rotor disk disappears

[lama12]

sometimes the rotor disc is not visible when flying on front of collision objects with ALPHA set to 0.5.
It is reproducible meaning that it always happens at the same locations but not everywhere or with all objects.

[lama12]

this screenshot shows a situation where half of the rotor disc is visible and half isn't, right on the corner of the skybox. The trees in this case are the collision object with alpha=0.5.

hopefully this will give some hint for where the cause may lie.