ARMY AL&T


FIGURE 1


Maintenance Free Operating Period


Failure Threshold


10 percent chance of failure


90 percent chance of success


future risk. With less variation in the predicted remaining useful life, a compo- nent will remain in service longer and the Army will see more benefits from PPMx.


Measurements Predicted Remaining Useful Life Flight Time SURVIVABILITY ASSESSMENT


Figure 1: Illustrates the probability of failure increasing with accumulation of wear during flight hours. (Graphic courtesy of the authors)


Risk mitigation is a core goal under PPMx. To maintain safety, most of the time the failure will never occur. For aircraft using PPMx in a maintenance-free operating period strategy, some near-but-not-yet- failed parts will be removed. In doing this, the maintainer is trading econom- ically useful life for uninterrupted flight operations. Tese parts would still be oper- ating in the prescribed limits, but will be removed because they are expected to fail before the next maintenance window.


As indicated by the blue hash area, the component has a 90 percent chance to remain functional by the end of the oper- ating period and, consequently, a 10 percent chance the component will fail. Te future risk is the product of this 10 percent probability and the criticality of the component. With this estimate of the future, a maintainer has the infor- mation necessary to make appropriate maintenance decisions today. If the risk is unacceptable, then the framework can be


queried for which components are driv- ing the risk over the acceptable threshold.


UNCERTAINTY IS COSTLY Under PPMx, the uncertainty about the remaining useful life adds complexity to the maintenance decision. Using the weather example, if the forecast gives a 40 percent chance of rain in five days, a trav- eler may pack an umbrella for a trip. Tus, the uncertainty about the prediction influ- ences actions today as we hedge against


Our inability to know the exact moment of component failure is quantified by the uncertainty. Te uncertainty about the health statement drives maintenance actions which then raise sustainment costs and keep availability low. Better diagnos- tic systems and prognostic models with reduced uncertainty maximize the use of the remaining useful life to save money and minimize mission disruption.


MODERNIZED THINKING PPMx is a concept that hinges on proba- bilities, yet thinking in probabilistic terms does not come naturally to humans. Te engineering community needs to strive to give accurate assessments of the risks, broken out into usable information. For example, the risks could be reported as


PPMx’s goal is to arm the maintenance manager with the information to make efficient decisions that avoid mission disruption and eliminate premature removal of components.


https://asc.ar my.mil 79


Probability of Failure


Recovery Period


1 percent


10 percent


10 percent


99 percent


90 percent


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