The inspection and detection of damage in composite materials using Lamb waves are particularly effective because Lamb wave can propagate over relatively large distance and hence can cover a large area with few testing time and equipment. However, comparing to the dispersion features that have been investigated systematically in the literature, predicting simply and reliably the spatial attenuation, which is the decrease of the amplitude of the propagating wave with distance, is still a challenge especially for structures large enough to industrial scale. In this paper, a simple model able to predict Lamb wave attenuation for different frequencies, which takes three damping models, Hysteretic, Kelvin-Voigt and Biot models into account, is derived directly from dispersion equations. Experiments on a practical aeronautical component, a Fan Cowl Structure, are carried out to validate the developed model. The merits of the proposed method lie in the fact that it is derived directly from dispersion equations instead of relying on complex finite element models and are thus simple to compute. Despite its simplicity, it is still effective when predicting attenuation coefficient for geometrically complex structures such as the Fan Cowl Structure.