APM 2.7 Trex 450 fly bar serious up / down oscillating and crash

Anticipating venture into traditional heli as a main VTOL machine, I learn to set up my old Trex 450 with APM 2.7. I manage to set the rate roll / pitch and yaw and use other parameter as per manual. I manage to get it hover in stabilized mode. When I switch to Alt Hold mode, the model spin up as I activate the switch, I advance the throttle / pitch over 50 %, nothing change or happen, as I continue to advance the throttle, to a point where the heli start to lift off, immediately it goes up and down violently and the main blade pull off from the blade holder within seconds and crash.
As I’m completely new in traditional heli, I’m not sure what parameter control this behavior and the manual seems do not offer more info in this area.
Any help will be much appreciate. Thanks

The logs from the APM would be your first point of analysis.
Have a look at your IMU data, especially the Z axis access vibration levels as I have found they have a profound effect on altitude fluctuations as you described.
With multi rotors I find the safest way to test modes is to take off in Stab, be at a safe height, and switch modes, being ready to switch back to Stab if things don’t go right.

Hi Keeyen, I assume the ACCEL_Z_P is still at the multirotor default of 0.500? Try changing it down to 0.100 and go from there. Usually it ends up in the range 0.100-0.200.

Thanks you Rob. I’ll change that after get it repair.

I wish we could fix the default param for helis, but it’s not easy because it’s shared with multirotors.