Field FBG monitoring of helicopter landing gear: data integrity screening and time–frequency characterization of operational events

Operational monitoring of helicopter landing gear can benefit from direct strain measurements able to resolve transient contact and roll-out dynamics. This work presents a field demonstration of a fiber Bragg grating (FBG) measurement and processing workflow applied to the main landing gear of an Erickson S-64 Skycrane operated by the Italian National Fire Brigade. Four gratings were bonded to the landing-gear structure, and wavelength data were acquired at 1 kHz during operational activities. The study does not aim to perform damage detection, health-state assessment, or hard-landing classification; rather, it evaluates whether field-acquired FBG signals affected by realistic acquisition imperfections can be transformed into interpretable time–frequency descriptors. Raw wavelength records were screened for dropouts, saturation artifacts, and spikes, then converted to normalized strain and high-pass filtered. Event-centered analyses based on Hann-window short-time Fourier transform (STFT), Thomson multitaper estimates, and ridge tracking revealed recurrent transient signatures and sensor-dependent frequency families in two representative datasets. The results support FBG-derived landing-event characterization under field conditions, while showing that synchronized avionics references, temperature compensation, and larger repeated datasets are required before validated Health and Usage Monitoring System (HUMS) or structural-integrity decisions can be made.