Fiber optic based in-flight structural health monitoring application implemented on a gearbox support beam of a helicopter

A critical crack was identified in the main gearbox support beam of the largest Israeli Air Force (Sikorsky CH53) Helicopter. The crack has developed on the main supporting beam months after it had been repaired. Interestingly, this crack was anomalous in its position and direction and could not have been predicted by conventional Finite Elements (FE) based fatigue analysis. As a necessary step to better understand the actual loading conditions acting on this complex structure and the implications on the growth mechanism of the crack, an inflight fiber optic based Structural Health Monitoring (SHM) concept was developed. Multiple Fiber Bragg Grating (FBG) sensors, embedded in composite "Smart Patches", were installed to fully monitor the structural behavior of the cracked beam during dedicated flight tests. These flight tests varied in their takeoff weights and involved carefully selected maneuvers, designed to have the highest load impacts on the beam. This paper reports the flight data evaluation as part of this SHM concept and introduces the new insights obtained throughout the entire process: from the design stage to the implementation. By analyzing the flight measurements obtained from all smart patches, the factor which most affected the crack growth could be directly inferred. It is expected that this successful experience will potentially lead to broader usage of in-flight SHM systems, which will serve as a preventive, as well as investigative, tool.