How to make the LinearVelocity constraint maintain a constant speed?

Hello
I have a plane/rocket which travels in world without gravity.
I want it to travel at a constant speed (10 studs/sec.) at the direction at which the plane nose is facing.
I tried with VelocityConstraintMode=Line and Relative to Attachment which is positioned at the center of the mass of the plane.
This works until any kind of external force is applied on the plane. After that the plane continues to move with a speed bigger or smaller than the speed, which I have set as Line Velocity.
Isn’t linear Velocity constraint supposed to keep a constant speed?

Thanks in advance