Where could I find parametєr RC_OPTIONS in List?
I need to ignore Receiver Failsafe.
You won’t find that parameter in Arduplane 3.9. But in V3.10.0-dev you will.
Where could download it?
As per James link or from Mission Planner enter Cntrl-Q to populate the selections with the latest Dev version.
Great thanks for this. Is V3.10 safe-ish ? I need a way to trigger a relay (on-board lights) from my RC controller but would like to minimise risk of course 
Well, V4.0 beta is out now use that. A Beta release would be more stable than a dev release.
Thanks Dave, will do.