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UK
[1].
However,
the
regulations
which
are
de.ned
concerning
HUMS address only the functionality and availability of the system. It does not specify formal procedures in the decision making process between HUMS output and possible maintenance responses.
In some cases, like the Eurocopter endorsed systems, the aircraft OEM and the HUMS provider is the same party. In these cases, the OEM can pro-vide maintenance recommendations in cases where the operator is in doubt. However, the customer support throughput is usually not su.cient to pro-vide diagnoses on .ight-to-.ight bases. As HUMS output should indeed to be analyzed between each .ight, this still leaves much of the decision making to the line personnel.
There are no formal procedures for reporting detections and non-detections. As a result, it is di.cult to create accurate statistics to determine which HUMS functions work and which do not. Some feedback is provided by the operators, but this information is highly biased and inconsistent. The follow-ing sections tries to extract whatever information possible, based on recorded data and expert opinions.

2.4.2 Safety
Helicopter accident rates have shown a clear downwards trend from the be-ginning of the eighties. Several measures, among them HUMS, where taken during the eighties to improve safety. Although it is di.cult to quantify the e.ect of each measure, the safety enhancing e.ect of HUMS is none the less signi.cant. The report "Helicopter Safety Study 2" by Sintef, states that HUMS is "the most signi.cant isolated safety improvement measure during the last decade". The CAA estimates that about 70% percent of all drive train
faults
are
uncovered
by
the
current
generation
HUMS
[38].
This
.gure
is equivalent to the detection statistics available for the Eurocopter endorsed systems.
Despite good diagnostics capabilities for a wide range of failure modes,

2.4. IMPACT OF CURRENT TECHNOLOGY
several in-service di.culties have been reported by the operators. Some of these di.culties are related to the fault diagnosis technology available. Others are related to more practical usability issues which were not foreseen during the design of these systems.
2.4.3 Maintenance Credit
Changes in maintenance procedures, removal of maintenance tasks, or ex-tension of component time between overhaul (TBO) due to the introduction of alternative monitoring techniques are referred to as maintenance credits. Maintenance credits to HUMS have been granted to the following functions:
.
Load Cycle Calculation

.
Exceedance Monitoring

.
Power Assurance Check (PAC)

.
Rotor Track and Balance (RTB)

.
Engine Vibration Monitoring (EVM)

The functions listed above are mandatory functions on most helicopters. The calculation of usage cycles on non HUMS rotorcraft is performed by another permanently installed device. On HUMS rotorcraft, this function is simply embedded into the HUMS. In the case of PAC, RTB and EVM, HUMS is certi.ed to replace temporarily installed equipment, used at .xed intervals. Performing these tasks on non HUMS rotorcraft require ground-runs of engines and / or rotors. In the case of RTB, test .ights are also required. On HUMS rotorcraft, the information needed for tasks is recorded during the normal operation of the helicopter. This is clearly a cost saver, both in terms of maintenance man hours and even pilot man hours (for RTB technical .ights).
　
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