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Maintenance can be divided in two main categories: Condition Based Maintenance (CBM), and Time Based Maintenance (TBM). CBM represents the maintenance tasks which are generated as a result of faults uncovered during inspections, faults uncovered by HUMS, and operational irregularities, such as torque limit overshoots or rotor over-speeds. TBM, on the other hand, is performed at various .xed intervals. Some are just a few hours apart, or even between each .ight. This is the tedious day-to-day work of inspections, to ensure that the aircraft is in an airworthy condition.
The TBM workload is very high on rotorcraft compared to most other vehicles, and is one of the main cost drivers in helicopter operations. This is due to the large amount of moving parts in the helicopter transmission system, as well as the lack of redundancy in the power path from engine

2.1. BACKGROUND
to rotor. Because of the lack of redundancy, several failure modes in the helicopter transmission system can be catastrophic. To minimize risk, very strict and expensive maintenance routines must be followed in rotorcraft TBM.
Every component on a helicopter has a safe life limit. Upon reaching this age, the component must be overhauled. The safe life limit of each component is derived from an expected usage spectrum of the aircraft, and then given a substantial margin. Consequently, most retired parts are in a perfectly good condition. However, if an aircraft is exposed to higher loads than what was anticipated when the maintenance schedules where created, components might
be
exposed
to
more
stress
than
they
where
design
to
handle
(Fig.
2.1).

Most of the inspections and overhauls performed as TBM are unneces-sary, in the sense that maintenance is performed on helicopters which are in a perfectly airworthy condition. This is however the proactive nature of TBM, if one is to ensure that the possibility of mechanical failure is mini-mized. Obviously, helicopter operating costs could be decreased dramatically if one were to perform maintenance only "on condition" (CBM), whenever a failure occurs. However, performing corrective maintenance after a fault has occurred will in most cases pose an unacceptable safety risk.
This is, of course, unless one has a reliable way, other than manual inspec-tion, to detect a propagating fault before it becomes critical. HUMS was, and still is, regarded as the answer to this problem. In addition to increase safety, HUMS was seen as the technology that would revolutionize rotorcraft maintenance, and shift rotorcraft maintenance strategy from TBM to CBM. For various reasons, these ambitions have so far not been reached.
2.1.3 Regulatory De.nition
The only formal de.nition of HUMS is maintained by the UK CAA, as the UK is still the only country where HUMS is mandatory. HUMS is mandatory for helicopters in the following category:
United Kingdom registered helicopters issued with a Certi.cate of Airworthiness in the Transport Category (Passenger), which have a maximum approved seating con.guration of more than 9 passengers.
　
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