EAA Sport Aviation - February 2010

MIKE BUSCH
Is Engine
TBO a Myth?
Reliability Centered Maintenance, Part 2
LAST MONTH WE EXAMINED the principles of reliability-centered
maintenance used by the airlines and military to achieve cost-effec-
tive maintenance. Now, let’s explore how RCM can be applied to
our small GA aircraft, especially our piston aircraft engines.
For three decades the airlines and military have been using
reliability-centered maintenance (RCM) to slash maintenance
costs and improve reliability. Most of these benefits come from
replacing fixed overhaul intervals with on-condition maintenance.

Unfortunately, RCM has not trickled down in the aviation food chain. Maintenance of piston general aviation (GA) aircraft remains largely time-directed rather than condition-directed. Most GA aircraft owners dutifully overhaul their engines at time between overhauls (TBO), overhaul their prop every five to seven years, and replace their alternators and vacuum pumps every 500 hours, just as Lycoming, TCM, Hartzell, McCauley, Kelly Aerospace, and Parker Hannifin recommend. Bonanza owners have their wing bolts pulled every five years. Cirrus owners replace their batteries every two years.

MAKE SENSE?

After analyzing reams of operational data from a number of
major air carriers, RCM researchers concluded that fixed-inter-
val overhaul or replacement rarely improves safety or reliability,
and often makes things worse. When does TBO make sense?

For fixed TBO to make sense, the component must have a failure pattern that looks like pattern B in Figure 1, where the component can be expected to operate reliably for some predictable useful life, beyond which the probability of failure starts to increase rapidly to unacceptable levels.

But piston aircraft engines don’t exhibit this kind of failure pattern. We know these engines suffer the highest risk of catastrophic failure not when they pass TBO, but rather when they’re fresh out of the factory or field overhaul shop. Look at the National Transportation Safety Board (N TSB) data for the five-year period 2001 through 2005 (Figure 2).

Figure 1

Fixed TBO may make sense for components with failure pattern B, but probably not for those with pattern A and definitely not for those with pattern F.