(e.g., ability to withstand g-loads), the acceptable
standard of performance is established by the FAA
during aircraft certification; in other cases (e.g.,
dispatch reliability), the acceptable standard is
established by the aircraft owner or operator. The
purpose of maintenance is to ensure that each
item continues to meet its performance standard.
Before we can establish a rational performance
standard for a component, we need to examine
the consequences of failure. For a component
whose failure is likely to result in death or injury
(e.g., a wing spar), the likelihood of failure must be
infinitesimally low. On the other hand, for a component whose failure is simply an inconvenience
(e.g., the No. 2 comm radio), a higher failure probability is acceptable.
From a maintenance standpoint, we must do
whatever it takes to prevent the failure of safety-critical items like wing spars and engines, even if
it’s expensive to do so. On the other hand, it’s usually not worth spending any time or effort to
prevent the failure of a non-critical item; we just
run the item to failure and then fix it when it fails.
Often, the consequences of failure depend on
the component’s operating context. The failure of
a dry vacuum pump is much less critical if the air-
craft has a standby vacuum pump or an electric
backup attitude indicator. The failure of an engine is considerably less critical
on a four-engine airplane than on a single-engine airplane. The failure of a
wing spar is less critical if the wing has a fail-safe multiple-spar design.
RCM classifies the consequences of failure into four categories, in
descending order of importance:
• Safety consequences. A failure has safety consequences if it could kill or
injure someone.
A strategy known as “reliability-centered maintenance” has drastically
reduced the cost of maintaining transport and military aircraft, while
simultaneously improving dispatch reliability.
• Operational consequences. A failure has operational consequences if it
prevents the aircraft from being operated.
• Hidden consequences. A failure has hidden consequences if it is not
apparent to the flight crew, but could cause a subsequent failure to have more
serious consequences.
• Non-operational consequences. Failures in this category are evident to
the flight crew, but impact neither safety nor operation and involve only the
cost of repair.
FEASIBLE? WORTH DOING?
RCM does not require that all failures be prevented. It recognizes that not all
failures are created equal, and that maintenance resources should be focused
on reducing failures that really matter. RCM concentrates on preventing
