"Potential thrust loss" almost every takeoff [solved]


Almost every takeoff, we get the “Potential thrust loss” message, although the drone is acting fine during the whole takeoff, flight and landing. i looked into the code, and i understand that negative z velocity with full power throttle is causing this. so i searched the logs and i see the our GPS (and GPS2 too) VZ value is negative when climbing and positive when descending. so that’s might explain the thrust loss message.
I guess that the VZ should be positive when climbing. am i right? if so, what might be the cause?

I attached the log file and a screenshot from the log showing the thrust loss message right after the take off and the negative VZ during takeoff.



I got this result (and a near crash last flight) with copter 4.0.4 DEV a few moments after triggering in flight learning offsets every single time. Copter behaves fine, imediatly after triggering in flight learning offsets led on the here GPS turn form green to slow flashing yellow copter behaves fine and about 10 seconds later copter has nearly total loss of control and in the logs I see the “Potential thrust loss” message. Copter is defenetly overpowerd!

Copter 4.0.4 DEV
Cube Black

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Ardupilot uses a NED reference frame for it`s internal calculations.
NED means North, east, down. It results in negative values for climbing and positive values for descending. So that is not the issue. Can you guys post binary dataflash logs?

Hi Guy,
I couldn’t really make sense of your battery voltage and current monitoring, you might want to check that’s all working as planned. This will enable you to do the Compass/Motor calibration which is definitely worth it. It helps with scaling PIDs as battery voltage changes too.

Also it looks like you’ve started with all default parameters.
Set these right away:

But definitely let us know more, like prop size and battery cells - or use this spreadsheet as a starting point and run a new Autotune (after fixing that voltage and current stuff)


@amilcarlucas, i knew that ardupilot uses NED, just thought from some reason that it will be logged as NEU. Still, i think that it’s probably somehow related, because if the Z velocity were non-negative the thrust-loss message wouldn’t happen (i’m attaching a code snippet from crash_check.cpp). and you can see in the log that during the whole takeoff phase the VZ is above zero.
i already posted the binary log in the original post (https://drive.google.com/file/d/1_bRgMwST1hhG9KywAIWcDNI_XjfzFCCI/view).

@xfacta, the battery voltage and current aren’t calibrated at the moment. and i don’t understand how can it cause the thrust loss message. can you please explain?

any other ideas, anyone?

Thank you all,

It really helps with scaling PIDs as battery voltage changes, and it will tell you more about how or why your craft is behaving the way it is, even if it’s just to rule out voltage or current issues - apart from the safety aspect.
Personally I wouldn’t be flying without first setting up the voltage monitoring and failsafes as a minimum, and preferably the current monitoring too.

I know that doesnt help directly with “thrust loss” but it might help to rule more things out.


I have the same message “EMERGENCY: Potential Thrust Loss” which appears after each automatic takeoff.
In Auto takeoff, the drone rises very quickly.
This is a function I use for the first time, usually I take off the quadcopter in Loiter mode and activate Auto mode afterwards.
I think it doesn’t matter because I’ve been using this quadcopter for several months and have been flying properly.

Quadcopter configuration:
Orange Cube
Firmware Copter 4.0.3
Weight: 1.8 kg

Could you tell me what you think about it?



What size motors and props?

My first guess would be either voltage sags a lot, or props are too big, and they don’t spin up as fast as the autopilot expects… Either one could result in the copter not increasing altitude as quickly as it expects.
(I can’t review logs at the moment)

You have an imbalance between motor thrust which is exaggerated enough at takeoff to generate the error.
So it IS warning you of a problem.
You can see here that on takeoff motors 2 & 4 go to full thrust while 1 & 3 do not.
Is the copter balanced about the centre point?
Are the motors level?
Flex in frame?
So there is a physical issue that needs to be tracked down.


Great, thanks for your help!

Ok, I understand, the center of gravity is slightly on the back.
I use 900kv motors with 10x5 propellers and in flight, this quad is quite responsive and flies very well.
I think the phenomenon is accentuated because of a auto takeoff that is too “abrupt”. At the very beginning, it took off like a rocket!
I did not find any information to configure a softer auto take-off.

@Dorian @Flytrex_Guy

I get the potential thrust message all the time on auto takeoff. It will even give me the potential thrust loss message before the props even start to spin. The auto takeoff in my opinion is way to abrupt, fast and there are no parameters to control the motors going to 100% so fast. I have posted about this issue.

It has nothing to do with the copter being balanced or anything like that. I can take off all day in Loiter mode as fast as I want and will never see the potential thrust loss message or in flight. In fact, I never seen the message but on auto take off in guided mode. I am running 4.0.4 Dev.

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You can limit auto takeoff vertical speed by setting wp_speed_up to a low value. The vertical acceleration is set to fixed 50 cm/s/s

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Having a similar issue with OctaQuad setup, firmware 4.0.3. During a recent flight I had the Potential Thrust Loss Error come up a couple of times. Flight ended with erratic behavior and rapid loss of altitude, even when applying full throttle. Looking at the log for RCOU PWM I see that there is a peak spread of about 400, which might be the cause of the problem. Just went through balancing and auto tune. The rapid loss of altitude occurs at 17:22.30. Usually when the Potential Thrust Loss Error arises I can just keep flying without issues. Not the case with this flight

Probably need to shave weight and go through balancing again.

Log is here


Drone: OctaQuad, X
FC: Cube Black
ArduPilot Version: 4.0.3
Prop Size: 13 inches
Weight: Close to 10lbs. (yea I know, its heavy)

Too heavy as you say. The motor outputs look terrible. With little demand other than hover one motor is commanded to run flat out at times. I would say your average PWM output is ~1750 at hover. The MOT_THST_HOVER value is in-line with that. You should set the MOT_BAT_VOLT_MAX/MIN parameters in any case.


@dkemxr Thanks for the response. Might look into ESCs also, since the motors are all running quite cool. The drone also seems to have quite a bit of power, no problem getting it up off the ground / climbing once in flight. Would I be correct in thinking that an overweight copter would lead to hot ESCs / motors? Thanks

Not necessarily. Describe the components in more detail (motors, esc’s). 6S Power?

Motors: TMotors Antigravity4006 KV380


Props: Eolo 13x5

Power: Drone is tethered to a power converter that outputs 24V with no load (current = 62.5A)

Assuming the dynamic power demand is being met by that supply by time it gets to the craft I would say it needs a ~2lb weight loss. More as you drag the tether higher. Under a ~25A demand it’s dropping 2V.

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I was just flying my newly built pixhawk 2.4.8 quadcopter running Arducopter V4.0.5 ; testing out various autopilot features like RTL. All of a sudden I get a “Potential Thrust Loss (3)” message preventing me from controlling the drone while in stabilize mode. How do I fix this issue? The drone seemed to be flying pretty ok before. It was built two days ago. Could it be because of a low battery percentage?

Yes, that warning pops up whenever the motors do not have enough power to stabilize the copter, like in the case of low battery.

Without that feature and that warning, your copter would have probably crashed