You never said what the prop size was you are using for the 55ax and what rpm's were you turning on the ground. A friend of mine and myself has Kaos's and he has a 55ax and I have a 45fx and we both use APC props and both turn ( on ground ) around 11,500rpm's and have no problems. So I am wondering if it was cold and you were turning high rpm's like we do and those master airscrew props flex a lot they could have broken from flex fatigue. Just a thought!