2.4 Impact of Current Technology
2.4.1 Reliability
Although e.ective in capturing several drive train failure modes, all existing HUM Systems are also responsible for generating a substantial number of unjusti.ed warnings. The total number of warnings is aircraft and system dependent,
but
is
reported
by
the
operators
[11]
to
be
somewhere
between
4.5 and 12 pr. 1000 .ight hours, as a global average. The number of justi.ed alerts is typically in the order of 1 or 2 pr. 1000 .ight hours. Obviously, this number of false warnings can be quite overwhelming for inexperienced operators and the cause of a frustration for both HUMS personnel and man-agement. Also, this creates a signi.cant unregulated void in the procedures of rotorcraft operations.
All aspects aircraft operations are highly regulated. What maintenance work to perform, when to perform it, how to perform it, and what information to report to regulatory bodies and Original Equipment Manufacturer (OEM) is de.ned in .ne detail. This applies of course also to any .y / no-.y deci-sion, based on the outcome of maintenance inspections. The practical use of HUMS as a maintenance tool is however somewhat in contrast to this level of regulation.
Operators in the UK are obliged to submit documentation of their HUMS organizational structure and handling procedures to the CAA. Be that as it might, the day-to-day use of HUMS still leave waste room for subjective in-terpretation when it comes to HUMS based decision making. Even though the Eurocopter endorsed systems display reference to working cards in re-sponse to HUMS alarms, these can not be followed blindly. Obviously, a false alarm rate in the order of 4-1 would generate an immense amount of added (and unnecessary) maintenance work, if the alarms / working cards were to be followed without question. This leaves important decision making to the line technician or in best case to the company HUMS expert. As there is no formal training or certi.cation for the interpretation of HUMS output, it is up to each operator to maintain a level of training which ensures that safety is maintained. Thus, there are in reality no formal procedures for HUMS based decision making.
The Norne accident in 1997 did highlight the need for regulation of HUMS. In the Norne case, the aircraft was .tted with HUMS, but the sen-sor adjacent to the failed component was unserviceable at the time of the accident. If the HUMS would have been able to detect the fault, given a serviceable sensor, has been subject to debate. Regardless, the accident displayed the need for formal HUMS procedures and regulations, and was probably one of the contributing factors in the mandatory introduction of HUMS
in
the
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