My Quad Copter had some other issue that cause it flied up in RTL mode. I tried lowering the throttle to the bottom, and switching to Loiter mode, however, by mistake, I switched to Stabilize Mode, and then the Quad Copter leaned and “Free fall” to the ground.
It seems minimum rate (MOT_SPIN_ARMED) can not Stabilize the Quad Copter at all, and I’m wondering whether Stabilize Mode should be designed so that even the throttle is lowered to the bottom, if the aircraft is still in air, Stabilize Mode can keep a reasonable throttle level, to provide minimum “Stabilize” (the drone keep level)