I was the crewman on Wazz& Zoom's aircraft. Just to back him up. the a/c was AP out so there should be no inteference from the AP system. We were AP out purely for a handling exercise for the student. The vertical bounce onset almost instantly and W&Z took appropraite action for us to clear a refuel area and in the process of landing/reselcting the AP in the problem solved itself. Witnesses from an adjacent building sadi they had never seen the underside or top of the a/c in such clear view in the space of a few seconds from about 50metres.
All I could do during the event was hold on ( was on harness in the cabin) and sit in a seat. The bounce was so severe that I could not focus within the cabin, only outside and I could not release my grip to strap into the seat.
Th incident has been reported to make sure people are aware of what happened.
A similar incident occured a while ago on the same a/c where instead of vertical bounce, whilst hover taxying downwind and AP out, there were oscillations from both engines with associated oscillations of RRPM and torque fluctuations and resultant yawing. This was reported to the engineers but no incident raised at the time. Again, no fault found!