An approach to fault diagnosis of helicopter planetary gears

Abstract

Failure of flight critical components on-board a helicopter could cause an accident resulting in loss of life and/or aircraft. It is imperative, therefore, that precursors of such failure modes be monitored continuously and remedial action be taken as soon as feasible in order to avoid catastrophic events. A crack in the planetary carrier of a UH-60 Blackhawk main transmission has received recently extensive evaluation through analysis of vibration data (J. Keller et al., 2003). The rotorcraft main transmission includes a planetary gear train comprising an inner gear surrounded by five rotating planets. Torque is transmitted through the sun gear to the planets, which ride on a planetary carrier. The planetary carrier plate, in turn, transmits torque to the main rotor shaft and blades. The U.S. Army Aviation Engineering Directorate conducted a series of experimental tests with faulted and unfaulted carrier plates on a test cell and also on-aircraft to determine if a fault (a plate crack) can be detected via vibration monitoring. This paper introduces a methodology to analyze raw vibration data provided to Georgia Tech from the carrier testing. The analysis approach consists of selection and extraction of appropriate features from vibration data indicative of the fault condition and the construction of an optimum feature vector. Test cell data sampled at 100 kHz for torque cases ranging from 20% to 100% were processed. On-aircraft data covered a limited torque range up to 30% due to safety considerations. Both raw and time synchronous data were considered and features in the time, frequency and wavelet domains were investigated. The analysis results indicate that a selected subset of features clearly distinguishes between the faulted and unfaulted cases.