Potential Thrust loss: Reason Unknown

Hey guys,
Started getting potential thrust Loss message mid-flight. First time having this issue, unable to spot the cause. Can someone help? Logs attached.

Arducopter 4.0.0?
Caused may not done ESC calibration and
Set MOT_PWM_MIN and MOT_PWM_MAX 1100 AND 1940 accordingly.

What’s ypur prop-motor-ESC combo and AUW ?

Yes, it is Arducopter 4.0.0.
Tried ESC Calibration after the above flights but the result is same, will try and set the PWM limits manually.

AUW: 5000g
Prop: 17x55 CF
Motor: Emax MT5210= 290 kV
Battery: 6s 10000mah 25C
ESC: Blheli 30A

That’s a lot of propeller for that puny ESC. And a lot of weight.

Your #1 motor sits at and around maximum thrust even when the copter is descending, and that’s what’s causing the Potential Thrust Loss detection to kick in. Your CCW motors are driven harder then your CW throughout the flight, so you have some alignment issues.

Also, there’s a 60-degree mis-alignment on the Z-axis between your first compass and the other two, and I think that’s what’s upsetting the EKF at power-on. You should try recalibrating.

So, We flew again the next day with AP4.0.0 but with PIDs that we saved after an autotune in 3.6.x (before we upgraded to AP.4.0). The flight was graceful.

Sorry for the delayed follow-up. Had other issues to fix and got busy.

@rmackay9 has posted a warning on ardupilot forums - Check : https://discuss.ardupilot.org/t/warning-autotune-bug-in-copter-4-0-1-4-0-0/51709

As he mentions, do not autotune in 4.0.1/4.0.0. we have had good stable flights with 4.0.0 but pids tuned in 3.6.x

Re the “Potential Thrust Lost” messages, we think they are a little too common and Leonard Hall is going to work on reducing them. In particular I think we’ve found that during fast descents they can appear and it’s just a bit scary when there is really no problem most of the time. Sorry for the noise, we will get the number of messages reduced.