Hello Experts... I was flying my Mavic Mini when it suddenly descended 80 feet into the lake at an unusually fast speed.
The drone is at the bottom of the lake.. and I can't remember ever feeling so helpless while this happened... I'm hoping someone can mildly explain why the sudden full speed 80 foot dive without an imput.

https://www.phantomhelp.com/LogViewer/WMB4TFJQRX4UR43HDUS8/#


Here's a cached video link
https://drive.google.com/file/d/1KZmhK8eqmWptm9YP0UPZmdX19DP3JzA9/view?usp=sharing


Here's the link to the actual flight data
https://drive.google.com/file/d/1p58xwHppPqV1gLcxA3hJwpGlxNPSJoAc/view?usp=sharing

thanks in advance for any help

"I'm hoping someone can mildly explain why the sudden full speed 80 foot dive without an input."

This was most probably nothing else than the "Uncommanded descent" phenomenon that was so common before firmware version 1.05. (v1.05 alerts the pilot about too fast rotating motors & prompts for a prop change).

The reason that have been established for this have been deformed props (especially those at the rear). The props can't generate enough thrust anymore & the Mini lose lift. Usually this is seen together with a great number of "NotEnoughForce" messages in the app/log ... this due to that the motors can't spin faster, they are already maxed out.

Here is your flight in a chart ... no graphs here, but all the green background color stripes are "NotEnoughForce", the first came already 18,6sec into the flight. (Click on the charts to make them larger).



Here below I've filled in some graphs that show the uncommanded descents in the end of the flight ...

The green graph is the vertical speed in m/s (descent=positive value, ascent=negative value)
The blue graph is your throttle command (left stick forward/back, ascend=positive value, descend=negative value)
The red graph is the height relative home point in meters

Have circled in 2 periods where the descend speed goes over the max speed of 1,8m/s in P-mode according to the Mini spec ... this also partial without any throttle input.

This isn't any pilot error ... contact DJI about this, they should help you.

slup Posted at 8-29 14:34
"I'm hoping someone can mildly explain why the sudden full speed 80 foot dive without an input."

This was most probably nothing else than the "Uncommanded descent" phenomenon that was so common before firmware version 1.05. (v1.05 alerts the pilot about too fast rotating motors & prompts for a prop change).

Thank You for Your time and help

Doesn't look very deep where the Mini went in. Did you try to retrieve it?

Hiya,

Had a look at your log as well, indeed (as Slup already wrote) a typical Uncontrolled Decent. (see the blue-red ALT line)

I had once myself a UD and found a error in the DAT file about this, is it possible to upload your DAT file for this flight?

DAT file on your mobile device in he MCDatFlightrecord map ; file = FLY051.DAT
Upload in the cloud and put a link on here. (DAT files shows motors RPM and more usefull info)
DJI should help you out on this imo warranty case.

PS nice area to fly, great video! (apart from the last section...)
cheers
JJB

GaryDoug Posted at 8-29 19:02
Doesn't look very deep where the Mini went in. Did you try to retrieve it?

First thing I did was put an arrow on the ground marking the direction of where it went in.  There were 3 people fly-fishing in little floats.. one of them came over and started looking for the drone (the water was very clear) however there was lots of vegitation right at the edge of where it went in, and He was unable to see the drone assuming it went under the plants.  I took my shoes and shirt off and attempted to, but the water was extremely cold and when I steped on the plants it felt like there was no bottom.. so then I found a 10 foot tree branch that sort of looked like a giant fork (was pretty heavy) and tried with that for a while.  It was a helpless, sad and lonely moment.. I was with my Uncle and He recorded my desperate attempts it's actually a pretty funny vid after the fact.

It's like Bermuda's Triangle,now we've a new disappeared aircrafts zone
it's not a coincidence about the most of the sinkings are over lakes,i guess because no waves or water movements are producing a mirror effect and the uncommanded landing ,, i think that they can put somekind of "fly over water" option removing the bottom sensor function if the box is clicked ON ... on this case we would avoid this kind of behaviour.
was you using Sport,Positioning or Cinema mode ?

JJB* Posted at 8-30 01:45
Hiya,

Had a look at your log as well, indeed (as Slup already wrote) a typical Uncontrolled Decent. (see the blue-red ALT line)

Thank You very much for Your time, kind and encouraging words

Hello there. I am sorry to read and to know what happened to your DJI Mavic Mimi. Since this unfortunate incident happened and you have lost the said drone. I would recommend for you to contact our DJI Support Team at https://www.dji.com/support?site=brandsite&from=nav for further assistance. We have a team that will do there best to help you and give out the best resolution for the said issue. Again, I am sorry for your loss and thank you for your understanding.

DJI Stephen Posted at 8-30 23:06
Hello there. I am sorry to read and to know what happened to your DJI Mavic Mimi. Since this unfortunate incident happened and you have lost the said drone. I would recommend for you to contact our DJI Support Team at https://www.dji.com/support?site=brandsite&from=nav for further assistance. We have a team that will do there best to help you and give out the best resolution for the said issue. Again, I am sorry for your loss and thank you for your understanding.

Thank You...   There are some amazing helpful people on this forum.

djiuser_tKwOmrDEUPk7 Posted at 8-31 03:43
Thank You...   There are some amazing helpful people on this forum.

Hi there. You are very much welcome and thank you for the reply. If you have any other concerns or inquiries with DJI. Please feel free to post it here at DJI Forum. We are all here to help and assist you. Thank you.

I believe this is the first time i hear about the Uncommanded descent.

Should this happen to me, should i get some sort of error message during the flight or will i simply loose the drone ?

Also in case of damaged propellers, will i get some sort of alert telling me there is an issue ?

Conte Mascetti Posted at 9-1 13:49
I believe this is the first time i hear about the Uncommanded descent.

Should this happen to me, should i get some sort of error message during the flight or will i simply loose the drone ?

There's no way this was caused by damaged propellers etc.. it seemed very much in control (not by me).. and stable..  but it suddenly decided to go down full speed.   I will never have the guts to fly a drone anywhere close to water other than a swimming pool or a very shallow pond etc.  Or have a full scuba gear and wetsuit handy just in case.

Conte Mascetti Posted at 9-1 13:49
I believe this is the first time i hear about the Uncommanded descent.

Should this happen to me, should i get some sort of error message during the flight or will i simply loose the drone ?

I believe this is the first time i hear about the Uncommanded descent.

Pretty known phenomenon, the plague started up broadly during the early spring 2020 ... read more about it here --> https://mavicpilots.com/threads/mavic-mini-uncommanded-descent-tests.86237/

djiuser_tKwOmrDEUPk7 Posted at 9-1 14:05
There's no way this was caused by damaged propellers etc.. it seemed very much in control (not by me).. and stable..  but it suddenly decided to go down full speed.   I will never have the guts to fly a drone anywhere close to water other than a swimming pool or a very shallow pond etc.  Or have a full scuba gear and wetsuit handy just in case.

There's no way this was caused by damaged propellers etc..

Interesting, & you base that firm belief on ..?

Recommend you to visit the provided link in post #14 above for more info.

Deformed prop(s) would certainly explain a sudden descent.  The only time that's happened to my Spark was when I was flying with LED props that had noticeably less pitch than stock

slup Posted at 9-2 01:57
There's no way this was caused by damaged propellers etc..

Interesting, & you base that firm belief on ..?

I am sorry.. I should have said in my opinion and also added the fact that I do not have anywhere close to the experience that most of the good and helpful people on this forum have.  
I'm assuming the people who think the propellers were the cause.. have not actually taken the time to look at the flight data.
The reason why I have a hard time with the propellers being the cause of the sudden "Kamikaze" aka "Uncommanded descent" and that thought has now been confirmed by the experts at DJI

My last flight lasted 10 minutes and 25 seconds
I pushed the MM to it's limits during the first 10 minutes of the flight.. and it performed flawlessly... wouldn't You think this would be the time for a bad propeller to show lack of response?
while hovering at 80 feet or 24 meters.. without any command from the remote control the MM decided to descend at above maximum speed in perfect control straight down into the lake...
This was not lack of control or wind or anything like that.

DJI has determined the crash to be a warranty issue and have now sent me a replacement MM.  It will be hard to ever trust the MM, and will try not to be around any water unless I have a scuba gear handy.

djiuser_tKwOmrDEUPk7 Posted at 9-14 11:11
I am sorry.. I should have said in my opinion and also added the fact that I do not have anywhere close to the experience that most of the good and helpful people on this forum have.  
I'm assuming the people who think the propellers were the cause.. have not actually taken the time to look at the flight data.
The reason why I have a hard time with the propellers being the cause of the sudden "Kamikaze" aka "Uncommanded descent" and that thought has now been confirmed by the experts at DJI

Hiya,

Good that DJI did replace your faulty MM.  If only we knew what the real error is in such dramatic Uncontrolled Descend (UD).

I had once a UD, just once! (landed in a green field, easy to recover)
Since then never ever had any major error with my MM (still use the original 8 blades),  having said that sometimes is see errors in the DAT wich link to that UD i had once....

So do i trust my MM, yes   as it does fly great since  (but a little voice in my head always speak   "what if.....")

Happy many landings with your replacement MM!
cheers
JJB

JJB* Posted at 9-14 11:26
Hiya,

Good that DJI did replace your faulty MM.  If only we knew what the real error is in such dramatic Uncontrolled Descend (UD).

This is what DJI said regarding the crash...

According to the data analysis, the flight accident was caused by a possible propulsion issue, resulting in the abnormal altitude control. There is no pilot's error and we determine to cover this case under warranty. For safety concern, it is not suggested for the pilot to fly too close to the surface of the water.

djiuser_tKwOmrDEUPk7 Posted at 9-14 13:09
This is what DJI said regarding the crash...

According to the data analysis, the flight accident was caused by a possible propulsion issue, resulting in the abnormal altitude control. There is no pilot's error and we determine to cover this case under warranty. For safety concern, it is not suggested for the pilot to fly too close to the surface of the water.

oke,  rather funny answer from DJI

as if the drone is aware that it is flying above water.....and if this issue happens at 100 meter above water ; it will still get wet feet!

Do you have the DAT file for this flight? [FLY051.DAT]

I do not think that it is a propulsion issue, but a fault in the height controlling channel.

JJB* Posted at 2020-8-30 01:45
Hiya,

Had a look at your log as well, indeed (as Slup already wrote) a typical Uncontrolled Decent. (see the blue-red ALT line)

This is great info. How did you generate this info from the flight data file?