Momentary control loss P3P

Damon0 · Sep 25, 2017

Thursday I had a bad experience, luckily for a short time and without any consequence.

I was on the sea, windy but not too much. After taking off all was ok, usually I stay in hover some seconds to check everything is ok before moving away. When I started moving, after some seconds, my P3P started to turn left, and the gimbal bent. It took 20 seconds, then everything returned to normality for the rest of the flight.

This is the video that I was recording when the problem occoured:

This is the flight log on Phantomhelp, please note from 0.30.9s to 0.50s the ATTI mode, that I hadn't touch, and that corresponds to the time of the abnormal flight:
DJI Flight Log Viewer - PhantomHelp.com

And this the flight log on Airdata UAV, same ATTI mode from second 30 to second 53:
Airdata UAV - Flight Data Analysis for Drones

I use Litchi app, version 4.1.2 and P3P firmware 1.11.20 with RC 1.9.2. No other problems in latest months.

I also downloaded the DAT file from the drone, and there was one error:
View attachment 88722

Can someone help me to understand what could have happened? Bad compass calibration is the cause I think, but Litchi app didn't displayed anything about it. Thanks

Meta4 · Sep 25, 2017

Damon0 said:
Thursday I had a bad experience, luckily for a short time and without any consequence.

I was on the sea, windy but not too much. After taking off all was ok, usually I stay in hover some seconds to check everything is ok before moving away. When I started moving, after some seconds, my P3P started to turn left, and the gimbal bent. It took 20 seconds, then everything returned to normality for the rest of the flight.

Can someone help me to understand what could have happened? Bad compass calibration is the cause I think, but Litchi app didn't displayed anything about it. Thanks

Even in atti mode, you still have full control of the Phantom.
The only thing you lose is position holding ability.
Looking at the flight data, the cause is a mystery.
There is no compass error, which you would expect to see and you have plenty of satellites.
It's almost as if you had switched the mode switch to atti but even if that happened by accident, you would remember switching it back.
It only lasts for 20 seconds and does not appear to be related to anything on the ground.
Your compass is fine, a bad compass calibration would have cause a lot more trouble than that and wouldn't have just gone away.
Perhaps it's just a mysterious one-off event.

BudWalker · Sep 26, 2017

Damon0 said:
Thursday I had a bad experience, luckily for a short time and without any consequence.

I was on the sea, windy but not too much. After taking off all was ok, usually I stay in hover some seconds to check everything is ok before moving away. When I started moving, after some seconds, my P3P started to turn left, and the gimbal bent. It took 20 seconds, then everything returned to normality for the rest of the flight.

This is the video that I was recording when the problem occoured:

This is the flight log on Phantomhelp, please note from 0.30.9s to 0.50s the ATTI mode, that I hadn't touch, and that corresponds to the time of the abnormal flight:
DJI Flight Log Viewer - PhantomHelp.com

And this the flight log on Airdata UAV, same ATTI mode from second 30 to second 53:
Airdata UAV - Flight Data Analysis for Drones

I use Litchi app, version 4.1.2 and P3P firmware 1.11.20 with RC 1.9.2. No other problems in latest months.

I also downloaded the DAT file from the drone, and there was one error:
View attachment 88722
View attachment 88724
View attachment 88725

Can someone help me to understand what could have happened? Bad compass calibration is the cause I think, but Litchi app didn't displayed anything about it. Thanks

The screenshots of the .DAT show the compass error that occured. Most likely it is coincident with the ATTI mode shown in the Litchi log

This doesn't appear to be one of those launch-from-geomagnetically-distorted-site incidents since the compass error did not occur immediately after launch. But, it could be similar to some recent incidents where there was an abrupt change in the compass. Would it be possible to Dropbox or GoogleDrive the .DAT and post the link here. It would also be helpful if you could include the .DAT for most recent flight before this incident, and the .DAT for a subsequent flight if any.

During ATTI mode there was a 2 second interval where there was a no stick input and the P3 appeared to be flying sideways. Most probably it was drifting with the wind. Was the wind out of the southeast at about 10 MPH?

Damon0 · Sep 26, 2017

Thank you very much for the answers

BudWalker said:
Would it be possible to Dropbox or GoogleDrive the .DAT and post the link here. It would also be helpful if you could include the .DAT for most recent flight before this incident, and the .DAT for a subsequent flight if any.

Of course, I'll do it this evening, Italian time

BudWalker said:
During ATTI mode there was a 2 second interval where there was a no stick input and the P3 appeared to be flying sideways. Most probably it was drifting with the wind. Was the wind out of the southeast at about 10 MPH?

Sure, when I realized the bird was drifting away I left the sticks for a moment to understand what direction it was taking. In that moment I was following the bird at sight, so I couldn't see on the tablet the ATTI mode. Only subsequently when I read the logs I understood it was in ATTI mode.

Damon0 · Sep 26, 2017

Here are the DAT files of 3 previous flights and 1 subsequent flight:

- FLY162
FLY162.DAT

- FLY163
FLY163.DAT

- FLY164
FLY164.DAT

- FLY165 (the accused flight)
FLY165.DAT

- FLY166
FLY166.DAT

BudWalker · Sep 27, 2017

Damon0 said:
Here are the DAT files of 3 previous flights and 1 subsequent flight:

- FLY162
FLY162.DAT

- FLY163
FLY163.DAT

- FLY164
FLY164.DAT

- FLY165 (the accused flight)
FLY165.DAT

- FLY166
FLY166.DAT

Thanks for retrieving and posting these. I looked at them all. In summary the incident in FLY165 should be considered to be a one-off as @Meta4 suggests. I've had this happen to my P3 a few times. My perspective is that the Flight Controller can't be perfect, and will sometimes erroneously detect the possibility of a compass error. By switching to ATTI mode the P3 is basically saying to the pilot "you fly this thing until I'm ready".

DatCon computes a signal called magYaw that is heading based just on magnetometer data and then corrected for roll and pitch. It's completely independent of Yaw which is computed by the Flight Controller. The two don't always agree. E.g. when launching from a geomagnetically distorted site. Also, once the geoDeclination can be derived from GPS data Yaw will be adjusted by that amount.

The FC must use magYaw (or something like it) to compare against it's Yaw value. When a difference is seen the FC can't really know which is correct and will declare a YAW_ERROR_LARGE compass error just to be safe. With the P3 the switch to ATTI is pretty immediate, the other platforms can vary in the amount of time it takes to switch. You can see this in your flight.

The problem is that magYaw doesn't always get computed correctly and this is the reason for these one-off compass-error-switch-to-ATTI incidents. I've been trying to figure this out for at least 6 months now. The actual magYaw computation has been verified correct in a couple of ways - the FC suffers from the same problem. I had suspected that the larger quantization errors that occur at the higher geoInclination latitudes (i.e. closer to the poles) was the problem. But, that was a science fair project that didn't work out. It does seem like there is some relation to abrupt changes in pitch and or roll, but I haven't gotten very far with that.

Got all that

, It's a one-off. It may happen from time to time in mid flight as it did in this flight. But, if you ever get a compass error immediately after launch that's probably for real. The P3 should be recovered ASAP, and don't initiate an RTH.