Sadly, my little quad has been lost in Davey Jones' Locker, I fear. I thought I'd try a longer programmed flight, and I didn't account for what would happen if the TX and/or the telemetry radio lost signal. At least that's my theory. I thought it would execute RTL, but it appears it didn't. A couple minutes into the flight (when I lost sight of it beyond some trees (but over the water), the telemetry radio stopped tracking. I went out to the beach and looked around, but the water gets really deep really fast out there, so I suspect it's at the bottom in 20-30 feet of water. I'm sad. Here's the last image from my computer...
 I have a new theory...see my comment on page 2. Maybe it's not in the ocean, after all? According to the source code, the default RTL altitude is "1500cm (=15m). If zero the copter will return at it's altitude at the time RTL was invoked." I'm not sure I ever changed the default, and if that's true and RTL did engage on loss of signal, it's almost definitely going to be in the trees.
I hope you find it, however the RTL logic is as follows: If the RTL height is set to 0, it will indeed fly back at the altitude it was at when it switches to RTL mode. If the default RTL altitude is 15m, then it will return at 15Mm ONLY if it is flying below 15m. It means RTL *at least* at 15m OR use current altitude if it is higher than 15m. RTL value in MP thus defines a minimum altitude. In your case, at about 100 ft, that is already pretty high, though I must say I have flown across some trees higher than that. I hope you find it back though! Check in MP with another quad if it properly switched to RTL when the transmitter is Off. You can check this when just connecting through USB and no battery, to ensure no flyaway. Arm, then turn transmitter off. That way you can check if your transmitter is sending a low enough PWM value for it to trigger RTL. If it doesn't, then it's unlikely that your fly away quad will have triggered RTL. On a side note, my hexa also decided to land in the water a week ago, I was ignoring the low battery level for a tad bit too long (RTL on low voltage was deliberately off - explain another time why). I tried diving in the lake to recover, but it was too deep. Tomorrow a second recovery attempt.
My five cents,
Next time you go to look for you copter bring your laptop and transmitter with you don't turn it off, if your copter or airplane is still alive and have battery you will be able to regain telemetry link, and as soon as you do you will have updated GPS location on your MP. If you lost it in dens foliage and you cant see it despite your close by, you can engage throttle for one second or two and listen for direction.
Don't ask how I know this ;-)
OK, I'm very encouraged. Here is a map showing what might have happened. The trees in this area are probably at least 50-60 feet high, and if the default RTL altitude of 15m was used by the copter, it's almost definitely in the trees.
I've been thinking all night about this situation, and now I have another theory: maybe RTL did engage, but the copter ended up in the trees that you can see to the right of Waypoint #3 in the picture. This is a stand of tall trees, and my assumption all along was that they were less than 100 feet (my RTL altitude). But I could be wrong about that, and if the quad did sense a radio failure, and did decide to RTL, it may be tangled up in the trees. I'm going to go out and take a look along that treeline, and now I have new hope that the copter may be recoverable. Of course, I'll need one of my other copters to recover it. Wish me luck!
Does anybody here have a deeper understanding of this logic in the APM hardware and software?
Ouch..... thats some expensive equipment . never fly over water when its not design to float
sorry to hear your loss .
a couple of weeks ago while flying waypoints (on a mission used before without problem ) my quad stop still in the air for around 20 seconds then carried on to next way point and landed .the problem being it still had around 10 waypoint left to fly and no delay were set on any WP .luckly it was over land and not water . battery still over 12v
So I am interested in what may have happened to your quad .
By the way I had just update mission planner that day ?
Steve, just to be sure, did you have the settings correct that it would RTL upon loss of signal?
Did you test RTL functionality?
well, drones usually crash a lot. quads could suffer from the control and telemetry failures, but also a single broken prop, or burnt motor or speed could make it fall like a rock. next time, don't fly it over water! and if you do, attach some floating device to it, so at least you could get it on the beach with the help of the waves.