The P-F Interval
According to NAVAIR 00-25-403 RCM
Guideline
NAVAIR P-F Interval
The following describes NAVAIR [1] methods
based on the classic CBM:
1. Determining the P-F Interval, and
2. Determining the Inspection Interval
[1] NAVAIR 00-25-403 RCM Guideline
OMDEC Inc
Performance
Session 4
Slide Number: 2
NAVAIR Methods for determining PF intervals:




Laboratory testing (in conjunction with accelerated life
testing (ALT)),
Analytical methods
Evaluation of in-service data - AE (Age Exploration),
and engineering judgement based on inputs from
operators and maintainer, (i.e. Classic RCM method)
and
Knowledge of the item’s design and of applications
consisting of similar components.
Note: NAVAIR does not provide details about the above
methods in its 00-25-403 guide.
OMDEC Inc
Performance
Session 4
Slide Number: 3
PF used only for starting Inspection
Interval

The PM task interval is determined by
using some fraction of the PF interval. This
fraction will depend on the consequences
of failure and the effectiveness of the
proposed task.
I = PF/n
Where:
I = Inspection interval
PF = potential failure to functional failure interval
n = number of inspections in the PF interval
OMDEC Inc
Performance
Session 4
Slide Number: 4
Determining n
n = ln (Pacc) / ln (1-θ)
Where:
 θ = Probability of detecting a potential
failure with one occurrence of the proposed
On Condition task, assuming the potential
failure exists
 Pacc = Acceptable probability of failure
Note: θ can only be determined practically in a LRCM program that
tracks CBM hits and misses.
OMDEC Inc
Performance
Session 4
Slide Number: 5
Error in the 00-25-403 Guide
The next slide illustrates an error by NAVAIR
regarding the P-F interval. It is a commonly held
misconception …
ON CONDITION TASK
Task Development
Functional Failure:
A
Functional
Capability
B POTENTIAL
FAILURE
DEFINED POTENTIAL
FAILURE CONDITION
DEFINED FUNCTIONAL
FAILURE CONDITION
C
OPERATING AGE
I
I
I
FUNCTIONAL
FAILURE
Inspection Interval
TASK INTERVAL PRACTICAL?
PF Interval
OMDEC Inc
Performance
Session 4
Slide Number: 7
ON CONDITION TASK
Implies that functional capability will drop off as the system
Task Development
Functional Failure:
approaches a potential failure state (point B).
In fact, in most cases, especially in complex systems, the
equipment's functions remain fully operational at the
potential failure point
A
Functional
Capability
B POTENTIAL
FAILURE
DEFINED POTENTIAL
FAILURE CONDITION
DEFINED FUNCTIONAL
FAILURE CONDITION
C
OPERATING AGE
I
I
I
FUNCTIONAL
FAILURE
Inspection Interval
TASK INTERVAL PRACTICAL?
PF Interval
OMDEC Inc
Performance
Session 4
Slide Number: 8
ON CONDITION TASK
Task Development
Functional Failure:
In the original on-condition model by Nowlan and
Heap, and in Moubray's book "RCM II", the
vertical axis is correctly labelled "Condition“ or
“Failure Resistence”
A
Functional
Capability
B POTENTIAL
FAILURE
Condition
DEFINED POTENTIAL
FAILURE CONDITION
DEFINED FUNCTIONAL
FAILURE CONDITION
C
OPERATING AGE
I
I
I
FUNCTIONAL
FAILURE
Inspection Interval
TASK INTERVAL PRACTICAL?
PF Interval
OMDEC Inc
Performance
Session 4
Slide Number: 9
Conclusion

The P-F Interval is a good, but subjective,
first approximation developed during an
RCM analysis in the absence of data based
on the recollection by the analysts of potential
and functional failures that have occurred.
 Age exploration techniques (e.g. EXAKT and
LRCM) during the In-Service (IS) phase of
operation should be used to confirm
inspection frequencies and refine predictive
models for the continuous improvement in
Remaining Useful Life Estimation (RULE).
OMDEC Inc
Performance
Session 4
Slide Number: 10