Damage Sensitive Features of a Repaired Helicopter Tail

Abstract

A helicopter tail, with a fatigue induced crack near the rear rotor gear, was repaired by a bonded composite patch that was instrumented with Fiber-Bragg- Grating (FBG) sensing net. A repair damage detection algorithm, based on the Principal Component Analysis, was used for tracking anomalies in the vibration signature as recorded by the sensors. Sensors' readings, taken during engine runs, demonstrated excellent mutual correlations, indicating that the repair still maintains its airworthiness capability after more than four years of service. Using this airworthiness assessment concept, the many advantages of bonded repairs (smoother load transfer, good fatigue and damage-tolerance behavior of the composite patch, ease of application on curved surfaces) are preserved without the need for complex inspections techniques, thereby minimizing maintenance costs and A/C down time.