Check this thread and see if it fixes your problem:
Thanks, but I'm not using PPM.
The TX and RX I tried are DEVO 10 and DX8.
Both haven't solved the problem.
I'm almost sure it has nothing to do with the TX or RX I'm using.
Only the APM's input ch6 is kind of faulty and causes the motor twitches when it gets a PWM input of about 75% to 100%. Below 75% are no twitches at all.
Everything else is working fine.
This problem is due to the internal PPM encoder in the APM. It comes into effect when you don't use CPPM to connect your radio (AFAIK). I recommend you update the PPM encoder firmware in your APM if you haven't. It's a separate operation from normal firmware flashing.
today I updated the PPM encoder firmware as you suggested and I went outside to test it.
The twitching it is gone !!
It flies perfectly now. I would've never thought to update the PPM encoder even if I don't use PPM at all.
It works so thank you a lot !!
That's great! Glad I could help.