Dear Vishvas,
“Yes,” FAST uses blade inertia in the equations of motion. And “yes,” the inertia changes with blade deflection, which is why the fixed (undeflected) value of RotIner that is written to the FAST summary (*.fsm) file is not used by FAST’s equations of motion.
More inform on the FAST theory basis is available in the following forum topic: Coupled blade modes in FAST.
Best regards,