Potential thrust loss (4) resulting in hard landing then crash

ArduCopter Copter 4.0
1

Hello and thank you for taking time to read this.

After 3 successful test missions following waypoints i wanted to test Battery Failsafe using guided mode. Failsafe triggered as expected (at 22.5V), but upon descent, the quad rolled 45 degrees left and right and had a sudden drop in altitude from about 10 meters (first potential thrust loss 4). After it hid the ground it bounced right back up almost 8 meters and then it descended rapidly until it hard landed again to a complete stop (breaking a leg and the battery case witch was 3d printed)
During this whole mission, battery has not been under 22 volts at any time, so i think it is safe to assume that voltage sag can be excluded.
Motors seem in balance to me, hovering at about 1500 without any noticeable difference between them.
Attached are the parameters used in this flight (resulted from a previous autotune)

4.0.7 AUTOTUNE SUCCESS
4.0.7 AUTOTUNE SUCCESS897Γ—809 156 KB

I appreciate any help figuring out what happened or if there is something concerning in the log i may have missed to have a safe flight.

link for log file:
https://drive.google.com/drive/folders/14awSEQ2Zhce4SoKiMhJiGyxWHBbYzxe_?usp=sharing

7.4kg AUW
Quadcopter using tarot x680 Frame
T-Motor mn501s 360kv motors
17 inch carbon props
Tmotor alpha ESC 40A LV
CUAV x7 PRO with dual GPS (CUAV neo V3 PRO and CUAV neo 3)
Turnigy Graphene 6s 12000 Mah Battery

2

you lost thrust in motor 4, have you got any low voltage protections on the escs that could have shut them off?

motor 4
motor 41705Γ—1117 124 KB

3

Not that i know of, the esc-s are all factory default using correct firmware and were calibrated in mission planner. As far as i know, default shutdown voltage seems to be 3.3 for those specific ESC-s, so maybe that was not the issue.

4

its possible it lost sync between esc and motor.

5

Given the fact I have a hard time understanding some parameters in the logs, i was wondering if you could help me with an input if anything look suspicious at a first glance? maybe desired roll & pitch vs actual values, etc (for the missions that completed successfully)
Thank you for your time.

6

it’s not a parameter that caused this, it’s been either a ESC, motor or propeller failure. ardupilot just sees a loss of thrust in that corner, without ESC telemetry there is no way to determine what caused the faulure from the log.

7

Understood. Thank you!
All the best,
Mihai