information Worksheet
Step 5: Failure Consequences
A properly written Failure Effect allows the Failure Consequence to be assessed. A Failure Consequence describes how the loss of function caused by the Failure Mode matters. There are four categories of Failure Consequences:
•Safety
•Environmental
•Operational
•Non-Operational
Step 6: Proactive Maintenance and Associated Intervals
After consequences are assessed, the next step in the RCM process is to consider proactive maintenance as a failure management strategy. In the context of RCM, the proactive maintenance tasks that may be identified include:
Scheduled Restoration A scheduled restoration task is performed at a specified interval to restore an item’s failure resistance to an acceptable level—without considering the item’s condition at the time of the task. An example of a scheduled restoration task is retreading a tire at 60,000 miles.
Scheduled Replacement A scheduled replacement task is performed at a specified interval to replace an item without considering the item’s condition at the time of the task. An example is a scheduled replacement of a turbine engine compressor disk at 10,000 hours.
Scheduled restorations and scheduled replacement tasks are performed at specified intervals regardless of the item’s condition.
On-Condition Task An On-Condition task is performed to detect evidence that a failure is impending. In the context of RCM, the evidence is called a potential failure condition and can include increased vibration, increased heat, excessive noise, wear, etc. Potential failure conditions can be detected using relatively simple techniques such as monitoring gauges or measuring brake linings. Additionally, potential failure conditions can be detected by employing more technically involved techniques such as thermography or eddy current, or by using continuous monitoring with devices such as strain gauges and accelerometers installed directly on machinery. The point of On-Condition tasks is that maintenance is performed only upon evidence of need.
In the context of RCM, all proactive maintenance tasks must be technically appropriate and worth doing. Chapter 9 details how to determine if a proactive task is technically appropriate and worth doing.
Step 7: Default Strategies
As mentioned earlier, RCM isn’t just about maintenance. There are a great many solutions other than proactive maintenance that can be derived using the RCM process. Examples include: Failure Finding tasks, Procedural Checks, no scheduled maintenance, and other recommendations such as modifications to operating procedures, updates to technical publications, and equipment redesigns. In the context of RCM, these recommendations are known as Default Strategies. Default Strategies are discussed in detail in Chapter 10.
It is often wrongly believed that FMEA and FMECA are analyses that are accomplished independently of, or in lieu of, RCM. On the contrary, the first four steps of the RCM process produce a FMEA. The steps to accomplish a FMEA are depicted in Figure 1.23.
Figure 1.23 First four steps of the RCM process produce a FMEA
Figure 1.24 First five steps of the RCM process produce a FMECA
Additionally,
