Unexpected drop of altitude

Hello, today we experienced unintentional hard landing.

While hovering in altitude around 1.5m drone suddenly dropped on the ground at vertical speed about 2.5 m/s according to logs. The pwm input didnt change the whole time during this event. What could be cause of this ? We looked into logs but cant find the reason for this, everything looks normal to us.

Thanks !


If the RCOUT (PWM outputs) did not change, you must have a ESC and/or motor problem.

sorry my bad, i meant PWM input, pwm output did change on all 4 motors

@rmackay9 could you please look into log if you find something that could be the cause ?

Thank you

Your throttle input is channel 1.

Log shows altitude went down because throttle went down. (Throttle in orange, Alt in Blue)

@manavgandhi17 No thats not the moment i was talking about. I did mark the moment when it happend. The one you mean was done on purpose, i stopped descent at arround 0.5m and then gained some altitude as you can see in logs. Then it dropped without change in throttle as you can see

Probably you need to do this:

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All I could tell thus far is that the autopilot decided to decrease that altitude at a rapid pace. Desired and actual altitude match well and desired and actual climb rates both drop to very large negative values. The rangefinder you have installed does seem to go out a couple of times right before the drop - but it also gets excluded from altitude every time that happens.

Barometer and RC-In don’t seem to be the culprits. Anyone see anything different?

I got a similar problem with no error messages and a copter falling down after 10min of flight… After changing ESC, pixhaw and motors I realized that the problem came from my battery which was dead. Check it with the internal resistance (should be < 7 mOhm/cell)

We dont use battery, it have constant power supply through cable so thats not what caused it

For me, I think the problem is the ground effect, I have seen that drop in a baro when the drone is very close to the floor, and you also have the same effect when you take off:

So a solution would be protect it with more foam, or put the FC inside a box…

Hello, today it happend again, but this time it did happen in altitude of 10 meters and i was able to stop the descend at 2m.

I climbed to 10m and then started to slowly descend, but suddenly drone started descending much more faster than usual (at arround 2m/s) without any change in throttle. Im attaching logs once again if somebody want to check it and maybe find something. (Btw high gains in rangefinder altitude at start was caused by reflection from irlock beacon protective lens).

Log 27.1.2022 (its tlog file, wasn able to find BIN log)

Can you get a .bin log file from the SD Card directly? Or download via MissionPlanner

@xfacta Yes, finally was able to download BIN file, here it is,

There are 2 files when it happend, one is from 1.2.2022 and another one from 27.1.2022

27.1.2022 BIN It happend at 11:43:05

1.2.2022 BIN In this log file it can be seen at time of 9:31:29 if you check GPS VZ then you can see it suddenly starts descending at speed of 2.5 m/s. Also happend one more time at 9:34:25

We dont think its ground effect, it did happen multiple times now at different altitudes

I had a look to your last log and I haven’t seen anything strange, when the throttle is under 1500us (orange line) the drone descends. Maybe I am missing something and @xfacta can see something more, but for me, in this case, the drone performs as expected.

Can you show us a photo of your flight controller and how it is mounted on the vehicle, so we can rule out aerodynamic issues effecting the barometer.

You also have a motor imbalance or one of your ESCs is not calibrated correctly .

Your rangefinder is giving faulty values and causing your altitude to change

The problem is that rate of descend changes on its own without change in the pwm throttle input. So for example lets say we are descending at constant speed of 0.5 m/s and sudenly drone starts descending at speed of 2 m/s without any change in pwm input.