Suddenly dropped from the sky while hovering, Mavic Air

[ValentinCM]

Hello,

I'll begin by stating the lesson I learned from this flight:
Don't fly in freezing high humidity(foggy) conditions.

In my case the temperature was around 0 Celsius. The humidity was visible, fog.

My assumption is that ice was formed on the proppelers and in the motors. The flight was short, 1 minute of air time, I was hovering in place taking pictures. I didn't notice any wobble before the crash, the Mavic Air was hovering in place and suddenly dropped from the sky on the frozen lake that I was trying to capture in the photos.

I did get the warning "Motor current error. Check your propellers and fly with caution." but I failed to act on it, the flight ended with "Motor is Blocked. Not Enough Force/ESC Error"

Below is the link to the AirData Flight Log:

The PhantomHelp LogViewer:

DJI Flight Log Viewer | Phantom Help

Upload and view flight logs from your DJI drones

www.phantomhelp.com

A have also attached the Flight Record and DAT file.

I would appreciate if my assessment could be verified and any extra information that could help diagnose the Mavic Air during this flight and for future flights. I did notice that the battery had some cell deviation before the crash.

Thank you!

 

[Olav van Gerven]

Looking at the weatherconditons, I think that you had an icingproblem. The temperature was just under 0° C (-0,1) which means that there was a serious risk of ice builing on the props an/or motor.
As ice in the motor will make it harder for the motor to function, it will not be a critical immediatly. With the ice on the props it's a different story. The flow of air is changed, the lift of the prop change and then there is no lift left.
This iceproblem is not something to be taken easy, "real" airplanes have to de-ice before flight depnding on the weatherconditions, and every once in a while, you have reports of big planes coming down in uncontrolled actions, because of the ice.

In your case, I think it was a combination of both, ice reducing the lift of the props, combined with ice in the motor causing the motor to be blocked and not enough power to overcome the resistance.

 

[Lastrexking]

Is it still flyable?

 

[Fly Dawg]

@ValentinCM This is not an icing issue. It appears that the FC concluded that the aircraft was on the ground and shut down the motors at ~6 meters. The last data point in the .txt logs shows this.

 

[old man mavic]

unfortunately it was the perfect storm,in relation to ice forming on the props and the sudden loss of lift

 

[Fly Dawg]

@ValentinCM Also, it is extremely unlikely for all four motors to shut down due to icing. The VPS and Barometer altitudes can be seen here just as the motors shut down.

 

[old man mavic]

if the drone flipped over as the lift was lost from say two props on the same side then the motors would have shut down

 

[Olav van Gerven]

at 1:11 there is an indication of a current error with the motor, this can be caused by ice, making it harder for the motor to function propperly.

at 11:22 there is a warning that the motr has not enough force, also possibly caused by ice

at 11:23 there is a final warning that the motor is blocked, combined with the message "not enough force"

Then the drone crashed. The switching of of the motor seems to me not to be caused by a data error (landed so shut off) but by obstruction of the motor. Most probable cause consindering the weatherconditions herefor would be ice.

 

[Fly Dawg]

The switching of of the motor seems to me not to be caused by a data error (landed so shut off) but by obstruction of the motor. Most probable cause consindering the weatherconditions herefor would be ice.

There is no indication of any motor anomoly whatsoever, looking at the motors speeds. They are perfectly normal considering a hover. One would see some sort of differential in the speed with an icing scenario. Particulary, speed increase due to drag.
The aircraft was hovering perfectly level. This air shutdown has been seen before, due to the VPS and the FC calculating that the aircraft was on the ground, hence the shutdown of all 4 motors. There is no deviation in the aircraft's attitude until the motor shutdown. You would also see a deviation there if there were icing.

 

[Deleted member 94047]

One of the motors stalled at 83s after take off and the FC issued a motor stop shortly after - leading to all four motors stopping while the AC was still in the air.

83.602 : 10683 [L-FMU/MOTOR]fault on , esc(3) is stall
83.602 : 10683 [L-FMU/MOTOR] Stop. reason:motor stall near ground
83.602 : 10683 [L-FMU/MOTOR]Total start times: 1, time: 83.82
83.602 : 10683 [L-PWM]set actuator mode:IDLE
83.602 : 10683 [L-N_MIS]req abort all for motor stopped
83.603 : 10683 [L-FMU/FSM]state changed. cur: motor off
83.603 : 10683 [L-RC]craft ctrl failed!!!

 

[RadioFlyerMan]

In addition to the motor speeds, look at the current draw on the battery even while hovering. Even as motor speed may have been maintained, current would spike with more resistance due to ice.

 

[ValentinCM]

Is it still flyable?

Yes, the hinge from one leg(antenna) is broken, and one release from the battery snapped out.

I've seen no other external damage. Also, in the app status is normal for all components.

if the drone flipped over as the lift was lost from say two props on the same side then the motors would have shut down

The AC fell flat on it's belly, no flipping involved, no strange noise from the motors. All 4 motors just stopped, and the AC fell.

 

[ValentinCM]

@ValentinCM Also, it is extremely unlikely for all four motors to shut down due to icing. The VPS and Barometer altitudes can be seen here just as the motors shut down.

As @Doppler mentioned:

One motor stalled, due to ice ?
83.602 : 10683 [L-FMU/MOTOR]fault on , esc(3) is stall

And then the FC proceeded to stop all motors because it concluded that the aircraft was "near ground" ?
83.602 : 10683 [L-FMU/MOTOR] Stop. reason:motor stall near ground

As @RadioFlyerMan says, the current draw would be interesting to see as the AC may be struggling to maintain the motor speeds.

Thank you for your opinions guys.

 

[Fly Dawg]

@ValentinCM ....Yes, I took a look at that as well. Current did increase, but that is normal with the voltage drop. The increase was negligable.

 

[Deleted member 94047]

And then the FC proceeded to stop all motors because it concluded that the aircraft was "near ground" ?

I don't think that is quite right. The FC decided to stop motors because one motor stalled while the AC was still near ground; not because it concluded the AC is near ground. I think this is an emergency, possibly safety feature kicking in.

 

[new2mavic]

Agree

And then the FC proceeded to stop all motors because it concluded that the aircraft was "near ground" ?

The VPS reading in the .TXT file shows that the craft was 4.5 m above ground when the message "esc stall" showed up in the event log of the .DAT file . That's definitely not "near ground".

 

[new2mavic]

I don't think that is quite right. The FC decided to stop motors because one motor stalled while the AC was still near ground; not because it concluded the AC is near ground. I think this is an emergency, possibly safety feature kicking in.

I am not sure If I have interpreted the data fields in the .DAT file correctly. The MotorCtrl fields show that no stop command has been sent to the motors. The command just stayed constant.

 

[Deleted member 94047]

The Reason for the motor stop is given in the DAT event stream log and it is clear.

83.602 : 10683 [L-FMU/MOTOR] Stop. reason:motor stall near ground

As for the NearGround flag, it was alternating between "True" and "False" even if the AC was hovering at a relative height of 3.5 - 4 meters for almost the entirety of the flight. I am not sure what threshold the FC uses to set this parameter to true or false, but in this case it was set to true for sometime before and at the time of the stall. We then have a motor stop command from the FC for reason "motor stall near ground"; and the AC falls to the ground. I am not entirely certain as to why the FC commanded this; hence my speculation that this could be an emergency procedure/safety feature kicking in.

 

[Deleted member 94047]

@sar104, if you have the time, can you take a look at this one?

 

[sar104]

@sar104, if you have the time, can you take a look at this one?

I'm not sure what everyone has been looking at, but it seems pretty obvious that this issue was due to icing. Look at the motor speed and motor current data:



Motor speed is basically constant, but as the flight progresses the motor current steadily goes up, which tells you that the load on the motors is going up significantly. That eventually leads to the FC concluding ESC stall at 83.6 seconds, followed by motor shut down.