ASFKAP, I believe the B4 model uses an aluminum fin and rudder, but I stand to be corrected.
It seems to me that if a "tap" test (and listening to the sounds it makes) has a degree of effectness in determining if voids exist between the honeycomb inner core and outer skin of a composite structure, then this screams out to me that imaging ultra-sound methods could be the anwser for inspecting composite airframe structures.