Experience in setting thresholds for mechanical diagnostics in the UH-60L fleet demonstration

The UH-60L Fleet Management Demonstration Program is a two phase, two year program to gain operational experience with an integrated Health and Usage Monitoring System (HUMS). Thirty Sikorsky UH-60L BLACK HAWK aircraft of the 4th Battalion of the 101st Airborne Assault Division have been equipped with Goodrich IMD-HUMS and Aero System Sapphire maintenance management information system (MMIS). The program is aimed at evaluating the IMD-HUMS and MMIS, exploring the impact thereof on maintenance and assessing their feedback on overall fleet management and OEM engineering inputs. One of the functionalities of HUMS is the mechanical diagnostic monitoring of the helicopter drive train components, including main transmission and tail drive. The main requirement of mechanical diagnostics is to detect when a component degrades from a healthy state to an intermediate non-health and ultimately faulted condition. The mechanical diagnostic monitoring is typically accomplished by accelerometer based vibration monitoring, with respect to appropriate thresholds on features (condition indicators) derived from advanced vibration signal processing. A formal procedure for setting thresholds was previously present by the authors ("Setting HUMS Condition Indicator Thresholds by Modeling Aircraft and Torque Band Variance", E. Bechhoefer, A. Bernhard, 2003 IEEE Aerospace Conf). That approach focuses on setting thresholds based on controlling false alarm rate (in the absence of faulted data statistics). This paper presents the successful implementation of that approach to the thirty U-60L BLACK HAWK aircraft fleet and will evaluate the robustness and merit of the approach. Review of selected raw, time domain data is used to provide further insight into the evolution of condition indicators and the validity of the thresholds