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operations
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[47].
To improve rotorcraft airworthiness, several steps were recommended. Among them was the permanent installation of vibration monitoring equipment.
Vibration monitoring of mechanical systems was at the time already an established technology. Although not previously tested on aircraft, such tech-niques had already proven their e.ectiveness in condition monitoring of in-dustrial machinery, such as paper mills and power plants. However, it was not until the eighties that the size and weight of the necessary numeric hardware were in such a manner that it could be .tted on a helicopter.
By the end of the eighties, two parallel trials were under way. Motivated by the HARP report, and largely sponsored by the petroleum industry, these programs aimed at testing the concept of in-.ight vibration monitoring. One of the programs was led by Steward Hughes Limited (SHL) / Teledyne, the other by Meggitt Avionics. The purpose of the trials was however more to create a proof of concept, than re.ning diagnosis algorithms. By the time the trials ended in 1991, the technology was, however promising, still regarded as immature.
In 1990, CAA issued new regulations making Flight Data Recorders
13
(FDRs) mandatory in helicopters operating in hostile environments. The avionics manufacturers participating in the HUMS trials saw this as an op-portunity to introduce their newly developed technology to the market. With the operators’ and the petroleum industry’s increasing interest in the tech-nology, creating combined FDR / Cockpit Voice Reorder (CVR) / HUMS systems had obvious competitive advantages. Thus, two FDR / CVR / HUMS systems were put on the market; SHL’s North Sea HUMS and Meg-gitt Avionics’ IHUMS.
Al though not mandatory by law, the oil companies’ great interest in these systems made them an important burgeoning point when negotiating service contracts with the rotorcraft operators. As a result, HUMS quickly became a reality for all operators involved in o.shore .ight, on both sides of the North Sea. In 1999, the CAA issued regulations making HUMS mandatory for all heavy rotorcraft registered in the UK.

2.1.2 Motivations
As already mentioned, the initial motivation for introducing vibration mon-itoring in helicopters was safety. However, it soon became clear that a tool capable of describing the actual condition of critical components had consid-erable potential in maintenance planning and cost reduction.
Aircraft maintenance workload is normally measured in Mean Man Hours / Flight Hours (maintenance) (MMH/FH). Maintenance workload is highly dependent on aircraft size and type, and can be found anywhere from less than one hour to several hours per .ight hour. Compared to equal size turbo prop airplanes, even the most economical rotorcrafts have very high operating cost due to maintenance. In fact, around 25% of the total life cycle cost (LCC) for most helicopters is maintenance, equivalent to the acquisition cost.
　
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