MM not responding normally after low battery + crashed and lost.

bror.00 · Oct 20, 2020

Hi All
I was flying my MM the other day and I ended up loosing it in some rough terrain and have not been able to locate it. I've filed a report with DJI to see if I can get some support from them because something strange was happening when I attempted the fly the aircraft home. I've uploaded my log to the PhantomHelp.com site, here is the link:

DJI Flight Log Viewer | Phantom Help

Upload and view flight logs from your DJI drones

www.phantomhelp.com

Not sure if I needed to upload any of the other files. Happy to do so if needed. I do have a video of some of this but not of the crash.

Here's a short explanation of what happened ...
I was flying around the back yard and I I decided to do a maneuver that I have done multiple times. I start at the far back of the yard and then fly up and over the house and then over the forest near a pond. It's a great cinematic that's fun to fly.

As I was up in the air, I got a low battery warning and the auto RTH came up on the screen. I canceled the auto RTH as I was not far and I wanted to make sure I landed safely (10:19 on the timeline). However, as I oriented the aircraft towards home and went to throttle forward, the aircraft would not fly forward. It was like it was stuck in mid air. This really confused me and I was a bit frantic trying to understand what was going on.

I was able to spin the aircraft around (left/right turn) but no matter what I did it would NOT fly back towards me. I tried to initiate the RTH via the software controls, but again it would not fly in the right direction. All this time the aircraft was slowly flying AWAY from me and getting further and further over very rough terrain. As the batter was getting lower, I knew there was a road back along the direction that the drone was drifting so I thought I would try and orient towards that and then fly to it and hopefully land in a place clear of trees.

Just about the time I figured out where I was, the battery had gotten so low that the auto landing initiated. I oriented the camera strait down to try and see where it was going and to see if I could negotiate a clear space. Fortunately/unfortunately there was a clear area right under the drone but ... it was a beaver pond on a neighbor's property! As the aircraft descended, I lost the video feed either because of LOS or because of the battery.

I was able to walk the ground of the last known location, but could not find the drone. I suspect it might have gone into the water. The area is very marshy and there is a big beaver ****/pond and the drone crashed somewhere near where a large steam runs into the pond.

Again, I have opened a case with DJI but I would LOVE independent analysis. Happy to answer any other questions and/or provide more data.

Thanks in advice!

Best
David

slup · Oct 20, 2020

bror.00 said:
Hi All
I was flying my MM the other day and I ended up loosing it in some rough terrain and have not been able to locate it. I've filed a report with DJI to see if I can get some support from them because something strange was happening when I attempted the fly the aircraft home. I've uploaded my log to the PhantomHelp.com site, here is the link:

DJI Flight Log Viewer | Phantom Help

Upload and view flight logs from your DJI drones

www.phantomhelp.com

Not sure if I needed to upload any of the other files. Happy to do so if needed. I do have a video of some of this but not of the crash.

Here's a short explanation of what happened ...
I was flying around the back yard and I I decided to do a maneuver that I have done multiple times. I start at the far back of the yard and then fly up and over the house and then over the forest near a pond. It's a great cinematic that's fun to fly.

As I was up in the air, I got a low battery warning and the auto RTH came up on the screen. I canceled the auto RTH as I was not far and I wanted to make sure I landed safely (10:19 on the timeline). However, as I oriented the aircraft towards home and went to throttle forward, the aircraft would not fly forward. It was like it was stuck in mid air. This really confused me and I was a bit frantic trying to understand what was going on.

I was able to spin the aircraft around (left/right turn) but no matter what I did it would NOT fly back towards me. I tried to initiate the RTH via the software controls, but again it would not fly in the right direction. All this time the aircraft was slowly flying AWAY from me and getting further and further over very rough terrain. As the batter was getting lower, I knew there was a road back along the direction that the drone was drifting so I thought I would try and orient towards that and then fly to it and hopefully land in a place clear of trees.

Just about the time I figured out where I was, the battery had gotten so low that the auto landing initiated. I oriented the camera strait down to try and see where it was going and to see if I could negotiate a clear space. Fortunately/unfortunately there was a clear area right under the drone but ... it was a beaver pond on a neighbor's property! As the aircraft descended, I lost the video feed either because of LOS or because of the battery.

I was able to walk the ground of the last known location, but could not find the drone. I suspect it might have gone into the water. The area is very marshy and there is a big beaver ****/pond and the drone crashed somewhere near where a large steam runs into the pond.

Again, I have opened a case with DJI but I would LOVE independent analysis. Happy to answer any other questions and/or provide more data.

Thanks in advice!

Best
David

Yeah, this was a "Blow away"... kind of at least.

Feeding in your log to my paid Airdata shows below wind speeds ... this to compare with the max wind resistance rating of 8m/s for the Mini.

But this wasn't the whole truth in this incident though ... the wind speeds was most probably not that high.

Looking at how your Mini performed compared to the spec ... especially how well it could pitch & by that resist the head winds there in the end shows another story.

Here below the end of your flight, from where the blow away started ... The white background is P-mode, the blue RTH & the pink low battery autolanding. Looking into a period where we know for certain that the AC is going max forward towards the HP is to look at a period of RTH (blue area). Have put the marker during the last RTH period, just before the autolanding ... it shows that the AC could just pitch (purple graph) to approx 15 degrees in average. But according to spec it should have been able to pitch to 20 degrees.

This meant that the Mini wasn't able to fight a wind speed up to 8m/s as the spec says ... it was much more prone to being blown away. The bad pitch was most probably due to deformed rear props making the motors rev to max in order to just keep it airborne, no headroom left to pitch properly rev wise... couldn't see any height instability during the flight, so the AC hadn't started to suffer from uncommanded descents (yet at least) but the pitch was rather unstable the whole time & it became worse towards the end.

So in short ... this was a clear blow away, but this time not due to flying in too strong winds. Instead due to deformed rear props making the Mini's ability to pitch down properly really bad & not on par with the spec ... & this lead to a bad ability to fight the wind.

Possibly you could have saved it back to HP by loosing altitude from those 74m you had the Mini at there in the end ...

new2mavic · Oct 20, 2020

When it's windy, get low and use sport mode, to home you go.

slup · Oct 21, 2020

new2mavic said:
When it's windy, get low and use sport mode, to home you go.

Yes, agree with get low... but this time, using Sport hadn't created any higher heading speed. If the AC can't achieve a proper pitch in P or RTH mode it will not do it in Sport either.

icecoldgin · Oct 21, 2020

Sorry for your loss.

Not 5 minutes ago I was already airborne with the Mini. However the winds here are still strong as a typhoon just passed the last 24 hours.

The Mini was struggling at about 40m altitude but well within clear VLOS. I decided to cut the flight and land it. It was the first time I had some difficulty hand catching it as it was really twisting side to side due to the wind.

bigswifty · Oct 21, 2020

I have no idea if it contributed to the problems with this flight, but as a general observation I think any time you start a flight with the battery pack in this condition you are pretty much asking for trouble:

Battery	Battery Voltage	Cell 1	Cell 2	Cell 3	Cell Deviation

68%

16.16V

7.938V

8.207V

0.015V

8.192V

... and only five seconds later:

68%

79.646V

40.45V

39.182V

0.014V

40.436V

slup · Oct 21, 2020

bigswifty said:
I have no idea if it contributed to the problems with this flight, but as a general observation I think any time you start a flight with the battery pack in this condition you are pretty much asking for trouble:

Battery Battery
Voltage Cell 1 Cell 2 Cell 3 Cell
Deviation

68% 16.16V 7.938V 8.207V 0.015V 8.192V

... and only five seconds later:

68% 79.646V 40.45V 39.182V 0.014V 40.436V

In general you're right ... if the the battery have been laying around for some time & self discharged and by that created an imbalanced state between the cells. Starting from 100% fly & land, shutting motors off ... then start up again & fly a second turn will not risk imbalanced cells & risk a sudden voltage drop in one cell starting a forced landing. We can't say from the limited info from the OP why it was at 68% when this flight started ...

But none of this was the cause for this blow away ... below all battery stats & the thresholds for gohome & low battery autoland. When the battery reached 7% it started to land as that was the threshold for low battery autoland.

All shown here below in the chart ...

bror.00 · Nov 16, 2020

Hi All
Apologies for the long delay in getting back to everyone who replied to this thread! Life got very busy with a number of things and I'm just getting back around to posting a reply.

First, thank you thank you thank you everyone for your replies! All the info was Very helpful and I feel like I'm a better pilot for having all the follow-up info.

@slup WOW! What great analysis! That was really helpful and I didn't realize the extent of different wind speeds at different altitudes. Where I standing, the wind was not that bad. However, where the drone was flying was along a rise and the wind was much stronger than I realized.

@new2mavic Yes, that is actually very helpful advise!! It never occurred to me in my panic to switch modes and get low. Something new to keep in my back of piloting tricks.

@icecoldgin haha! at least you got your drone back!

@bigswifty Yes, that is a really good point! I know the battery was a little low AND I had not intended for a bigger flight. So, my own fault in terms of pushing the drone when I should have been more conservative.

UPDATE: I did contact DJI and got a replacement drone at a discount. I have also signed up for the insurance, something I didn't know about in advance!

So, some sessions learned ...
1) Pay attention to the wind warnings!
2) Use sport mode when you need to get home quick
3) Don't fly on partially charged batteries
4a) know what you are up to BEFORE you fly
4b) stick within your self imposed limits for that flight

Also, I have re contextualized how I think about my mini ... I need to think about my mini as a small camera that I need to keep close and be VERY conservative with! It is a great drone and very capable BUT it is not a robust platform that should be sent off far into the wilds!

Thanks again all!
Best
David