1. Method to operate an air spinning machine, in which the air spinning machine has at least one spinning unit with a spinning nozzle ( 2 ) for manufacturing yarn ( 6 ), in which a fiber strand ( 3 ) is fed to the spinning nozzle ( 2 ) through an inlet ( 4 ) during the operation of the spinning unit, in which the fiber strand ( 3 ) is imparted a twist inside a swirl chamber ( 5 ) of the spinning nozzle ( 2 ) by means of a swirled air current, so that a yarn ( 6 ) is formed from the fiber strand ( 3 ) that finally leaves the spinning nozzle ( 2 ) through an outlet ( 7 ), and in which an additive ( 9 ) is added with the help of an additive dispenser ( 9 ), at least temporarily, to the spinning unit while the air spinning machine is operating and applied on the fiber strand ( 3 ) and/or the yarn ( 6 ) or on sections of the spinning nozzle, characterized in that at least one physical parameter of the yarn ( 6 ) leaving the outlet ( 7 ) is monitored with the help of a sensor system ( 11 ) whereby, based on at least one measured value supplied by the sensor system ( 11 ) correlated with the abovementioned parameter, it is determined whether and/or how much additive ( 9 ) was applied on the fiber strand ( 3 ) or the yarn ( 6 ) manufactured from that passed through the sensor system ( 11 ).