Copter Disarmed and Crashed in Auto Mode immediately after No RC Receiver Failsafe

We have completed 100’s of Auto Missions with this same hardware/software setup and never had issues before.
Running 3.5.7 Firmware
Hardware is as follows:
Here GPS
Horus X10S Transmitter
FrSky L9R Receiver

We were in the middle of an Auto flight mission and received a No RC Receiver Error Message and it went into RTL Failsafe Mode, then it Disarmed and fell from the sky. It was a total loss because it caught fire and everything burnt to a crisp. We have the tlog only, link below:

Would this have not happened if we were running 3.6.5 or newer Firmware?
It appears that there was an RC input fix which stated “Mode and Auxiliary switch range check to protect during FrSky SBUS failsafe recovery”.


It is not clear for sure for me from tlog what was the exact reason but I suppose one (or more) following reasons:

  • FrSky receivers failsafe behavior is bad, when the receiver is connected via SBUS. Sometimes receiver could report to FC that RC link is restored but still provide bad RC channel values.
  • your THR_FS is 925, but FrSky can output 984 while in near-FS state
  • because of the above reasons flight mode was changed to Stabilize, instead of RTL or AUTO. May be throttle channel was also at low level so copter flipped and autodisarmed.
  • There is at least one funny bug was fixed recently: if you will switch AUTO-STABILIZE-AUTO really fast - copter may abort AUTO mission with just a descending with no obvious reason. That’s because ardupilot don’t likes rapid mode changes. Who knows if there some another related issues remaining…

And about the “Mode and Auxiliary switch range check to protect during FrSky SBUS failsafe recovery”
I believe it will not help because it only fixes aux switches. See

So for myself I have two rules that I hope shall help avoiding such problems:

  • turn of RC transmitter when in AUTO to prevent RC failsafes
  • never set Stab or Acro or any manual flight mode as the first mode. Better to have RTL or HOLD as first mode.

Hi Sergey,

Thanks for the quick response!
Yes, we had the FrSky receiver connected via SBUS.
Although, we’ve never had this problem in the past and have had many “No RC Receiver” Failsafes during 100’s of AUTO flights…
The throttle on the transmitter was at the midpoint during the entire AUTO Mission, even when it dropped from the sky. The transmitter was never touched as it all happened in a matter of seconds.
We do have Stab as the first flight mode, but again, we have had it that way for years.
Also, Turning Off the Transmitter during an AUTO flight seems a little extreme?

Which Receiver do you recommend that has a good failsafe behavior?


yes, maybe. But unstable RC during AUTO could be even worse sometime. I can turn on the transmitter in a few seconds.

I’m only using different models of FrSky.
Also one more important thing I remembered is to set “hold last values” as a failsafe action of the receiver.
Here is the explanation


How has your experience been as of late with the X10S and Fail safe? Any better luck? Advice?