I cannot understand what went wrong. Pilot error, rookie mistake, interference ?

My Mavic Air, only 1 month old, crash landed on water and sank. After 10 hours, a diver was able to recover the unit but I don't think I will try to make it fly again.
I have placed the Mavic Air in a rice box to get as much moisture away. It was in a river, so no salt water. Then I will probably send it to DJI with hope to find out what happened and maybe get a replacement unit. But first, I would like to understand what went wrong.
When the drone was recovered from the water, the battery was removed immediately, it still blinks but I do not dare try to charge it.

Main controller is connected to an Iphone 8 using the side cable.
DJI GO 4 v4.3.9 (4269)

I have the last flight log and the last video before I lost signal.
I was also able to recover all the photos & videos from the SD card but I did not record the screen since I was not going to do any special manoeuvers.






It seems the drone did receive the order to restart filming as I found a corrupted video file on the SD card. I was able to repair it and I have the actual footage of the water landing...


And here is the narrative...

On January 4th I did 2 flights. The first one went well, no issue

For the second flight, I was at the top of a small waterfall. I used a fresh battery. I took off from a flat rock bed, used intelligent take off and the flight lasted 2 minutes 21 seconds. At that point, the drone was 10 meters away from home point and below at altitude -2.5m. It was in position to film my friends who were ready to jump into the water fall. So I decided to stop recording so I could start a new record file.

Just after I stopped the video recording, the screened showed a connection loss. I had visual line of sight with the drone which was still only 10 meters away. The antennas were perpendicular and as I tried to reconnect and could see the MA slowly descending toward the water about 3 meters below…

I was never able to regain connection, and it finally and slowly ‘landed’ on the water. Propellers stopped and it sank into the river. It was already 4pm so too late to dive (4 to 5 meter deep – no visibility). It was recovered only the next day at 7am.

• Why did the remote loose connection just after I decided to stop recording? Is it an isolated event? I did a similar flight the day before at another location and it went very well. I did go below the home point in altitude and I did also stop and restart recording. I never lost connection.
• It seems losing the connection triggered an auto landing where it was rather than a RTH. However, I thought such behavior was only if the drone is between 0-5m from home point. But It was 10 meters away, as recorded in the flight log.
• Also, it was less than 20 meters from home point, so according to the manual, it should have returned to home at the same altitude. If so, because it was 2.5m below the waterfall, it would have hit the stones. But instead, it just entered into auto landing without moving, or retracing the route.
• If you are wondering: I always use the mode RTH when connection lost (not hover or land).
• Safe RTH altitude was set at 40 meters (confirmed in the log)
  

So is there a weekness in RTH fail safe when the drone is in the 5-20m zone but with a negative altitude?? Clearly it did not try to climb or realign itself to the takeoff heading. It just decided to land right there, into the water.

What do you think: Pilot error? wrong configuration? missunderstanding of RTH?

https://www.phantomhelp.com/LogViewer/82FB5WNW3E804L7NPBWF/#
Link to the detailed log file before connection loss

The negative altitude hypothesis is intriguing.

I'll actually try that with mine tomorrow to see if it behaves the same way (not over water, of course).

Sorry about your drone.  I delight in seeing water "landings" when people do stupid things.  I hate seeing situations like yours where the operator is seemingly betrayed by the technology.

Skeeter Honey Posted at 1-5 21:07
The negative altitude hypothesis is intriguing.

I'll actually try that with mine tomorrow to see if it behaves the same way (not over water, of course).

Thank you for your support!
It's not like I got tricked by the wind after a 3 km range testing over the ocean or the 10% battery left forced landing.
That place has got no electronic interferences, there is no electric cables, wifi, or even phone coverage!
I hadd 13 satellites, 2.8Ghz frequency for better signal. intelligent takeoff as always, 40m RTH altitude, etc.
I really played safe and tested many situations before on a football field. I even turned off my controller on purpose to witness the track back safe mode and RTH.
But that autoland over water is really getting me puzzled.

This morning, I tried to switch the Mavic on and... it did light up.
The gimbal is shaking all over the place but I was able to download the flight log from DJI Assistant. Unfortunaltely, these are DAT files that I cannot read or interpret. I suppose I will send it to DJI but I would rather have also a third party opinion. They could very well argue pilot error by lack of evidence otherwise.

Did you have loss of signal set to land, there are 3 options hover land or RTH ?

hallmark007 Posted at 1-6 03:50
Did you have loss of signal set to land, there are 3 options hover land or RTH ?

In case of loss of signal I always set it to RTH.
After I lost the drone I went back to Go4 to check it is was indeed set to RTH, safe altitude 40m

hallmark007 Posted at 1-6 03:50
Did you have loss of signal set to land, there are 3 options hover land or RTH ?

With all due respect, did you not fully read his post sir? It quite clearly indicates it was not set to simply land, but RTH at 40m.

Not trying to be difficult, but as a DJI Captain I guess I just expect one to fully read posts about drone-loss issues. It is not a small matter. He lost his drone.

JohnnyMac41 Posted at 1-6 04:20
With all due respect, did you not fully read his post sir? It quite clearly indicates it was not set to simply land, but RTH at 40m.

Not trying to be difficult, but as a DJI Captain I guess I just expect one to fully read posts about drone-loss issues. It is not a small matter. He lost his drone.

Sorry my bad, read the top part of the post missed the bottom part.
Was not in anyway trying to make light of the situation.

Eclair29 Posted at 1-6 03:57
In case of loss of signal I always set it to RTH.
After I lost the drone I went back to Go4 to check it is was indeed set to RTH, safe altitude 40m

Sorry my bad didn’t read all your post, it doesn’t show much in your log as to why your aircraft landed in the water, at loss of signal it should have climbed 2.5 metres above take off point and flown home and landed, this looks like a failure of auto system.
As regards signal loss, flying below take off point you should always have antennas pointed in direct line with your craft. I’m not implying you didn’t, but only you will know. Best to start a case with dji they will have all the info contained in your dat file. Good Luck.

Yes sad story. For me looks that there was no pilot error

Sorry to see this. Hopefully you'll be back in the air soon.

Hello and good day Eclair29. I am sorry to know what happened to your previous flight with you DJI Mavic Air. Since this crashed happened. The best thing you need to do is to contact DJI Support at https://www.dji.com/support and and create a ticket for your drone for additional help. Please sync the flight records as well using your DJI Go 4 for additional analysis. Thank you.

DJI Stephen Posted at 1-6 06:43
Hello and good day Eclair29. I am sorry to know what happened to your previous flight with you DJI Mavic Air. Since this crashed happened. The best thing you need to do is to contact DJI Support at https://www.dji.com/support and and create a ticket for your drone for additional help. Please sync the flight records as well using your DJI Go 4 for additional analysis. Thank you.

[view_image]

Yes I did just that the same day, but because of the week end I have no answer yet and I was hoping the forum would help me shed some light on what happened.
And because only DJI can decipher the original DAT log files from the Mavic, I will have to trust they will look into it seriously rather than claim pilot error. I did not take the DJI care insurance... So it could cost me $700 for another unit...
So if the waranty does not work, it's not like I can argue their diagnostic...

Eclair29 Posted at 1-6 06:51
Yes I did just that the same day, but because of the week end I have no answer yet and I was hoping the forum would help me shed some light on what happened.
And because only DJI can decipher the original DAT log files from the Mavic, I will have to trust they will look into it seriously rather than claim pilot error. I did not take the DJI care insurance... So it could cost me $700 for another unit...
So if the waranty does not work, it's not like I can argue their diagnostic...

Hi there Eclair29, we're sorry for what happened to your drone. Can you provide your case number? we would like to look at it and see what we can do. Thank you.

Eclair29 Posted at 1-6 06:51
Yes I did just that the same day, but because of the week end I have no answer yet and I was hoping the forum would help me shed some light on what happened.
And because only DJI can decipher the original DAT log files from the Mavic, I will have to trust they will look into it seriously rather than claim pilot error. I did not take the DJI care insurance... So it could cost me $700 for another unit...
So if the waranty does not work, it's not like I can argue their diagnostic...

DJI will take the issue very seriously and if deemed hardware issue will replace your air no hassle whatsoever. Worry not, they are to be trusted.

Hi Eclair29,

Sorry for your loss!

Mayby this is what happend.
At disconnect MA was 10.1 meter away from homepoint.

If the RTH at Current Altitude was disabled in your App, the AC lands immediately.  (2 b ii)
If enabled if flys back to HP, see the copy paste of the manual.

I live in a country with lots of canals, rivers, lakes etc. If i can i never takeoff close to a waterfront, if possible always > 20 meters away, if not possible > 5!.

cheers
JJB

JJBspark Posted at 1-6 10:03
Hi Eclair29,

Sorry for your loss!

I think your taking that from an older manual or the spark manual, the Mavic Air up to date manual makes no mention of having RTH Altitude being disabled.

hallmark007 Posted at 1-6 11:17
I think your taking that from an older manual or the spark manual, the Mavic Air up to date manual makes no mention of having RTH Altitude being disabled.[view_image]

Thanks, you are right, found it in my first MA manual. Downloaded the latest version now.But what if the OP uses older FM ?
I will check this out, see if the manual represents the actual behaviour.

Or mayby the GPS position calculation went wrong, if less accurate than 5 meters offset means that software thinks its within 5 meters of homepoint.Guess the DAT file will tell more.

cheers
JJB

JJBspark Posted at 1-6 10:03
Hi Eclair29,

Sorry for your loss!

I live in a country with lots of canals, rivers, lakes etc. If i can i never takeoff close to a waterfront, if possible always > 20 meters away, if not possible > 5!.

Good advice knowing inaccuracy of consumer level GPS.  Factor in accuracy of GPS system itself (not GPS receiver), and you get "Missed it by that much".

Government commits to broadcasting the GPS signal in  space with a global average user range error (URE) of  ≤7.8 m  (25.6 ft.),  with 95% probability.   Actual performance exceeds the specification. On May 11, 2016, the  global average URE was ≤0.715 m (2.3 ft.), 95% of the time.

JJBspark Posted at 1-6 10:03
Hi Eclair29,

Sorry for your loss!

I did print the user manual when I bought the drone, and that 2.b.ii is definitely NOT in there! My manual is V1.2 (2015.05). So if this is how the drone reacted, then of course I should have known about that option. But the whole point of safe RTH is to come back at a safe altitude.  That option contradicts the setting of the minimum RTH altitude, so of course, it is always off in my case.
To land immediately 20 meters away is a big risk. Especially since the Mavic will only wait for 3 seconds! Not enough time to reboot the controller or adjust your position/antenna...
Because not matter why the drone decided to land, I am still puzzled as to why I lost connection 10 meters away in perfect line of sight! There is a solid 20 seconds between the loss of connection and the crash. I was never able to regain control. On my phone, the only flight log is the initial 2 minute long pre-positionning.
As for the Mavic firmware, I was able to connect to DJI Assistant 2 and the version is V01.00.0500
Finally, the last log record shows that the MAvic had 13 satellites and the recorded distance to Home Point is 10.1m  So this is the distance used to choose between the case 0 to 5m or 5 to 20m or more than 20m from HP right? No matter is GPS is accurate or not.

PS: The Mavic seems to work as normal (lights, gimball movement, startup sound) and I can connect it via the USB. But the one thing that does not work is wifi connection to the RC. The rear button does not even bip when pressed 2 seconds or longer...

Eclair29 Posted at 1-6 18:06
I did print the user manual when I bought the drone, and that 2.b.ii is definitely NOT in there! My manual is V1.2 (2015.05). So if this is how the drone reacted, then of course I should have known about that option. But the whole point of safe RTH is to come back at a safe altitude.  That option contradicts the setting of the minimum RTH altitude, so of course, it is always off in my case.
To land immediately 20 meters away is a big risk. Especially since the Mavic will only wait for 3 seconds! Not enough time to reboot the controller or adjust your position/antenna...
Because not matter why the drone decided to land, I am still puzzled as to why I lost connection 10 meters away in perfect line of sight! There is a solid 20 seconds between the loss of connection and the crash. I was never able to regain control. On my phone, the only flight log is the initial 2 minute long pre-positionning.

Hi Eclair,

Sorry to mis inform you in my post #16, see #17,18 too.

Puzzled here too why you did loose connection at that short range. I tried myself to move around with my RC (pointing all directions away from my drones, adjust antennas wrongly, never loose connection at short distance)
As you can connect your drone to the DJI assistent, can you download the DAT file from this flight?

Last position hovering with 13 sats and good reception (5bars). Question is what happend after the disconnection. Flying at 10.1 meters from HP it should fly back home after turning towards home, raise height to 2.5 meters. So why did it autoland?  Mayby the DAT file stored in the MA has the answer.

My guess that its likely it has something to do with (a change) GPS position after the disconnect (think its < 5 meters away from HP), cannot think of another reason why it enters an autolanding. See also the blue circle changing diameter in the app video, afaik it say something about accurancy of the GPS location, in this case about your mobile device.

cheers
JJB

JJBspark Posted at 1-6 23:59
Hi Eclair,

Sorry to mis inform you in my post #16, see #17,18 too.

No worry, I just want to get to the bottom of this incident (not the bottom of the river... )
So every lead is interesting.
I did wonder what the blue cercle was representing in the go4 flight log replay... So this could be the accuracy of the RC gps? On some flight replays I do not have this circle at all and here, it changes size at least twice during the flight.
Yet, the home point was recorded properly on takeoff and should not be affected by the RC location.

As for the internal DAT flight log, I was able to retrieve it and send it to DJI, but I cannot interpret the data on it because it is not the sme TXT format as what can be found in the Go4. So only DJI will be able to see what it recorded.
DJI contacted me today so I could send them all the useful files, which I did.
Wait and see... I'm glad I was able to fish it back (it cost me $70 for the diver) and it can still connect to DJI Assistant! After 17 hours under water! Lucky this was not salt water.

But it shows some fatal errors, and there is still moisture trapped inside so it will not fly again. Gimbal is also acting weird.

Eclair29 Posted at 1-7 03:11
No worry, I just want to get to the bottom of this incident (not the bottom of the river... )
So every lead is interesting.
I did wonder what the blue cercle was representing in the go4 flight log replay... So this could be the accuracy of the RC gps? On some flight replays I do not have this circle at all and here, it changes size at least twice during the flight.

Oke, guess you know but the RC has no GPS, its the mobile device GPS (blue dot).

Ofcourse HP setting has nothing to do with mobile device, but GPS reception of sats yes, not only how many sats is important but how many sats far away from each other in the sky gives accuracy.

If that affected your mobile device it could have effected your drone, flying low and mayby the surrounding higher ground shielded some sats away, but enough to show directly in the app.

cheers
JJB

Sorry about your loss.  In your oringinal post, you mention the drone was only 10m away.  But away from what - you or the home point?  The manual says that if it's within 5m of the home point, it will descend immediately if RTH is triggered.

Of course that does nothing to explain why the connection was lost - hopefully the answer to that is in the logs you recovered from the soggy drone.

BK

Eclair29 Posted at 1-6 06:51
Yes I did just that the same day, but because of the week end I have no answer yet and I was hoping the forum would help me shed some light on what happened.
And because only DJI can decipher the original DAT log files from the Mavic, I will have to trust they will look into it seriously rather than claim pilot error. I did not take the DJI care insurance... So it could cost me $700 for another unit...
So if the waranty does not work, it's not like I can argue their diagnostic...

In hindsight, always have insurance, and a way to recover the drone, when flying over water.  

Flying radio control models is risky anyway regardless of how landing friendly the terrain is.  Flying over water with radio control models is for the brave owners with plenty of spare money.

Dynamic Home Point tell the drone it was close enough to the remote control that landing where it was would be less risky than climbing and heading home?  Climbing and possibly hitting trees, to go a few meters sideways, being less safe than just landing near the pilot.  The GPS having and the software acknowledging low accuracy is interesting.  

The drones need water below (water in landing area) detectors on them?

BKahuna Posted at 1-7 08:16
Sorry about your loss.  In your oringinal post, you mention the drone was only 10m away.  But away from what - you or the home point?  The manual says that if it's within 5m of the home point, it will descend immediately if RTH is triggered.

Of course that does nothing to explain why the connection was lost - hopefully the answer to that is in the logs you recovered from the soggy drone.

I am just quoting the flight log, as interpreted by the online web site Phantomhelp.com:
The column says "home distance" so I assume it is either from the home point (fixed at take off) or dynamic (the RC). Depending on the option you select.
In my case, the last record before connection lost is 32.8ft or 10.1 meters, well within the 5m-20m radius where the drone is supposed to either:
- return to home at current altitude (at -2.5m below it would have hit the stone wall) or,
- climb to a minimum of +2.5m if below (my case) and then RTH

I don't think the accuracy of the GPS signal is a factor here. I had 13 satellites and the accuracy would only affect the precsion of the landing or how well it can find the home point. But the behavior of the drone shows that it did not engage into a RTH but rather into an immediate landing as if it was less than 5m from HP. This contradicts the recorded distance of 10.1meters...

One more thing puzzles me from the log report. The last entry does not mention the connection lost. I checked some previous flights when I did encounter interferences and got disconnected. These records show the disconnections, the intereference warning and the RTH mode being engaged. In one case, I did turn off the RC on purpose as the drone was only a few meters from HP. Sure enough, it landed immediately but the log does show the connection lost message. Is it because in that case I turned the RC back on and as it reconnected with the Mavic, the application retrieved the missing data from the unit?
For sure, as soon as I'm back in the air, I will do further tests in all kind of situations. But safely above a football field!...

PS: since everything happened so fast, I really recommend to all pilots to use screen record to be able to go back and review all the information displayed on the app. And especially that "lost connection" message and if there was other important clues. If was "lucky" to be able to finally have the video of the crash from the SD and the flight log from the internal memory. Otherwise, very difficult to go back and analyse anything just based on the App flight log.

I'm just curious why is your home point set to the water? Didn't you notice that when you started? Or did it change later?

him7403 Posted at 1-8 00:55
I'm just curious why is your home point set to the water? Didn't you notice that when you started? Or did it change later?

No! I did not takeoff from the water!!... Check the first 2 minute video at 0:11 and the picture below. It may help understand the configuration.



I took off from the dry river bed above the water fall.

hmmm more than 48 hours since DJI Drone Investigation Unit has been working on my case.
No news = good news right?
At least if I did something wrong they should have found it already...

One piece of the puzzle I did not study is the remote control unit. Does anybody know how to retrieve a log file (if any) from the RC? I tried to connect it to DJI Assistant 2 but it is not recognized.
Only from Go4 do I know that it is running version 01.00.0200

Eclair29 Posted at 1-10 01:44One piece of the puzzle I did not study is the remote control unit. Does anybody know how to retrieve a log file (if any) from the RC?

Follow these instructions to retrieve those logs:
http://www.datfile.net/DatCon/retrieveV3DatfromTablet.html

Eclair29 Posted at 1-10 01:44
hmmm more than 48 hours since DJI Drone Investigation Unit has been working on my case.
No news = good news right?
At least if I did something wrong they should have found it already...

Don't expect too much I have an issue and the customer services and data analysis team are there to cover up for the company they will say it's pilot's error no matter what.

Check what happened to my MA https://forum.dji.com/thread-177108-1-1.html

Eclair29 Posted at 1-6 18:06
I did print the user manual when I bought the drone, and that 2.b.ii is definitely NOT in there! My manual is V1.2 (2015.05). So if this is how the drone reacted, then of course I should have known about that option. But the whole point of safe RTH is to come back at a safe altitude.  That option contradicts the setting of the minimum RTH altitude, so of course, it is always off in my case.
To land immediately 20 meters away is a big risk. Especially since the Mavic will only wait for 3 seconds! Not enough time to reboot the controller or adjust your position/antenna...
Because not matter why the drone decided to land, I am still puzzled as to why I lost connection 10 meters away in perfect line of sight! There is a solid 20 seconds between the loss of connection and the crash. I was never able to regain control. On my phone, the only flight log is the initial 2 minute long pre-positionning.

I'm very sorry to hear your misadventure, the way he landed just freaked me out.
I think you're right, the manual doesn't explain well what the "return to home at current altitude" do and a lot of people misunderstand same as like you, take a look on this thread: https://forum.dji.com/thread-176913-1-1.html
Furthermore, you're not the first one that landed accidentally his aircraft on the water, I think they really have to add some feature to toggle as like "NEVER force-autoland, just hover after RTH-" this could save a lot of baths.

TwentyThrill Posted at 1-10 16:00
I'm very sorry to hear your misadventure, the way he landed just freaked me out.
I think you're right, the manual doesn't explain well what the "return to home at current altitude" do and a lot of people misunderstand same as like you, take a look on this thread: https://forum.dji.com/thread-176913-1-1.html
Furthermore, you're not the first one that landed accidentally his aircraft on the water, I think they really have to add some feature to toggle as like "NEVER force-autoland, just hover after RTH-" this could save a lot of baths.

Actually I'm ok with the process described in the manual with the different scenarios like 0-5m / 5-20m / beyond 20m. Once you know the rules, you play by them.
But my MA did not behave as explained. It was at 10 meter distance, it should have climbed to +2.5m and RTH - not immediate landing.
I did not activate the "RTH at current altitude" option because in my case, the drone was lower than the Home Point at -2.5m so it would also have resulted in a crash. Same would happen if the drone is downhill. Me filming a waterfall from above is not an out-of-the-box situation. I did not feel I was taking a risk. The lost connection at 10m distance in perfect line of sight with no interferences whatsoever is also a factor here and needs to be explained. As you can see from the video I was at the edge of the stone structure. The record shows the disconnection was simultaneous with restarting the video recording.
What DJI could improve is the time frame between loss of connection and decision to RTH. I think 3 seconds is too short. You have no time to move yourself, readjust the antennaes or worse restart the controller or the app. This parameter should be opened and I would personnaly put 30 seconds (unless you also hit the point of no-return and then force RTH)

Additional question: Let's say you select "hover" in case of lost connection in the option menu. What happens when the drone hits the RTH trigger because of low battery? Does it RTH?
Also I would suggest for the drone to emit some kind of "alarm" when it is engaging in a forced landing so it would warn people around.
Finally, I am pretty sure the drone detected the terrain was unsuitable to land because it kind of hesitated to land for a second. But by lack of connection to the RC the question "terrain unsuitable, do you confirm landing?" went unanswered and again, the drone decided to land anyway without allowing more time for the RC to re-connect. Yet it still had 80% battery, so there was no rush to land.

Eclair29 Posted at 1-10 20:18
Actually I'm ok with the process described in the manual with the different scenarios like 0-5m / 5-20m / beyond 20m. Once you know the rules, you play by them.
But my MA did not behave as explained. It was at 10 meter distance, it should have climbed to +2.5m and RTH - not immediate landing.
I did not activate the "RTH at current altitude" option because in my case, the drone was lower than the Home Point at -2.5m so it would also have resulted in a crash. Same would happen if the drone is downhill. Me filming a waterfall from above is not an out-of-the-box situation. I did not feel I was taking a risk. The lost connection at 10m distance in perfect line of sight with no interferences whatsoever is also a factor here and needs to be explained. As you can see from the video I was at the edge of the stone structure. The record shows the disconnection was simultaneous with restarting the video recording.

Very unlucky situation, I really hope that DJI will consider to fix your case as a warranty issue.
Anyway, I'm not totally sure but I think that if you set "hover" in the situation of low battery the MA will force land anyway, they really should explain better on their documentation how MA would act in all situations possible.

One thing is for sure, I will test all possible situations once I get my hands on a drone again!
Also it seems there are many differences between models. Mavic pro manual does not mention anything about a distance between HP and drone when RTH is triggered.
Older manuals mention different behaviors. This adds to the confusion.

Did the drone disconnect from the remote's control link and video link?  Or just the video link?  Video is lost first in marginal radio environments.  

This isn't the common problem with drones in stable hovers over water being fooled by the reflections and transparent nature of water?  The drone changing the hover from four feet over the reflective surface to four feet over the pond's bottom (well underwater) because it can't see the surface?  

Gunship9 Posted at 1-10 23:15
Did the drone disconnect from the remote's control link and video link?  Or just the video link?  Video is lost first in marginal radio environments.  

This isn't the common problem with drones in stable hovers over water being fooled by the reflections and transparent nature of water?  The drone changing the hover from four feet over the reflective surface to four feet over the pond's bottom (well underwater) because it can't see the surface?

After I set the MA to hover, I decided to stop recording. The log shows that order was received.
I then hit record to start a new video file. The log does not register that order, but the recording did start because there was a new video on the SD card, which actually recorded the sinking.
At that very same moment, the video screen went gray with the message "disconnected" not sure in what terms. I believe I lost both the control link and the video because I immediately looked at the drone and when I saw it was descending I tried to bring it up of the move sideways hoping I could still control what I thought was a sudden loss of power and I thought I could bring it closer to the river bank. I was never able to regain control and the video does last a good 30 agonizing seconds!
Upon connection lost, I guess the drone decided to land immediately (as if less than 5m from HP) instead of RTH at either safe set altitude of 40m, same altitude or climb at minimum +2.5m.

The last entry in the Go4 flight log shows it was at 10.1m from HP and at a height of -2.5m from HP as well as 5.3m from the "ground" (recorded by the sonar data).
Radio signal was 100%, battery 88%, 13 satellites in P-GPS mode.

Sorry to hear what happened, just something from my own experience, everyone should take into account.
In my early RC modeling days, I build a water plane, with frsky receiver (2.4GHz system) And I really had a weak signal when plane sitting on to the water, 5 steps or so behind me. Same at low altitudes gliding, the plane was unresponsive. But after altitude gaining, the control goes back to normal.
I believe, this phenomena because of radio signal bounced the water surface at opposite phase, can cancel your signal from controller. This so called fresnel blind zone, happened at distances at multiplier of radio wave length . It can be avoided by directing the antennas in different ways or polarization. Unfortunately you cant change antennas arrangement on mavic air.
Well, this is my assumption why you lost the link. And this is why I never going down to the water less than 10m.

Why does it landed, have no good explanation. I did the lost signal scenario test on my air right after purchasing. When remote was turned off, and the drone was aprox  2m away from me at may be 2 - 2.5m height, it landed right down. But with 5 or 10 seconds delay.  Have you noticed some delay when link was lost.  Or it started to descend immediately?


P.S. just not to add more confuse, the lost signal scenario I did on very first firmware, so things can be changed since then.
Your post is a good warning also to me just to check again on drone behavior as I updated the firmware already twice...

Thank you for your support guys, this just landed into my mailbox

Your request（#1645268）has been updated, please reply the email below:
We're happy to inform you that we will cover the warranty for repair at this time. Please follow the guidelines below and route the unit to our facility.


I hope that by "repair" they actually mean "replacement" because atter 17 hours in water, there is not much that can be salvaged in the long run. I hope they replace the battery as well because it is dead.
No further explanation on the reasons for the disconnect and the forced landing.
Also, they don't seem to implicate the RC for the loss of connection.
To be continued...

I tried to replicate this and it did not perform as yours did.

I was about 40 feet away from "home" and -5 feet in elevation.  I turned off the controller and it climbed to its 42M RTH altitude and started flying away.  In a panic, I turned on the controller as it started to leave and waited a few seconds (which seemed like an eternity) for it to re-bind.

So yeah...not the same behavior as what you saw, but momentarily terrifying nonetheless.