Yesterday I experienced a really strange behaviour from my Revo Board on a ZMR250 with M8N GPS & I2C MAG, OPLINK control & data, flying in RATE mode (LP "next" r957). I have to add that this flight was with higher AUW than usual (5200mAh Lipo & 50gr. HD camera). But I've flown numerous time with this setup using r957 and previous "next" build versions without problems. Also with lighter batteries: 2200mAh & 1800mAh.
The purpose of the flight was to test different QuickSmoothValue setups after AutoTune to achieve a balance between best handling and smoothest video shooting possible with this heavy frame.
The first glitch occured during a moderate accent, maybe 30 meters above ground. There was a brief slight roll (10-20 degrees maybe) to right, without my command. After the jerk the flight continued normally but I got worried. Made some circles around at this altitude and decided to land. The QuickSmoothValue of 0.5 appeared to be too high and inappropriate for a smooth video shoting as I felt the copter little "nervous" in the wind (8-12km/h, just a light breeze). So I descended cautiously and when the copter was maybe a meter over ground, just a second after I raised the throttle (definitely not full throttle!) to stop the descent, then the copter rolled to the right on its own at about 120-130 degrees, stayed (stopped rolling and stabilized itself) in this orientation for a fraction of a second and then went to the ground. I've managed to cut throttle fast enough so the only damage was the GPS stand, not a big deal.
The crash was definitely neither a pilot error, nor a mechanical or electrical failure too. And I'm almost sure it was not a desync issue too. And the motors and ESCs were cool. Motors are Emax MT2204/2300KV, ESCs - LitteBee 20A.
Later I examined the telemetry record from the GCS and the video from the HD camera. Link to the video of the crash is attached.
Playback of the telemetry record shows that when these sudden rolls occur, they are preceded (or happen at the same time) by alarms in SystemHealth Display. First, ATTI goes orange and STAB goes black with red X.
Next ATTI goes green, STAB goes orange. After a while the STAB goes green too. There is also a CPU Overload alarm recorded
immediately after ATTI recovers. It seems that for some reason the Revo loses momentarily the ability to stabilize the frame. No other alarms (including NoGPSFix) are recorded at this moment (checked this in a Scopes window showing all possible alarms).
The raw data flow from gyro sensors seems to be uninterruped, as far as I can understand the telemetry record.
I attach screenshots of the telemetry playback at the moment of the crash, also the telemetry record from the GCS itself. The alarms are present at 248-250 and at 401-403 seconds since start.
I have no ideas where to look for the problem resolution. I can replace the Revo board with a spare one I have but this doesn't seem to me to be a hardware problem. I can also try the latest RC2 hoping that if this was a bug in the Revo firmware r957, it will not persist in the latest builds. But as the problem is not easily reproducible, I would not know with confidence that the same crash won't happen again, this time from hundred meters above ground or over some water areas.
Crash video:
GCS snapshots and telemetry log are in attachments.
I hope that someone would be able to analyse the log file and give a reasonable explanation of what happened.
P.S. For some reason youtube link is considered "unsecure" and not displayed by default. Here is the link itself, just remove dashes between "https" letters.
h-t-t-p-s://youtu.be/ZN4sSoIA-vM