Maybe I was lucky and got the Altitude Hold (AH) worked the very first time.  After I have the V2.5.5 firmware installed and fixed my latest crash, my AH no longer work.  Once I switch to AH mode, the heli immediately slammed onto the ground with negative collective pitch.  It happened once way before V2.5.4 and I thought it was an isolated incident.  Luckily I already have a very strong 500 land…