Dear Syed,
As discussed in the following forum topic: FAST linearization V7, once you linearize the FAST model, apply MBC3, and azimuth-average the resulting matrices, you’ll end up with a linear state-space model without the azimuth state. Thus, when comparing the linear and nonlinear models, I would suggest applying MBC3 to the result of the nonlinear model in order to compare to the result of the linear model.
Best regards,