If we take the time-based model as an example (see figure 1), the degradation will
increase over time. The initial period can be described as ‘good performance’ where
the degradation rate is low, or even very low. During this period, classic visual inspection techniques may even fail to identify that degradation has started. It is during this
period where many structural inspections feature an interval threshold to account for
the good initial performance of an aircraft. However, as time progresses, degradation
is inevitable and the acceptable condition of the structural item is questioned.
In case the inspection finds degradation which has passed the predefined limit that
ensures safe operation until the subsequent inspection, the items must be restored.
The respective benchmark is named ‘restoration limit’ in the example model.
Consequently the progressing degradation after Check #2 in our example should stay
above the operational safety limits before Check #3, when the restoration limit was not
reached at the time of Check #2. With this, the safety margin between the safety limit
and failure are maintained. The restoration limit and the repeat interval are interdependent and set by trading of economics.
这是第一个图附带的说明

A second aspect is that restoration does not often bring the item back to its initial state
(a restored Item is normally close, but not exactly the same quality of a new item).
This conservatism however is necessary to build robust inspection programmes on the
current fleet level condition monitoring approach. Inspections resulting in ‘no finding’ could
possibly have been deferred on that aircraft, but there is however still potential for findings
on other aircraft triggered by the variation of an individual aircraft’s operational conditions
and occurrences. In other words, effectiveness of the inspection programme is validated by
the ‘no finding’ rate at fleet level - the current fleet level condition monitoring concept.
Switching the global concept to CBM brings maintenance from monitoring fleet
level conditions by inspecting every aircraft of a fleet, to managing the individual
aircraft airworthiness against predetermined safety and economic limits with built-in
monitoring capabilities.
Let’s look into the semantics of such an approach (see figure 2). The on-board system
monitors the relevant parameter over time and as from the initial detection, a prognostic
function provides a status input on the overall condition monitoring system for the structural
item. When the ‘maintenance notification limit’ is reached, the airline’s Maintenance Control
Centre (MCC) is notified by the aircraft system. Maintenance can now start planning the
restoration slot within the ‘maintenance window’ (e.g. packaged together with other
deferred items in a dedicated shop visit) to optimize the aircraft availability. A cockpit
message informs the flight crew and the MCC if the degradation is about to approach an
‘operational limit’, providing a clear status of the individual aircraft’s remaining operational
capabilities (e.g. number of flight cycles/hours before the item needs to be restored),
limitation of the aircraft’s specific capabilities (reduce flight level and/or reduced load), etc.
这是第二个图的说明