the only time i've experienced a boom strike is by holding the heli and repeatedly going from full positive to full negative several times in a row for about thirty seconds. Why do this you ask. Strength testing the head unit, and seeing how much the head can take. i would rather do this in a controlled enviroment than 30 40 50 feet in the air. Your blades could have also been out of tracking which caused them to become closer to the boom when you threw it into negative pitch. Or to much slop in the head unit causing the strike. Or it could have been a loose part, loose link, and some part that was not in alignment. None the less sorry for the loss. luckilythe parts are not too expensive. Good luck on your next invert. And i'm glad to see people starting to get these things inverted and posting on here.