I recently upgraded my Turnigy 9x radio with the FR-SKY DHT-U radio module. This upgrade gave me access to quite a bit of extra functionality, telemetry, range check and fail-safe. Stuff you don't have with the stock radio.
The 9x doesn't usually have fail-safe setup, though it does at least pull the throttle channel to zero. The Ardupilot can detect this, and take action upon this condition. In my case, it will go into circle mode for a few seconds, and if still no signal is found, it will RTL (return to launch). Pretty handy.
The DHT-U module, on the other hand, defaults to keeping all channels at their previous setting, including the throttle. This of course, won't do, so I decided to set up the fail safe.
I set it up to all sticks neutral, throttle to zero and though I was good to go.
One little thing I failed to realize, was that the channel controlling the autopilot mode, was also recorded at a fail-safe position. At the time of recording, that channel was set to "manual mode"...
Sooo, I went flying. The mission I planned took the plane out of LOS (for the first time ever) and I lost radio contact to the plane. The DHT-U module started beeping (loss of signal to the receiver) and I got really nervous, but the plane returned. So I thought my fail safe was working. First mission done, I took it out a second time. Same thing. Beep, beep, beep, beep.
The beeping never stopped, and the plane never came back.
Panic ensued. I started running in the direction the plane left, radio in hand. Fortunately, I got a directional patch antenna for the module and as soon as I regained signal, I could use the directionality of the antenna to home in on the general direction of the plane. Soon I was close enough that I could get telemetry with the laptop again (which was carried by my poor out of breath brother in law) and could pinpoint the plane.
I found the plane in a Macadamia orchard, basically intact. Later investigating the on-board logs of the AP revealed the error I made, causing the fail-safe of the radio to switch the AP into "manual mode". It then glided, down wind, into the orchard and hit the trees at around 70km/h ground speed. Considering the speed, the damage was really minimal (about 5mins of hot-glue work).
Check the link below for the story and an on-board video of the crash (you can see how fortunate I was in the way it hit one tree, that bled some energy, and then entered a second tree slower, further cushioning the crash):