How to protest a DJI crash analysis?

[sar104]

OK - that shows conclusively that the aircraft had a yaw oscillation. Did you notice that - you did not mention it in your description. Anyway, both the IMU yaw and magnetic yaw recorded the oscillation. The IMU source was the z-axis rate gyro, that clearly shows the rotations. The direct cause is also clear - computing the difference between the CW and CCW prop rotation rates gives us a measure of the total motor angular momentum which will turn the aircraft both due to the change in angular momentum and the difference in prop drag. The graph below is somewhat busy, but that's necessary to show the relationship between all the plotted values, and it's simple enough if you refer to the legend.



The motor speed changes correlate perfectly with the yaw oscillations. Since they were not commanded by rudder input, the FC was directing them which, to me, looks like a gain issue. However, this still is not the direct cause of the lateral motion and crash, which was caused by the stick inputs.

 

[7thstring]

Well there are multiple mistakes then. If you used auto-took off only the inward move should work (otherwise the same move you used to start).
But as Kilra points out, the default setting for CSC is only in case of a failure, so in practice you did exactly as I described.
Perhaps the flyaway wasn't caused by you (other than the concrete thingy), but you certainly didn't know how to operate it. Now you do.

**** happens. I learned not to use RTH for instance. And to prefer hand launch and catch.

 

[SteveH]

OK - that shows conclusively that the aircraft had a yaw oscillation. Did you notice that - you did not mention it in your description. Anyway, both the IMU yaw and magnetic yaw recorded the oscillation. The IMU source was the z-axis rate gyro, that clearly shows the rotations. The direct cause is also clear - computing the difference between the CW and CCW prop rotation rates gives us a measure of the total motor angular momentum which will turn the aircraft both due to the change in angular momentum and the difference in prop drag. The graph below is somewhat busy, but that's necessary to show the relationship between all the plotted values, and it's simple enough if you refer to the legend.

View attachment 42495

The motor speed changes correlate perfectly with the yaw oscillations. Since they were not commanded by rudder input, the FC was directing them which, to me, looks like a gain issue. However, this still is not the direct cause of the lateral motion and crash, which was caused by the stick inputs.

As for yaw oscillations I can't attest but I do know that it at least yawed in one direction as it also moved approximately 5 to 10 feet laterally before I tried to gain control with the sticks. Thanks for all your analysis, now I've got to somehow convince DJI to at least look at your conclusions. They have both the Mavic and the controller. The other odd thing is that today I looked at the mode settings in the DJI Go 4 app and it was set to mode 1. I haven't even been to that page before or since the crash. Plus I flew the drone just moments before and it obviously wasn't in mode 1 or I wouldn't have ever gotten the craft back. Thanks again for your help.

 

[sar104]

As for yaw oscillations I can't attest but I do know that it at least yawed in one direction as it also moved approximately 5 to 10 feet laterally before I tried to gain control with the sticks. Thanks for all your analysis, now I've got to somehow convince DJI to at least look at your conclusions. They have both the Mavic and the controller. The other odd thing is that today I looked at the mode settings in the DJI Go 4 app and it was set to mode 1. I haven't even been to that page before or since the crash. Plus I flew the drone just moments before and it obviously wasn't in mode 1 or I wouldn't have ever gotten the craft back. Thanks again for your help.

I don't see any evidence of movement prior to your elevator input at 6.7 s.



Was it your impression that it moved significantly before that? Neither the IMU data nor the GPS data support that.

With respect to the stick mode - if you launch the DJI GO 4 app without being connected to the RC then it always shows mode 1, even though it also states that mode 2 is the default.

 

[QuadKid]

This should be the flight log.

Took a look at your log, there were multiple GPS errors throughout the flight, according to DJI NFZ's you were in class B airspace, surprised it even took off for you.

 

[SteveH]

I don't see any evidence of movement prior to your elevator input at 6.7 s.

View attachment 42502

Was it your impression that it moved significantly before that? Neither the IMU data nor the GPS data support that.

With respect to the stick mode - if you launch the DJI GO 4 app without being connected to the RC then it always shows mode 1, even though it also states that mode 2 is the default.

 

[rfc]

Can one assume from the fact that the OP posted the DAT file from the flight in question, that DJI, upon repairing a bird, leaves the on board files alone? I.e. does not wipe them.

 

[SteveH]

My impression was that it moved off laterally 10 feet or more before I tried to gain control. Without the RC connected is that also why it's not possible to enable Vision Positioning and activating some of the other settings?

 

[sar104]

Took a look at your log, there were multiple GPS errors throughout the flight, according to DJI NFZ's you were in class B airspace, surprised it even took off for you.

What do you mean by "GPS errors"? The Class B airspace warning is not an error, and that is the only GPS-related issue that I can see, and it had no effect on the flight.

 

[QuadKid]

What do you mean by "GPS errors"? The Class B airspace warning is not an error, and that is the only GPS-related issue that I can see, and it had no effect on the flight.

If you look at the 3rd graph down (lower section) you will see the weak GPS lock(s) during the flight (Yellow Line should be solid green) as far as the Class B airspace it is just a warning and yes it would not affect flight. Just surprised it allowed you to take off in Class B space. The User also initiated a RTH at around the 108 marker.

 

[Cheech Wizard]

That would explain the values, but the OP clearly said down and inwards so I had discounted that possibility.

Instructions to retrieve the DAT file are here. You won't be able to upload it to this site with the .DAT extension - change that to .txt first.

On the RC mode, your attempted CSC was recorded as full negative input on the elevator, throttle and rudder (nearly), and full positive on the aileron. That's not consistent with mode 1 down and inwards, which would be negative elevator throttle and aileron, with positive rudder. Please check the mode setting on the RC too.

Sar104, I hope this is kind of on topic...I use Mode 3 because I like to pull back the left stick to gain altitude, all other stick input is default. My question is... does Down and Out/In still stop the motors in a custom mode? I have never had to shut the motors down before and don't want to do some wonky input to make matters worse in an emergency. Would the left stick now be pushed Up and Out/In?

 

[QuadKid]

Sar104, I hope this is kind of on topic...I use Mode 3 because I like to pull back the left stick to gain altitude, all other stick input is default. My question is... does Down and Out/In still stop the motors in a custom mode? I have never had to shut the motors down before and don't want to do some wonky input to make matters worse in an emergency. Would the left stick now be pushed Up and Out/In?

You could always just try it on the ground, both sticks down and in/out should start the motors, the same command should shut down the motors if held for 3 seconds. If it does it on the ground it will do it in the air.

 

[Kilrah]

Without the RC connected is that also why it's not possible to enable Vision Positioning and activating some of the other settings?

Most of what's in the app is only valid when connected to the rc and aircraft. What you see without them connected is meaningless.

Would the left stick now be pushed Up and Out/In?

It is possible yes, there are known weird things around that when using custom modes. You need to thoroughly test.

 

[Cheech Wizard]

You could always just try it on the ground, both sticks down and in/out should start the motors, the same command should shut down the motors if held for 3 seconds. If it does it on the ground it will do it in the air.

OK, I didn't think of that... But, I don't want to be the Guinea pig...ha. Can you do that with no propellers?

 

[QuadKid]

OK, I didn't think of that... But, I don't want to be the Guinea pig...ha. Can you do that with no propellers?

If you do a manual start by a down and in command with both sticks the motors should start and just idle ( it will not take off until in your case you move the left stick down, Mode 3) then just perform the command again and the motors should shut off. Never have to leave the ground.

 

[Cheech Wizard]

If you do a manual start by a down and in command with both sticks the motors should start and just idle ( it will not take off until in your case you move the left stick down, Mode 3) then just perform the command again and the motors should shut off. Never have to leave the ground.

See my dilemma ? You used the word "Should," Start and just idle. " Your drone "Should" have shut off the motors in this post... but.. didn't in your case. I'm just trying to get the "Mode" operations down. If I test this my drone might flyaway, but not with no props. Will the MP Pro start up with no props so I can test this is my question.

 

[QuadKid]

Yes

 

[sar104]

If you look at the 3rd graph down (lower section) you will see the weak GPS lock(s) during the flight (Yellow Line should be solid green) as far as the Class B airspace it is just a warning and yes it would not affect flight. Just surprised it allowed you to take off in Class B space. The User also initiated a RTH at around the 108 marker.

If you look at the first graph in post #22 you will see the number of locked satellites and the GPS health.

 

[sar104]

Sar104, I hope this is kind of on topic...I use Mode 3 because I like to pull back the left stick to gain altitude, all other stick input is default. My question is... does Down and Out/In still stop the motors in a custom mode? I have never had to shut the motors down before and don't want to do some wonky input to make matters worse in an emergency. Would the left stick now be pushed Up and Out/In?

As suggested, it's easy to test without the props. In any case, the answer to your question is that the stick mode doesn't change the CSC pattern; it's always as shown in the diagram.

 

[sar104]

Can one assume from the fact that the OP posted the DAT file from the flight in question, that DJI, upon repairing a bird, leaves the on board files alone? I.e. does not wipe them.

That DAT file was from the mobile device, not from the aircraft. With regard to your question, I doubt that DJI wipes the flight logs from the aircraft when they repair it, and some users have even reported receiving refurbished units with old flight logs.