Most research work has been performed within laboratory environments with seeded defects or accelerated testing and there is little published in the public domain on in-service defects with operational helicopter HUMS data. This study focuses on the actual service experience gathered from HUMS equipped AH64D helicopter belonging to the Republic of Singapore Air Force. Operational HUMS data from three helicopters with similar in-service defects found on their Tail Rotor Gearbox are analyzed and correlated with tear-down inspection findings. From analysis of the HUMS vibration spectrum, fault patterns which show the progression from localised to generalised damage in the bearing can be observed consistently across the defective gearboxes. The observed fault patterns are extracted as Condition Indicators (CI) to diagnose the different stages of bearing degradation within the gearbox. As the defects between the gearboxes are similar, the historical trends of the extracted CIs are used to developed prognostic model using parameter estimation approach.

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