EAA Sport Aviation - January 2010

introducing infant-mortality risk into an other-
wise reliable system or component.
RCM shows that fixed age limits and sched-
uled overhauls are technically feasible only if:
1. There is an identifiable age (TBO) after
which the item shows a rapid increase in the con-
ditional probability of failure (i.e., an obvious
wear-out zone); and
2. Most of the items survive to that age; that
is, there are relatively few premature failures.
To learn more about RCM, read:

• Reliability-Centered Maintenance, John Moubray. Second edition. 1997. ISBN 0-8311-3078-4.

• Reliability-Centered Maintenance. The original 1978 Nowlan & Heap report is available from the National Technical Information Service ( www.NTIS.gov). The document number is ADA066579.

On-COnditiOn MaintenanCe

On-condition maintenance consists of checking
for potential failures so that action can be taken to
prevent functional failures before they occur.

Although RCM has revealed that there is often little or no relationship between time-in-service and likelihood of failure, most failures give some sort of warning that they are about to occur. If we can detect these warnings in time, we may be able to take maintenance action to prevent the failure and avoid its consequences; see Figure 5: FIGURE 5

P-F Interval

Condition
Time

performed at intervals that are less than the P-F interval. In practice, it is usually optimal to implement a task frequency that corresponds to about one-half of the P-F interval. (If the P-F interval is 100 hours, we need to inspect every 50 hours to ensure that we will detect a potential failure in plenty of time to avert a total failure.) Such condition monitoring is considered to be technically feasible if:

FIGURE 1

• It is possible to identify a well-defined and
reliably detectable potential failure condition;
Conditional
Probability of Failure

• The P-F interval is reasonably consistent and predictable; and

• It is practical to inspect or monitor the item at an interval approximately one-half the P-F interval.

Useful Life Wear

Out Zone

Next month, we’ll get down to the nitty-gritty of how we can apply RCM principles to the maintenance of our piston-powered general aviation airplanes, with special emphasis on piston aircraft engines.

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Figure 5—The P-F curve. P is the point where an incipient ( potential) failure becomes detectable, and F is the point where total (functional) failure occurs.

FIGURE 3

If a developing failure can be detected somewhere between point P (where it first becomes detectable) and point F (where total failure occurs), it may be possible to take action to prevent the consequences of the failure. Whether or not it is technically feasible to do this depends on how quickly the failure occurs, how far in advance it becomes detectable, and how difficult it is to detect the potential failure.

Conditional
Probability of Failure

Infant Mortality

Wear Out Zone

AGE

The warning period between the occurrence of a detectable potential failure and its decay into a total functional failure is known as the “P-F interval” in RCM speak. It may be measured in hours, cycles, calendar months, or any other appropriate metric. To detect failures reliably before they occur, on-condition maintenance tasks must be

FIGURE 4

A

4%

FIGURE 2

Mike Busch, National Aviation Maintenance Technician of the Year for 2008, has been a pilot for more than 44 years and has logged more than 7,000 hours. He is a certificated flight instructor and an airframe and powerplant mechanic with an inspection authorization. Questions for Mike may be e-mailed to mike.busch@savvyaviator.com.