EAA Sport Aviation - January 2010

Many aircraft owners, mechanics, and aeronautical engineers still believe the best way to optimize reliability of complex aircraft systems like engines is to do some kind of proactive maintenance on a routine basis. Conventional wisdom is that this should consist of overhauls or replacement at fixed intervals. Figure 1 illustrates this fixed-interval view of failure:

FIGURE 1

Conditional
Probability of Failure

Useful Life

Wear
Out
Zone

-

Figure 1— The traditional view of failure, in which components are overhauled or replaced at a fixed interval (TBO). FIGURE 2

This traditional view assumes most items operate reliably for some fixed period of time (“useful life”), after which the probability of failure starts to increase rapidly (“wear-out zone”). It is predicated on the notion that analysis of failures will allow us to predict the useful life of an item and take scheduled action to overhaul or replace it before it reaches the point where risk of failure becomes unacceptable.

Conditional
Probability of Failure

This is valid for components that have a single, dominant, age-related
failure mode. For example, the failure pattern illustrated in Figure 1 is appro-
priate when considering an item that normally fails from metal fatigue due to
repetitive stress, such as a wing spar or cylinder head.

Safe

Earliest

Life

Known

Limit

Failure

In this view, the probability of failure during the item’s useful life is usually small but nonzero. Therefore, a modest number of premature failures can be expected before the item reaches the end of its useful life, at which point the probability of failure starts increasing.

AGE

Automated Engine Monitoring...

..of up to 29 parameters with 62 alarms. From RPM to peak-detection leaning, the EIS does it all. Includes graphical and digital displays, customizable screens, and alarms with external warning light. Models for all engines up to 9-cylinders. Find out why the EIS is the choice of thousands of pilots.