Effects of anti-icing fluid secondary wave on wing section lift degradation during simulated takeoff were studied in Aalto University 2 m x 2 m Low Speed Wind Tunnel during 2012-2014 and 2016-2017. The existence of secondary waves has been frequently reported since the first studies of anti-icing fluid effects on the wing lift degradation. However, the studies on its quantitative contribution to the lift degradation are scarce. The effects of wing section geometry and configuration on the secondary wave have not been addressed in the previous studies. In the present study two different wind tunnel models were used: a DLR-F15 wing section with a chord of 0.65 m and a modified HL-CRM wing section with a chord of 0.63 m. Both were two-dimensional three-element models. The lift coefficient measurements suggested that the secondary wave may have a dominating contribution to the fluid-induced lift degradation. The slat size and geometry turned out to have a significant effect on the formation and size of the secondary wave.