2.3 Health and Usage Monitoring Tasks
A HUMS is responsible for alerting the operator of any problems which might threaten the airworthiness of the aircraft. To accomplish this, the HUMS
2.3. HEALTH AND USAGE MONITORING TASKS
use a sensor array covering most critical components on the aircraft. Some of these sensors are part of the standard avionics package, such as airspeed sensors and engine temperature probes. Others, like accelerometers and rotor indexers, are proprietary to the HUMS.
A HUMS works both reactively and proactively. The reactive approach allows the HUMS to detect any faults present in rotorcraft, while the proac-tive methods allow faults to be anticipated before they occur.
2.3.1 Sensors and Acquisition Procedures
The vibration monitoring part of a HUMS uses three types of data; ac-celerometer and tachometer signals, as well as contextual parameters such as airspeed, temperature and torque. The need for the latter category of data will be explained later. Accelerometers are mounted on all critical compo-nents, including gearboxes, engines and the bearing block for the tail drive shaft (Fig. 2.2).
The rotors are covered by accelerometers mounted on the airframe. Speed sensors are mounted on each engine compressor, engine out-put turbine, and on each rotor. The rotor speed sensors generate only one pulse per rotation, making it possible to know the position of the rotors relative to the vibration phase.

A HUMS solution for a large aircraft can require more than thirty ac-celerometers, making it impossible to acquire all accelerometers simultane-ously without generating enormous volumes of data. To counter this, all commercial HUMS solutions acquire only one component at a time with a .nite length acquisition. During .ight, the HUMS airborne segment cycles a preset program acquiring data from all components, one at a time.

2.3. HEALTH AND USAGE MONITORING TASKS

2.3. HEALTH AND USAGE MONITORING TASKS
As the transmission system of every rotorcraft is di.erent, so is the sensor positioning. Figures
2.3
and
2.4
shows
sensor
positioning
for
the
HUMS
EuroARMS
when
.tted
on
a
AS332L2
rotorcraft.
Figures
2.5
and
2.6
shows
an acquisition from the AS332L2 left hand ancillary intermediate gear in the temporal and the frequency domain.
2.3.2 Usage Monitoring
Two proactive methods exist. One is estimating the load on key each compo-nents, and integrating this over time to see the total stress the components have been subjected to. This allows the HUMS to estimate the remaining safe life limit for the components. The other method is simply detecting obvious misuse, such as engine overloads and over speeds.
Parameter Exceedance
Which parameters to monitor for exceedances and how to monitor them is aircraft dependent, but usually involves engine torque, engine temperature and rotor speed. Parameter threshold overshoots are automatically logged by the HUMS, together with additional information such as time, time over threshold, max value, etc. Traditional aircraft avionics displays a warning directly to the pilots whenever an event is detected. The pilots must then re-lay this information to the maintenance crew. The advantage of HUMS when recording excessive use is automated logging, more precise logging, as well as logging of additional information which helps determine the seriousness of the event, and consequently the best choice of corrective maintenance.
　
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