As manual reference tells “Throttle Failsafe Value (ArduCopter:FS_THR_VALUE). The PWM level on channel 3 below which throttle failsafe triggers”. But if you have changed RCMAP parameters to custom channels mapping you’ll have a troubles. Because throttle failsafe will sense throttle channel value and set 3rd channel value with FS_THR_VALUE regardless of RCMAP settings. In my case 3rd channel was mapped to roll and I had permanent attempt to flip over right side. FS_THR_VALUE need to be set to throttle channel regarding to RCMAP settings.
I may misunderstand the principles of FS_THR_VALUE, but I had roll mapped to channel 3 and my quad with Pixhawk and APM:Copter 3.2 wanted to flip over right side when I increased throttle from minimum. Only with full stick left I could takeoff for a while. When I looked over log, desired roll was at fixed value near 25°(?). Than I changed RC mapping to default (throttle at 3rd channel) and remapped channels on my TX, and everything went OK. So I say that something about throttle is hardcoded to channel 3 regardless of RCMAP and I think it’s a throttle failsafe function.
It seemed I faced a known bug github.com/diydrones/ardupilot/issues/1661
There’s a workaround but you should be aware of it if you use RCMAP
There’s a bug in AC3.2 in that it’s not using the correct RCX_MIN and RCX_MAX parameters when RCMAP is used. The workaround is to set the range (on the transmitter) of the roll, pitch, yaw and throttle channels to all be the same, and then re-do the radio calibration and ESC calibration.