2.3.3 Health Monitoring
The reactive part of a HUMS consists in detecting faults in the drive train as they occur, but before they become critical. This is a challenging task, as the system must be able to detect early in the propagation process, while at the same time not generate unjusti.ed alarms.
Engine Vibration Monitoring
During engine power up and stabilized speed, temperature and vibration lev-els must be within certain limits de.ned by the engine manufacturer. These levels must be monitored at regular intervals to maintain airworthiness. For most HUMS, this task is performed automatically at each engine startup. On rotorcraft not equipped with HUMS, this procedure must be performed on the ground, temporarily installed equipment.
Transmission Monitoring
The health monitoring function tries to capture component condition using accelerometers mounted on the engines, gearboxes and shaft bearings. Chip detectors are also used on engines and gearboxes. A chip detector is capable of detecting metal debris in the lubrication. All rotorcraft are equipped with chip detectors generating cockpit warnings.
Rotor Track and Balance
In order to avoid violent vibrations at once per revolution of the main and tail rotor, the rotors must be well balanced. In addition, the track of each
2.4. IMPACT OF CURRENT TECHNOLOGY
blade must be adjusted, relative to the mast. Balance adjustments are made by adding or removing weights in the blades. Track is adjusted by changing the blade angle and pro.le. The vibration recordings required to calculate these adjustments are acquired during normal .ight.
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