1. Unit III Module 3 - On Condition Task
The P-F Interval
According to NAVAIR RCM Guideline

2. Unit III Module 3 - On Condition Task
NAVAIR P-F Interval
The following describes NAVAIR [1] methods based on the classic CBM:
Determining the P-F Interval, and
Determining the Inspection Interval
[1] NAVAIR RCM Guideline

3. NAVAIR Methods for determining PF intervals:
Unit III Module 3 - On Condition Task
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 guide.

4. PF used only for starting Inspection Interval
Unit III Module 3 - On Condition Task
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

5. Unit III Module 3 - On Condition Task
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.

6. Unit III Module 3 - On Condition Task
Error in the Guide
The next slide illustrates an error by NAVAIR regarding the P-F interval. It is a commonly held misconception …

7. Unit III Module 3 - On Condition Task
Functional Failure:
Task Development A
Functional Capability B
POTENTIAL
FAILURE
DEFINED POTENTIAL
FAILURE CONDITION
DEFINED FUNCTIONAL
FAILURE CONDITION
-403, C
FUNCTIONAL
FAILURE
OPERATING AGE I I I
Inspection Interval
TASK INTERVAL PRACTICAL?
PF Interval

8. Unit III Module 3 - On Condition Task
Implies that functional capability will drop off as the system 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
Functional Failure:
Task Development A
Functional Capability B
POTENTIAL
FAILURE
DEFINED POTENTIAL
FAILURE CONDITION
DEFINED FUNCTIONAL
FAILURE CONDITION
-403, C
FUNCTIONAL
FAILURE
OPERATING AGE I I I
Inspection Interval
TASK INTERVAL PRACTICAL?
PF Interval

9. Unit III Module 3 - On Condition Task
Functional Failure:
Task Development
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
Condition
Functional Capability B
POTENTIAL
FAILURE
DEFINED POTENTIAL
FAILURE CONDITION
DEFINED FUNCTIONAL
FAILURE CONDITION
-403, C
FUNCTIONAL
FAILURE
OPERATING AGE I I I
Inspection Interval
TASK INTERVAL PRACTICAL?
PF Interval

10. Unit III Module 3 - On Condition Task
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).