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In reality, HUMS is demanded on all o.shore .ights by the petroleum companies operating in the North Sea. Consequently, HUMS becomes a requirement for heavy helicopters operating in both British and Norwegian sector.
The CAA de.nition divides HUMS in two main subsystems; a Vibration Monitoring System (VMS), and "existing established techniques". The latter part covers functions such as temperature-and torque monitoring, magnetic plugs and chip detectors, thus corresponding to the Usage Monitoring System (UMS) of HUMS. It is worth noting that these functions are mandatory on all helicopters, regardless of whether a HUMS is installed or not. In case no HUMS is installed, these functions are maintained by other systems.
The VMS addresses the Health aspect of HUMS. The de.nition applies to
all
rotorcraft,
and
is
thus
not
very
precise.
The
CAA
directive
[1]
reads
as follows
Vibration monitoring System (VMS) should monitor :
.
Engine to main gearbox input drive shafts

.
Main gearbox shafts, gears and bearings

.
Accessory gears, shafts and bearings

.
Tail rotor drive shafts and hanger bearings

.
Intermediate and tail gearbox gears, shafts and bearings

.
Oil cooler drive

.
Main and tail rotor track and balance

Further, the HUMS Minimum Equipment List (MEL) states that
Depending upon system installation, if the data analysis (or fail-ure indication system) indicate a malfunction of any system or

2.2. ROTORCRAFT FAILURE MODES
sensor, i.e. accelerometer, then the maximum period that the
item or system can be deemed unserviceable would be as follows:
(1)
25 .ying hours

However, if the speci.c item has been under investigation due to adverse trend identi.ed by the HUM system, the maximum period of unserviceability would be as follows:

(2)
10 .ying hours

 

2.2 Rotorcraft Failure Modes
The transmission system of a heavy rotorcraft is highly complex, and has a high number of possible failure modes. Failure scenarios are typically com-plex, in the sense that one propagating fault tends to trigger other failures. This is especially the case for gearboxes. Still, it is possible to distinguish some classical fault types, and their symptoms.
2.2.1 Engines
Helicopter jet engines consist of two stages. The .rst stage includes com-pressor, combustion chamber and turbine, and resembles the design of a traditional .xed-wing engine. This assembly is followed by the second stage, which is an additional turbine. The second stage delivers power from the engine to the transmission system.
Engine Compressor and Turbine Unbalance
Engine turbines and compressors rotate at very high speeds, and must be per-fectly balanced. Problems like disk cracks, blade cracks and broken blades typically produce unbalanced rotation, and are uncovered by monitoring vi-bration energy at the frequencies corresponding to the compressor and tur-bine rotating speeds.
　
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