I've been building and flying drones since October of last year without any major incidents, at least not until today. I have a Flamewheel 450 running a pixhawk FMU a Frsky Taranis transmitter and a Frsky Delta 8 receiver - it works great and I have lots of flight hours on it. I have tested the radio fail safe on the ground several times and it all seems to work as advertised. So is says to myself, I wonder if the radio fail safe really works in the air? Seems like a reasonable thing to test. Not that I would every get far enough away to loose radio contact given the power of the Taranis and the new FAA regulations on VLS flight. I've been out 500 meters with no problems, so hey, why not give it a try - Bad Idea!
I understand exactly what happened - now. In fact, I had one of those "Oh Crap" moments of awareness as by beloved copter was free falling from 20 feet up with the motors off. Here's what happened.
First, let me say that the Pixhawk did EXACTLY what it was advertised to do in a radio fail safe situation. So no problem there. The Frsky Taranis is an awesome transmitter for sure, but it has this feature on power up where it checks that all the switches are in the down position and the throttle is off before it will turn on the RF. I was flying it Position Hold mode (middle mode switch position) when I started the test by turning the transmitter off. Sure enough, the copter went into RTL mode and started to come home. Imagine my joy. Everything would have been fine except I tried to regain control by turning the transmitter back on (you know what happens next don't you). The transmitter started complaining, verbally, that the switches were in the wrong position and the throttle was not down. Again, exactly what it is advertised to do. So, I pulled the throttle down and put the mode switch back to it's down position which was Stabilize mode. That was the fatal move. The receiver woke back up and sent the new mode to the Pixhawk which forced it out of RTL mode (as advertised) and put it into Stabilize mode (as I commanded) with the throttle off.
The rest is history. The bill of materials for my 20 foot free fall was not that bad considering.
1 3DR telemetry radio; 2 props; 1 motor boom; and 1 GPS mast.
A bunch of other stuff came apart, but it was designed to do that so no harm. This would make an interesting study in energy distribution in random events. Pieces were spread out all over the place with the furthest piece, my RunCam 2, landing a good 15 feet from the crash site. The good news is it was still running.
Bottom line - DO NOT live test the radio fail safe mode with a Frsky Taranis transmitter - Crap!