Chapters 2 through 8 detail the Operating Context and the seven steps of the RCM process. The following discussion briefly introduces each concept.
Figure 1.19 The RCM Process
Operating Context
An Operating Context is a document that includes relevant technical information such as the scope of analysis, theory of operation, equipment description, and RCM analysis notes. In essence, it is a storybook identification of the system to be analyzed. The Operating Context also documents notes and assumptions regarding analysis decisions. It is an important source of reference for working group and validation team members.
In the interest of time, the Operating Context is typically drafted by the facilitator before the analysis begins and is then reviewed with the working group before the first step in the RCM process (identifying Functions) is accomplished. During this time, the working group reviews and revises the Operating Context, as required. The Operating Context is considered a living document; it is edited as more is learned about the equipment and additional issues come to light during the analysis.
Step 1: Functions
The intention of RCM is to determine what solutions must be put in place to ensure an asset meets the requirements of the organization. The air tanker crashes and the Aloha Airlines disaster detailed previously illustrate how critical it is to understand what is required of an asset so that it can be determined if the asset is capable of fulfilling those requirements. For this reason, the first step in the RCM process is to identify Functions.
Functions and associated performance standards are always written to reflect what the organization requires from the asset rather than what the system is designed to provide. During Function development, it is often noted that the organization’s expectations of the equipment exceed the actual capabilities of the asset. As depicted in Figure 1.20, the Primary Function (the main purpose the system exists) and Secondary Functions (other Functions of the asset) are recorded.
Step 2: Functional Failures
Step 2 in the RCM process is to identify Functional Failures for each Function. Nowlan and Heap define Functional Failure as an unsatisfactory condition. As depicted in Figure 1.21, both Total and Partial Functional Failures are recorded for each Function. A Total Failure means no part of that Function can be performed. Partial Failure describes how the Function is still possible but is performed at an unsatisfactory level.
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