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Load Cycle Calculation
All rotorcraft parts have a safe life limit. For most parts, the safe life limit is de.ned in .ight time. For more critical components, mainly engines, rotors and gearboxes, a safe life limit is also de.ned in load cycles. Al though .ight time gives a good pointer to the total stress a component has been subject to, .ight time does not re.ect the severity of the actual use of the aircraft. For this, a more reliable metric must be de.ned. The load cycle scale is a metric which more accurately re.ects actual accumulated component strain.
Load cycles are calculated using reliable metrics such as torque, engine temperature and rotor speed. The HUMS then keep accumulative counters for the components for which load cycles are used, and alerts the maintenance crew whenever a component is about to reach its safe life limit. Load cycles must be calculated on all aircraft regardless of whether a HUMS is installed or not. For aircraft with no HUMS, this task must be performed by other systems or by manual calculation.
Engine Power Assurance Check
Engine performance is gradually degraded throughout the lifetime of the engine. Performance must however not be allowed to drop below a certain minimum threshold. To ensure this, the engine Power Assurance Check (PAC) is performed at regular intervals, calculating the performance of each engine. The PAC consists in measuring the exhaust temperature needed to produce a given torque. On rotorcraft not equipped with HUMS, this procedure must be performed with engines running on the ground, using temporarily installed equipment.

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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