According to quality parameters, DS472 (Danish Standard) and GL rules, wind turbines must have two independent braking systems. However, the IEC 61400-1 requirement does not specify what kind of two braking systems must have, but requires the protection system to remain effective even after the failure of any non-safe-life protection system component.
Usually, it makes sense to supply both aerodynamic and mechanical braking. Aerodynamic brake is more benign than mechanical braking, so it is always used in normal shut-downs. On the other side, although a braking system must be used during shut-downs, sometimes technicians allow rotor free turns (idle) within low winds. This strategy reduces the frequency of imposition of braking loads on the gear train.