Dear Fangfang,
I’m really sure I understand your question, but “Aero_Torq” is not actually an output of FAST. My post dated Jun 10, 2010 in the following forum topic explains the difference between RotTorq, which is an output of FAST, and the applied aerodynamic torque (AeroTorq): FAST-Aerodyn Beddoes Stall Mode.
Our next release of AeroDyn within FAST v8 later this summer will include applied aerodynamic load outputs.
Best regards,