Downloaded from https://www.everyspec.com ADS-79D-HDBK APPENDIX E VIBRATION BASED DIAGNOSTICS E.1 SCOPE This Appendix addresses Vibration-Based Diagnostics. It covers the use of sensors, acquisition systems, and signal processing algorithms to detect, identify, and characterize faults in aircraft mechanical systems. The process involves extracting features from the vibratory data and comparing the feature characteristics to a baseline set of limits (or thresholds) which indicate the severity of a potential fault. The diagnostic algorithms should also indicate a recommended maintenance action.
Another application for vibration-based diagnostic systems is rotor track and balance, or rotor smoothing, to reduce rotor vibrations. Rotor smoothing is applicable to both the main and tail rotors. Tracking and balancing a rotor is done by adjusting weights, trim tabs, wedges, and pitch link length to minimize the rotor‘s fundamental harmonic vibrations. Rotor smoothing is important to minimizing loads on life-limited dynamic components in the rotor system, improving aircrew human factors, and reducing vibration in non-rotor system components which reduces vibration induced failures. Rotor smoothing and balancing procedures are discussed in Appendix F.
Vibration measurements are collected from sensors such as accelerometers at periodic intervals under specific aircraft operating conditions. This accounts for the effects of variations in aircraft loading and drive train torque on the characteristic vibration signatures. Raw vibration data from the sensors is collected in the time domain then typically transformed to the frequency domain to obtain the vibration spectrum. The vibration data may be synchronized with at least one tachometer that produces a pulse at the same rate as the fastest rotating component of interest (order ratio analysis).
This synchronization process will permit effective filtration of spectral content from other components not of interest for the most accurate calculation of fault features. Features are then extracted from the spectrum and used to calculate the Condition Indicator (CI). One or more CIs may be used to calculate an aggregate Health Indicator (HI). The CIs and HIs, or HIs are then compared to thresholds to specify the component condition and maintenance status.
E.2 REFERENCES AND APPLICABLE DOCUMENTS (Copies of these documents are available online at http://www.faa.gov/regulations_policies/ ) (Copies of this document are available from http://www.sae.org/standards/ or SAE World Headquarters, 400 Commonwealth Drive, Warrendale, PA 15096-0001 USA. Phone (US) 1-877-606-7323 100 FEDERAL AVIATION ADMINISTRATION FAA AC 29-2C, Chg 3 Certification of Transport Category Rotorcraft. 30- September 2008. FAA AC 27-1B Certification of Normal Category Rotorcraft, 30 September 2008. SOCIETY OF AUTOMOTIVE ENGINEERS (SAE) INTERNATIONAL SAE Aerospace, Aerospace Recommended Health and Usage Monitoring Metrics Monitoring the Practice ARP5783 Monitor. 19 Feb 2008.