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.
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).
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.