HANDS ON / SAVVY AVIATOR
Time to apply it to general aviation?
n 1974, the U.S. Department of Defense commissioned United Air Lines to
prepare a report on the techniques used by the airline industry to develop
cost-efficient maintenance programs for civil airliners. The resulting report,
titled Reliability-Centered Maintenance (F.S. Nowlan & H. Heap, National
Technical Information Service, 1978), described a radically different
approach to aircraft maintenance based on rigorous analysis of traditional
maintenance practices and evaluation of their shortcomings.
Traditionally, a major emphasis of aircraft maintenance programs had
been defining specific overhaul and retirement intervals—time between
overhauls (TBOs)—to achieve a satisfactory level of reliability. However,
engineering analysis of reams of operational data from a number of major
air carriers produced fascinating insights into the conditions that must
exist for scheduled maintenance to be effective. Two discoveries were especially surprising:
1. For a complex item (like an engine), scheduled overhaul has little effect
on the overall reliability unless the item has a single dominant failure mode.
2. For many items there is simply no form of scheduled maintenance that
is technically and economically feasible. For example, reliability-centered
maintenance (RCM) researchers determined back in the 1970s that scheduled overhauls on turbine engines do not produce any reliability or economic
benefit, and that maintaining such powerplants strictly on-condition provides longer life, reduced maintenance costs, and improved reliability.
RCM has resulted in huge cost savings for the airlines. For example, the
pre-RCM DC- 8 required 4,000,000 man-hours of structural inspections during its initial 20,000 hours of operation, while the post-RCM Boeing 747
required only 66,000 man-hours over the same interval. That’s a reduction of
nearly two orders of magnitude.
Not only are these cost savings immense, but
they were achieved with no decrease in safety or
dispatch reliability. To the contrary, safety and
reliability actually improved in almost every
instance when emphasis shifted from scheduled
retirement-overhaul-replacement to on-condition maintenance.
This month I’ll talk about some of the fundamental principles of RCM, and next month I’ll
explore how they can be applied to our piston-powered airplanes. As we delve into the theory of
RCM, keep in mind that this is hardly a theoretical matter. My maintenance management firm
manages 150 piston-powered, owner-flown aircraft using RCM, and we’re saving the owners
thousands of dollars each year in maintenance
costs. The airlines and military have been using
RCM for decades and saving a fortune, and it’s
high time that this approach trickled down to
FUNCTIONS AND FAILURES
Each system and component of an airplane performs one or more functions. The purpose of
maintenance is to ensure that those items continue to perform their functions to an acceptable
standard of performance. In some cases