Hi everyone, I thought it might be interesting to post what it just happened to me last Friday. I was flying my Octa-Quad with AC 3.4.dev (an old version that I use for testing, but the very same happened with the most recent 3.4.6 when I tried in another test), Taranis X9D and receiver X8R. 

I set the receiver failsafe to NONE and I disabled the failsafe on the autopilot, as I was about to do an Auto Mission. My flight modes settings are such that STABILIZE is engaged when CH5 < 1200. 

The issue: when the radio is at a range where the connection gets weak (so it keeps connecting/disconnecting), the system is driven to STABILIZE....! Looks like the Autopilot actually listens to a NONE signal or, most likely, the receiver "ramps the PWM up"  from none to the transmitter's value. This weird behavior does not happen if I turn the radio off, only when the reception is poor.

In the flight log you can see the vehicle coming back into RTL but then switching to STABILIZE. Plotting the CH5 you can see the signal going from 874 to some other value (in this case the one to trigger the RTL), and the transition to STABILIZE just when the CH5 ramps up. 

During that day I tested the system extensively (on the ground...). I set the Taranis in RANGE and put the system far enough to make the radio transmitter disconnect/connect continuously. That behavior happened very often, independently from the Autopilot version or board (tested on two different multirotors).

The TURN AROUND: I set the receiver failsafe so to hold the CH5 and NONE to the throttle. In this way the autopilot understands the connection has been lost, though there is no more messing with the flight mode.

Any ideas or thought? I see this argument has been probably discussed in the documentation, even though it looks it is referring to earlier versions)

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Apparently your receiver is set up to failsafe. Nothing to do with Ardupilot really.

Thanks, I figured it is something due to the receiver. I thought it would be better to share my experience to warn other FrSky users

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