From Bell:
The OSN quite clearly states the Bell Helicopter position as firmly
against the use of any of our helicopters for aerobatics or any
operation prohibited by, or beyond the guidelines of the approved
Rotorcraft Flight Manual. Aerobatics is a form of flight for which Bell
Helicopter has no flight test criteria, knowledge of the imposed loads
or the potential fatigue damage that may result. From the very limited
flight test performed by Bell in severe maneuvers, it has been proven
that flight loads encountered are highly variable and very damaging.
Through engineering flight test communications, we are advised that
disposition of virtually every helicopter that has performed aerobatic
maneuvers has been scrap or at least permanent removal from service.
For these reasons, Bell cannot over-emphasize the danger associated with
performing aerobatics or other flights beyond the approved operating
envelope.