AC 3.1.2 Quad - Rapid decent/forced land at full throttle?

Had an interesting issue happen yesterday. I had my quad sudden start rapidly descend from 49m down to the ground while I had throttle at 100%. Once it “touched down” on snow thankfully, I was able to get it back up again and finish flying back to home.

Version 3.1.2 Quad Stable version
APM 2.6 with external 3DR Gps/Compass & radio

Here is the sequence of events (I have video from OSD, GoPro and will have logs downloaded soon):

  • Height was 49m

  • Mode was Loiter

  • RSSI was showing 55 to 60% but this is due to the Frsky 3.3v issue that will be resolved in 3.2

  • Failsafe modes are all disabled except for low throttle PPM and is set to RTL (not Land).

  • Pitch was 5 to 8 degrees yawing around

  • Throttle (by OSD) shows at 100%. Now that I think about it, this must be in error somehow as 100% throttle I would be climbing in loiter mode???

  • Battery was at 64% (Damn minimOSD config, accidently set it to % instead of mah on last update)

  • Minim OSD had battery low message on screen but that was due to voltage dropping under load

  • GPS sats were 10

  • 0 to 3 km/h forward speed

  • The hex started quickly from a yawing hover at 0 vertical speed to -230 Meters/minute.

  • With my Pan/Tilt camera I looked over at one of the motors and it was turning at a good speed (or I would have dropped like a rock lol).

  • Just before I “landed” I switched into Stab mode to try to recovery but it did not respond even to max throttle. Though it did seem to let me cushion the landing as the Vertical speed dropped (or rose actually) to -30 ish.

  • Once on ground I cut throttle.

  • I switched into AltH mode (likely by accident lol) and throttled up and climbed again and flew back home without issue.

EDIT: I see that Randy has posted an update to Github:

ArduCopter 3.1.3 7-Apr-2014
Changes from 3.1.2

  1. Stability patch fix which could cause motors to go to min at full throttle and with large roll/pitch inputs

Sounds very related.

Here is the APM log. You can see just after the 6 min mark the rapid decent. Throttle In and Throttle Out are high so that is strange (until I chop it when on the ground).

The errors at the end was due to a crash (damn tree branch) which unplugged the GPS.

However in the area of the decent I don’t see anything that jumps out at me.

Found another possible clue. When reviewing the flight video the current draw sitting idle on the ground not even armed bounces around from 300mA up to 1.64A for short periods.

Reviewing my past video did not show this, always well below 300mA at idle/not armed.

Just did a quick bench test and used my Taranis 40amp sensor as well, powered up just fine, 0 to 50mA or so at idle… weird.

FYI using APM Power Module (3DR version).

Sorry, you’re not logging much of anything. Pretty much impossible to say what’s going on. All I can say is your copter is way too heavy or way too underpowered. Not enough margin to fly properly. That bug I found could be contributing, but I doubt it. It takes some extreme circumstances to trigger it.

An oscillation from an over-tuned controller could cause this.

She flies great normally, that was a horribly windy day so I was taking it easy and just scouting out the field next to me (not yet a crazy pilot lol).

I’ve reviewed all my OSD videos and never have I had the idle (before arming) current be over 1 amp so I’m going to keep an eye out for that in the future. Might be a clue.

I’ll turn on more logs on for future ref. in case something similar happens again. I was wondering if it was due to low voltage (OSD was indicating the condition but I have no failsafes turned on for low voltage or low RSSI, just min. pulse width and that would be return to launch not land).