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Although an e.ective cost saver in some areas, HUMS contribution to reduced TBM is a di.erent matter. As mentioned in previous chapters, the probability of a technical failure in rotorcrafts is minimized through regu-lation. The consequences of system fault in a given component is put in one of the following categories; Catastrophic, Hazardous / Severe, Major or Minor. The probability of component failure must be no greater than 10.9 , 10.6 or 10.3 pr. .ight hour for the three upper categories respectively. For the rotorcraft transmission system, most components fall into the two upper categories. This means that a HUMS function set to monitor a component which is "only" of Hazardous / Severe criticality must still have a probability of failure less than 10.6 / Flight Hour. This is a long way from the average detection rate of 70% experienced with the current systems. Although some of the diagnostic functions are well above 70%, there are still large regulatory boundaries which must be overcome on order to have any credit granted.
A major cost-driver in avionics development is the problem of hardware and software certi.cation. A HUMS system which is to be quali.ed to Haz-ardous / Severe for a given function, must have airborne software certi.ed in accordance to DO -178 B Level B, which in itself is a feasible task. However, system criticality assessments are performed end-to-end. For instance, if a fault is captured by the airborne segment, but lost at the ground station due to buggy software, safety is obviously not maintained. For a Hazardous / Se-vere certi.ed HUMS, this translates into level B software also on the ground station. As no operating systems are certi.ed above level D, the entire ground station software, including operating system and hardware drivers, must be built from scratch. Further, all this software must also be certi.ed to level B, which is a very expensive and time consuming task for such a large amount of software.
In theory, some mitigating solutions can be made to avoid this problem. This can for instance be to develop the software for two di.erent platforms (OS + HW), and show that both solutions create identical results. Unfortu-nately, the Federal Aviation Authority (FAA) does not allow Commercial O. The Shelf (COTS) solutions containing software below level B in these cases. This means that custom made hardware must be ordered and certi.ed for the ground station. Such a procedure would probably be even more costly than a level B software solution.
Given some improvements in detection reliability, HUMS has in theory a clear potential in the reduction of TBM. It is however di.cult to see how any progress can be gained under the current regulatory regimes.
2.5 Objectives
The focus for this study is identifying methods which will improve fault de-tection rates and reduce false alarm rates for the health monitoring functions of EuroARMS and M’ARMS, two commercially available HUMS implemen-tations manufactured by Eurocopter. An additional objective is to increase the autonomy of these solutions, so that they require little or no con.gura-tion by the user. The main axis of research is improving the fault detection methods which are based on vibration monitoring. Other sensor technologies for detecting propagating damage will also be discussed brie.y. Further, the
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