The invention relates to an ascertainment method (20) for determining a final angle (E) of vanes that are adjustable between an initial angle and the final angle (E) and are part of a row of leading vanes (12) which are arranged upstream of a row of trailing vanes (13) of a compressor (12) in a gas turbine (10), in order to prevent ice from forming in the compressor (11). In the method of the invention, during operation of the gas turbine (10), a current value of at least one process parameter is determined in a parameter ascertainment step (22), whereupon the final angle (E) is defined in accordance with said value in a final-angle defining step (23), an aerodynamic speed being determined as the process parameter.