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Acquisition Rate
The systems currently in use perform a maximum of .ve acquisition cycles per .ight. This choice was made due to hardware limitations at the time when the .rst systems were created. In addition, it was believed that vibration-based condition indicators could be reliably interpreted like any other .ight parameter. Experience has however shown that condition indicators have considerable scatter, and are usually best regarded as signals themselves, subject to further signal processing and statistical analysis methods.
A way to increase system reliability is to increase the number of acquisi-tions per .ight. Due to the false-alarm rate of existing HUMS, the probability of a fault being present on a component, given an alarm, is not very high. With repeated alarms, the probability of a fault being present will however quickly
converge
[37].
The
more
data
is
acquired
per
.ight
hour,
the
quicker,
in terms of .ight hours, a reliable diagnosis can be produced. This is pro-vided that the recorded data is representative, and not polluted by contextual variations.
Evaluating data over a large number of acquisitions with current system does however pose a serious threat of overlooking rapidly propagating faults, due to the low acquisition rate of these solutions. Acquisition rate is however in part a limitation of the airborne hardware, and can not necessarily be altered by simply modifying software or con.guration. Increased acquisition rate should however be kept in mind when designing the next generation airborne segment.
Sensor Fault Detection
It is well agreed that the most .awed component monitored by the HUMS is
the
HUMS
itself
[11].
Although the ARMS solutions possess self test functions capable of identifying problems like dysfunctional circuit boards, capabilities for robust detection of partially damaged sensors or harness are less developed. A consequence of this is that the changes in vibration sig-nature caused by sensor and harness degradation are frequently interpreted as mechanical damage by the ground station. E.orts should therefore be made to develop indicators capable of distinguishing between electrical and mechanical problems.
Due to lack of relevant data, research in this area will require extensive test rig experiments.
Contextual Data Correction
The current implementations of Eurocopter Aircraft Recording and Moni-toring System (EuroARMS) / Modular Aircraft Recording and Monitoring System (MARMS) use contextual validation to decide when acquisition is enabled. This is to limit the number of environmental contexts for when acquisition is performed, thus reducing indicator scatter. There are however still signi.cant variations within this window, contributing to considerable variance between acquisitions.
E.orts should be made to investigating the correlation between di.erent environmental contexts, .ight stages, and vibration signatures. If such cor-relations can be modeled, these correlations can also be compensated for. Environmental normalization of condition indicators has already been pro-posed
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OPTIMIZATION OF FAULT DIAGNOSIS IN HELICOPTER HEALTH AND USA(30)
