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It is a well known phenomenon in vibration monitoring that damage to one component can cause perturbations in the condition indicators of adja-cent components. This might cause confusion as to what the actual fault is, and where it originates from. Fuzzy logic solutions tying together the indi-vidual classi.cation systems of components into system-wide or aircraft-wide diagnosis
has
been
suggested
to
amend
this
[24]
[13].
The
physics
involved
in inter-component vibration propagation are however quite complex, and not always well understood by the OEM. Moreover, it is di.cult to gather su.cient relevant training data to create and validate a robust solution.

3.4.4 Prognosis
Threshold testing can in combination with trend analysis be used for prog-nosis. This is the case both for single indicator thresholds, as used with traditional threshold testing, and multidimensional thresholds, as used with clustering systems. Given the value and the slope of an indicator progression at a point in time, it is possible to estimate the time remaining until the threshold will be breached. In a clustering system, this corresponds to the estimated time remaining until the system leaves the normal state cluster, given the current position and gradient. This estimate obviously assuming that the progression slope or gradient remains constant.

3.5. COMMERCIAL SOLUTIONS
For this to have a signi.cant operational interest, the link between the threshold, or cluster, and the mechanical state it represents should be clear and well understood. If classi.er training is based purely on statistical anal-ysis of a normal state indicator distribution, the prognosis will simply give the estimated time until the vibration signature changes from normal to ab-normal, given an arbitrary de.nition of normality and abnormality. As the mechanical state associated with the threshold or cluster border is unknown, the estimated time until the mechanical system reaches this point will have less operational interest.

3.5 Commercial Solutions
Several commercial HUMS implementations exist from several manufactur-ers. This section introduces a few of them. The selection of systems discussed here is however biased toward systems developed for heavy rotorcraft operat-ing in hostile environments, as this was the origin of the HUMS development. Numerous implementations and manufacturers have since joined the HUMS marked, addressing both the original audience, as well as new markets such as military operators and medium or even light rotorcraft.
3.5.1 IHUMS
IHUMS is manufactured by Meggitt Avionics in cooperation with rotorcraft operator
Bristow
[2].
It is a .rst generation system, and was one of the .rst two systems on the market. IHUMS ground stations with graphical user interface exist for both UNIX and Microsoft Windows NT. Diagnosis is based on condition indicators processed by a fuzzy logic-like matrix system. The matrix system aids in suppressing spurious alerts due to indicator spikes.
　
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