In recent decades, the aviation industry has increasingly adopted composite materials for various aircraft components, due to their high strength-to-weight ratio and durability. To ensure the safety and reliability of these structures, Health and Usage Monitoring Systems (HUMS) based on optical fibers (FO), particularly Fiber Bragg Grating (FBG) sensors, have been developed. However, both composite materials and optical fibers are susceptible to environmental factors such as moisture, in addition to the well-known effects of mechanical stress and thermal loads. Moisture absorption can lead to the degradation of mechanical properties, posing a risk to the structural integrity of aircraft components. This research quantifies the impact of humidity on FBG sensors and introduces a new formulation of the Bragg equation, which accounts for the effects of mechanical strain, thermal expansion, and hygroscopic swelling. The developed mathematical model, derived by differentiating the Bragg equation and applying the rules of mixtures, provides an accurate measurement of the Bragg wavelength variation, addressing potential measurement errors caused by moisture absorption. This approach is crucial for improving the performance and reliability of HUMS in monitoring composite structures, ensuring long-term safety in extreme environmental conditions.