Weird loss of roll control in FbwA, then RTL got away from home!

Hello to all,
I would appreciate your help with this weird crash we had with a Bormatec Maja plane with arduplane 3.7.1 in a Pixhawk.
We checked everything before take off, and launched the plane in FbwA. The aileron control did nothing and plane flew in a very large radius circle and we couldn’t change cruise. Then we switched to RTL, the plane gained altitute as expected but it flew away from home, still in the same large radius circle. In RTL mode, however, a slight movement of the roll joystick produced a sharp turn, like a very sensitive “nudge”. Throttle control was also weird, with constant changes of throtle, it finally crashed.
Looking at the logs, it seems that navROLL in telemetry logs and desROLL in datasflash roll are always zero (exact zero), even when trying desesperately to steer the plane in FbwA. After the crash, the ailerons moved ok with the transmitter joystick in manual mode, but did not move at all in FbwA.

Any help to understand what happened will be very much appreciated. We have tried to fly this plane three times, everytime it crashed in a new, unexpected and weird way, I don’t mind the crashes, but I would like to learn why!!!

Thank you very much in advance,
parámetros maja después estrellamiento no giraba.param (11.3 KB) 20-14-44 estrellamiento no giraba.tlog (813.2 KB)

Not a direct answer to your question, but can you fly the plane in manual? Do you know how to fly in manual?
Ardupilot is great, but going back to manual is very handy when it’s not doing the right things. Sometimes you don’t have enough time to switch back to manual and recover, but it sounds like in your case, you did.
Knowing that the plane flies properly in manual would be a first step in confirming that the basics are working.
(also, and you probably know this, but using any FBWA or other, when you haven’t done an autotune yet, is not great news since ardupilot doesn’t really know how to “fly” your aircraft until you’ve autotuned it)

Thankyou macmerlin for your reply. You are right, manual mode could have saved this crash. I don’t even try to use manual mode because, in my experience, FbwA even with default paramaters fly planes much better than me in manual mode.
In this case, the transmitter can control all the control surface in manual mode, but in FbwA mode, the ailerons do not move at all, the weird thing is that it was working before with the same parameters, I would like to know if there is a parameter that “connect” de CH1 from the transmitter to the ailerons that could be wrongly configured somehow.

Ok, first you indeed should not even take off if you cannot control the servos in FBWA mode, because indeed it’s not going to work.
Can you control both elevator and rudder in FBWA?
Is your aileron servo connected in channel 1?
Is your other aileron in channel 5?
I see you have RC5_FUNCTION,4 so that’s good.

Thankyou Marcmerlin,
I just found the problem! I had changed the parameter LIM_ROLL_CD from 45 to 35deg trying to avoid a previous crash that seeemed to be related to a stall, but I put 35, not 3500 centi-degrees!! so the maximum roll was 0.35deg, not a huge margin … a rookie mistake.
You are right about checking the servos before take off. … we just missed that because they were moving fine in manual and in AUTO mode (as I understand that the steering or nudge is not limited by this parameter), and we tested FbwA but only its own stabilization, in fact, I remember my friend saying “stop moving the joystick! I can’t see if it stabilizes ok!” … :grin::grin::grin:

Thank you and regards,