DO_CHANGE_SPEED didn't work with plane

Hi,
I just try to fly a plane with arduplane beta firmware.
I program some waypoint with DO_CHANGE_SPEED using GS, 12m/s, and 60% throttle… But i see that it didn’t change at all… it is always >14m/s with throttle >65%… is there a way to set the speed to some constant value?

Thanks…