Nice and calm this morning so decided to have a quick flight before work. Attached props as usual, completed usual pre-flight checks. Q500+ positioned in open space with no obstacles. Started motors and all running OK. Increased revs on motors and drone lifted to around 3 feet when it went 'BANG' one of the arms holding a 'A' motor snapped causing it to come crashing to the ground. Spoke to Yuneec UK and they have never heard of this before - any suggestions would be useful. Thanks.