A Case for Health Indicators vs. Condition Indicators in Mechanical Diagnostics

摘要

Health and Usage Monitoring Systems (HUMS) have been proven to be indispensable in improving aircraft safety, readiness and reduced maintenance cost. While there is some consensus in the helicopter community as to what constitutes HUMS functionality, there is still a question as to what mechanical diagnostics (MD) is and how MD information is displayed to the user. MD has at least two paradigms for the evaluation of a monitored component's condition. The first paradigm will be identified as Condition Indicator (CI) based thresholding, and the second paradigm will be called Health Indicator (HI) based thresholding. In most HUMS systems, a vibration signature is processed into a set of Condition Indicators (CI). For many components, the CI have physical meaning. As an example, shaft order 1 (SO1) is the magnitude of the shaft vibration in inches per second of the first harmonics. Other CIs' have limited physical meaning, such at gear distributed fault. In the first paradigm, these CI can be viewed and trended on a ground station. In some cases, the original equipment manufacturer will supply limits for the CI. As an example, SO1 limits may be given for the engine input drive shaft. In most cases, however, there are no established limits and a statistical procedure is used (e.g. a threshold is set such that the probability of the component being normal, when exceeding the threshold, is small) to trigger maintenance In the second paradigm, multiple CI for a given component are fused into one HI. The paper outlines the advantages and disadvantages of these methodologies and makes the case for the HI.