I've built a APM 2.5+ based quadcopter running AC 2.9.1 firmware. I got a gps lock before I took flight, flew around a bit quite successfully (landing several times).

I then switched it into RTL mode to test it out - it increased altitude, rotated towards and flew what seemed to be home, but once it reached there it veered off right. Unfortunately I didn't have enough room to let it continue to see what it was up to, so I switched it back to stabilised (and failed to control it and hit a tree!) No major damage though, battery survived the fall, one prop chipped.

I'm trying to figure out what went wrong, can anyone help?. It's set to loiter for 5 seconds once it reaches before descending to 2m and at first I thought maybe it tried to start circling (the circle radius is set to 10 metres, pretty big for a qc) but arducopter shouldn't circle in loiter mode, only arduplane should do that i think. arducopter loiter should just stay in place as I understand.

I've got a shakey video of the flight (my cameraman understandably panicked a bit as my flying blender suddenly changed course :)

https://www.youtube.com/watch?v=cKS2v3xb3Bg

have attached the trace. You can see the RTL kick in when the altitude increases. Any thoughts? Thanks in advance!

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Tested again today with much more space. First time, it was successful. loitered above home for the 5 seconds I have it set to, then lowered to 2m and held position.

The second time I tested, it just headed in the opposite direction, to a point about 25 metres away from the launch point. I had to take control back before I could see if it was going to land there as I was a bit worried it wasn't going to stop! I was able to take control and bring it back though.

Could this just be a jittery gps?

Look at your tlogs.

Verify you have a 3D fix on the GPS during RTL and when it flew away. If it was 3DFix the whole time, then it maybe a compass issue. If you lost a 3DFix, you lost GPS precision (you need a 3D fix for autonomous modes). 

If that's the case, down load your parameters and look at the compass offsets: if any # is > 20.0 in the offsets, there's a problem. Basically you want to find the correct offsets and then *turn off* the auto compass calibration feature. The compass can go nuts on 4 things typically: magnetic interference, I2C interference (if you're using a 900Mhz radio), bad solder joint of compass (APM1), or defective board (APM2).

I leaning toward compass offset issues (from the auto-tuning) since RTL was showing the "great arc" trajectory and speeding off (which is another form of the 'spiral of death' issue folks are having)

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