I had a really scary flight today. The flight path is a routine loop where I check our place and surroundings for fire dangers. I have done this countless times. I did not change any of the settings since quite a while and I fly without any wifi or server connections (flight mode is enabled and no phone feature on this tablet). I have done zero up or downgrade and never had any hard landing or anything which could make my system unhappy.
My story today went like this:
full system check, all parameters were normal. My home screen has  wallpaper that reminds me to check the RTH setting and I religiously verify this each and every time before I take off. I have it almost always set that a disconnection would trigger RTH. Weather was fine almost no wind no suspicious behaviour whatsoever during start and first part of the flight. At about 1.3 k distance the connection got disrupted  green indicator turned red (not too surprising at it is CE and hilly heavily forested country - has done that before). I expected that the drone would trace backward to the last connection point and/or return straight home. Would have taken less than 4 mins. Did not happen. I waited full tem minutes and realized that some serious trouble was present. I had the disconnect coordinates and I have also a Marco Polo tracker fitted and had that active. I was about to jump in the car with gps, and marco polo locator when I noticed that the green connection light on the RC was again there - the app remained disconnected. Eventually the app reconnected as well and showed that the drone was straight overhead on its way to decend and land. The battery warning stated that there was only sufficient charge left to return home. The drone did come safely home with remaining 17% battery at touchdown.
And here is now the scary part: I have after landing re-checked the action that should occur when connection is lost and it was set to RTH. I had the camera running all the time and the video shows that the drone stopped mid air when the connection was lost and kept hovering at 200m altitude for almost 12 minutes. it then turned around and went home. I guess that the low battery triggered coming home. Nothing which could explain this malfunction (hover instead of RTH at disconnect) as far as I understand things.
My confidence has taken quite a battering.

I don't have a sure explanation.   I'm sure there is one though.  It could have been that the control signal was interrupted which stopped the mission, but then reconnected before RTH started.  Meanwhile, you lost your video/telemetry feed and so had no idea what was happening.  So the AC, sensing the control signal, hovered as it is programmed to do, awaiting your command.  

I am just guessing.  Post a link to your log via this site and we can check it out:  http://www.phantomhelp.com/LogViewer/Upload/

You might consider fitting a live 3G tracker on it if you have 3G.  It has a way of calming the pilot when there is no RC FPV/telemetry connection.  

Good luck

site credit:  msinger

Don't let it shake your confidence.  Flukes happen from time to time for no apparent reason. I have had strange things happen that could not be explained. Dji drones are a package of sensitive electronics
and we all know that nothing is perfect.

It sounds like the only issue was not fully understanding how things work.
There are two signals, video and control signals.
The Phantom will RTH when the control signal is lost.
It's not uncommon to lose video signal but still have control signal.
You probably could have moved the Phantom, climbed higher or initiated RTH.
Your Phantom was probably just sitting there waiting for you to provide some joystick input the whole time.

If you follow the instructions from post #2, it will be easy to confirm this.

Labroides Posted at 2018-7-27 05:40
It sounds like the only issue was not fully understanding how things work.
There are two signals, video and control signals.
The Phantom will RTH when the control signal is lost.

Things are not that trivial. I am not talking video live feed but control signal. The RC light was red.
Point is that AC was programmed to RTH when control is lost (confirmed before flight and after return) but it kept hovering after disconnect  (confirmed with video on sd card) until low battery failsafe triggered RTH. RTH behaviour is stored in the drone and it should have executed different - that is the scary part.

oldhasbeen Posted at 2018-7-27 23:14
Things are not that trivial. I am not talking video live feed but control signal. The RC light was red.
Point is that AC was programmed to RTH when control is lost (confirmed before flight and after return) but it kept hovering after disconnect  (confirmed with video on sd card) until low battery failsafe triggered RTH. RTH behaviour is stored in the drone and it should have executed different - that is the scary part.

Instead of wondering what happened and not knowing, you could do what I suggested and get the recorded data to see what actually happened.

oldhasbeen Posted at 2018-7-27 23:14
Things are not that trivial. I am not talking video live feed but control signal. The RC light was red.
Point is that AC was programmed to RTH when control is lost (confirmed before flight and after return) but it kept hovering after disconnect  (confirmed with video on sd card) until low battery failsafe triggered RTH. RTH behaviour is stored in the drone and it should have executed different - that is the scary part.

Sir, if you'd simply post the log as explained in post #2, the mystery could possibly be resolved.  Otherwise we have nothing to work with.  So why not post the log?  It takes less time to post the log than it takes to write your above post.  

oldhasbeen Posted at 2018-7-27 23:14
Things are not that trivial. I am not talking video live feed but control signal. The RC light was red.
Point is that AC was programmed to RTH when control is lost (confirmed before flight and after return) but it kept hovering after disconnect  (confirmed with video on sd card) until low battery failsafe triggered RTH. RTH behaviour is stored in the drone and it should have executed different - that is the scary part.

The aircraft can lose radio contact briefly and then regain it, and I suspect that is exactly what happened. If it is a brief loss, the aircraft does not necessarily RTH, but simply stops and hovers in one place waiting for you to send it a command. In later versions of the firmware for P4Ps, it will even backtrack along the path it came, looking to re-establish a signal. It has to be a sustained loss of control signal to trigger RTH.

Mark The Droner Posted at 2018-7-28 04:15
Sir, if you'd simply post the log as explained in post #2, the mystery could possibly be resolved.  Otherwise we have nothing to work with.  So why not post the log?  It takes less time to post the log than it takes to write your above post.

I have uploaded the relevant log. Looking at the extended log file there is no new information above the already studied flight record within DJI Go4 for me to solve the issue. No flight record data between approx. 3:21min and 17:21min. 14 minutes are not accounted for. Needed would be the flight log decoded that is stored in the drone. Is there a way to do this without DJI support?

oldhasbeen Posted at 2018-7-29 00:40
I have uploaded the relevant log. Looking at the extended log file there is no new information above the already studied flight record within DJI Go4 for me to solve the issue. No flight record data between approx. 3:21min and 17:21min. 14 minutes are not accounted for. Needed would be the flight log decoded that is stored in the drone. Is there a way to do this without DJI support?

One thing to bear in mind is that the aircraft will only initiate its RTH if it has sufficient GPS/GLONASS satellites in view. Could there have been anything that perhaps caused it to wait for sufficient GPS signals ?

oldhasbeen Posted at 2018-7-29 00:40
I have uploaded the relevant log. Looking at the extended log file there is no new information above the already studied flight record within DJI Go4 for me to solve the issue. No flight record data between approx. 3:21min and 17:21min. 14 minutes are not accounted for. Needed would be the flight log decoded that is stored in the drone. Is there a way to do this without DJI support?

It's really hard to provide any comment when you still haven't posted a link so anyone else can see the data.

Aardvark Posted at 2018-7-29 01:17
One thing to bear in mind is that the aircraft will only initiate its RTH if it has sufficient GPS/GLONASS satellites in view. Could there have been anything that perhaps caused it to wait for sufficient GPS signals ?

17 sats before disconnect 18 after reconnect. RTH was initiated most likely due to low battery and this did work.

Labroides Posted at 2018-7-29 02:10
It's really hard to provide any comment when you still haven't posted a link so anyone else can see the data.

I have tried several times to upload the spreadsheet via the "Upload" option here but it does not seem to do this. How else can I publish the flight record on this very forum?
ok went again to Phantomhelp
here is the link
http://www.phantomhelp.com/LogViewer/KG6FH9E1FRZ5GZYS1ONJ/

oldhasbeen Posted at 2018-7-29 03:38
I have tried several times to upload the spreadsheet via the "Upload" option here but it does not seem to do this. How else can I publish the flight record on this very forum?

It's not a spreadsheet.  It's a text file.  It's located on your mobile device.  This is explained in the page the link sent you to.  Read the page from the top and then continue reading down the page and don't skip anything.  Once you do it, you'll realize it's a very simple process.  

I like to copy my log to my desktop's hard disk and then upload from there, but you don't have to do it that way.  But you must navigate to the directory/folder where your log is located and click on it to upload it.  Good luck.  

Mark The Droner Posted at 2018-7-29 03:50
It's not a spreadsheet.  It's a text file.  It's located on your mobile device.  This is explained in the page the link sent you to.  Read the page from the top and then continue reading down the page and don't skip anything.  Once you do it, you'll realize it's a very simple process.  

I like to copy my log to my desktop's hard disk and then upload from there, but you don't have to do it that way.  But you must navigate to the directory/folder where your log is located and click on it to upload it.  Good luck.

thank you for your help. I did not mean the uploading to the phantomhelp but wanted to post the spreadsheet which I had already obtainted from Phantomhelp to this thread and have tried the "upload" option but this kept going dead. See the link to phantohelp in my last post.

Okay... once you have uploaded the info and you're staring at the map and the data below it, and you want to share the info with the forum, you copy the url at the top of your browser.  Then paste the url to this page.  It should post as a hot link.  Good luck.

oldhasbeen Posted at 2018-7-29 03:38
I have tried several times to upload the spreadsheet via the "Upload" option here but it does not seem to do this. How else can I publish the flight record on this very forum?
ok went again to Phantomhelp
here is the link

At 17:17.6 when the connection is restored, the message shows Downlink Restored.
Your Phantom had lost the downlink signal.
That's the video and telemetry signal from the Phantom back to your controller.
It is normal to lose the downlink before you lose Control Signal (from the controller to Phantom).

You would have had to lose the control signal to trigger RTH.
Your Phantom just sat there for 12 minutes wondering when you were going to do something.
Anytime during that period you could have flown the Phantom back or hit RTH.
Eventually it gave up as the battery level was getting too low and began RTH.

All completely normal and just as someone suggested in post #4.

You might consider using Litchi for your future missions.  Unlike the Go app, with Litchi, the Phantom will continue its mission regardless of whether it senses the control signal.  In this case, had you used Litchi, the Phantom would have completed its mission at the expected time just as it had in the previous missions.  Good luck

oldhasbeen Posted at 2018-7-29 03:38
I have tried several times to upload the spreadsheet via the "Upload" option here but it does not seem to do this. How else can I publish the flight record on this very forum?
ok went again to Phantomhelp
here is the link

Haha I see you edited the post after I had posted.  You likely didn't realize I had posted because you didn't refresh the page.  With windows, you can press F5 to refresh the page and see if any posts had been made after yours.  Good luck