Potential Thrust Loss: Motors maxing out while descending at the end of flight

I’m using Quadrotor with following hardware
Frame: Custom Made X
Motors: T motors U8 lite KV85
Propellers: T motors 29x9.5 foldable
ESC: Alpha 60A HV
Battery: 12S (2 x 6S, 30Ah, 5C, T-Drones)
Cube Orange with 4.1.5 firmware
MTOW: 11 kg

In open plain areas (<4000 ft AMSL) my copter flies very well without any problem. I even took it to 700m AGL and vertically descended it without any issues. I took my copter to a little trip of mountainous terrain. While test flying the copter at high altitude (8000ft + AMSL), I faced potential thrust loss warnings on multiple motors towards the end of my flight (48 mins) Battery Voltage was around 41V. I had limited the descent rate to 1.6 m/s but it was not maintaining it; sometimes reaching 2.5 to 3 m/s. It was getting difficult to control descent rate of the copter and it landed very hardly on surface. Also I want to mention that I have increased the ATC_THR_MIX_MAX to 0.7 to give priority to attitude control over throttle. Why my motors were maxing out while descending even when my copter is adequately powered. What could be the probable reason for such behaviour?

  1. Drained Battery.
  2. Poor Tuning.
  3. Some external wind effects at this altitude.
  4. Low Thrust to Weight ratio of copter.
  5. Some other aspect

Please give your insight to the issue!
@Leonardthall, @rmackay9, @xfacta, @hosein_gh

Log file:


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ArduCopter 4.2.2 has improved support for 12S batteries.

Please check now!
Link updated!

in your log we can see that your copter was under powered

as you can see motors was maxed and FC was trying to use full throttle to keep the altitude

in fact in your last part of flight (near and during landing) your voltage was about 40V

40v / 12s ~= 3.33V per cell and its meaning that your battery was near empty and this is the reason of why your copter was under powered

and your question about wind and sudden air pressure change , i cant see any abnormal data in your barometer press records

but after downloading a 500MB log :grimacing: i decided to do more analysis on this log and i found these problems in your build

first you have vibration problem that affected your PID loop and total flight performance

you can see that on your PIDs output:

also on your rate values:

and this is the result on motor outputs:

and the vibration measurement by Ardupilot:

totally these problems make more battery pressure and it will make hard to keep altitude for FC and more pressure on motors and make them hot by amplifying vibration by D Term

and note that your motors will spin more when you are flying upper mountains because of lower air pressure and it will affect your flight performance that’s its common to me that i fly in Tehran 1400m upper than sea level


I agree with what Hosein has said. That flight should have ended about 10 minutes earlier. The five points you’ve suggested, I would say that all of them apply. The battery was run far too low. Altitude will decrease performance so what you’d get away with at sea-level or even 1000mASL, won’t work at 2500m.

Have you run auto-tune on this? The PID values look very default. Pitch and roll do not track well with their desired values. The overall flight performance would improve with proper tuning.

You should update to 4.2.2, and finish all the required tuning steps.


Can you elaborate what support for 12S batteries are added in 4.2.2 firmware?

I wouldn’t worry about that when almost everything else is wrong with this craft in it’s current state.

Please elaborate!
Need your valuable feedback and suggestions how to improve it!

You received a lot of advice already in this thread, act on that. But if the thrust/weight isn’t addressed nothing else will matter.