Okay DJI sleuths, help me understand why this might happen:

Recently I visited an abandoned Railroad Tunnel and tried to fly my drone through it.  The tunnel is about 50 meters long and made of brick/stone. I was in T (Tripod) flight mode to maintain control.  I was fully expecting that as I got deeper into the tunnel I would lose satellite contact and some flight stability--so I was on guard and ready to abort quickly.
When I was about 2/3rds of the way through the tunnel and nearing the exit, my drone suddenly accelerated like a bat out of hell. VERY FAST! Luckily, it went straight forward and not up into the roof of the tunnel. I reacted quickly and got it shut down and landed before anything tragic happened.
Of course, like any fool who touches something hot and gets burned, I tried it again.  Only difference was I went in opposite direction. This time I didn't get quite as far but the same thing happened.  I started out creeping carefully through the tunnel in complete control, then all of a sudden...ZOOM!  My Mavic shot out the end of the tunnel at top speed like it saw a ghost.  Again I was able to land it quickly without crashing.  It all happened very fast.
Why would it do this? My theory is that I was originally in Tripod mode and therefore was giving significant forward throttle but still moving slowly. Once the Mavic got deep into the tunnel it lost satelite contact and for some reason that exited Tripod mode and suddenly I was giving lots of throttle to a suddenly energized drone that was now in normal P or Sport mode.   Does that make sense?  Any other theories?
I need to figure this out because I actually have an indoor flight project planned and this would be a disaster in that environment.  Appreciate whatever guidance or ideas you might have.   Keep Calm and Fly On!

In a dark tunnel the OA will be ineffective and with loss of GPS it would have entered atti mode. With your pitch stick pushed forward to the max as it entered atti mode I can see why that may happen.

If I were you, I would just carry the drone and film it that way through the tunnel.

It probably got into ATTI mode when it lost GPS. If you could upload the flight data, would shed some light on what happened.

I need to figure this out because I actually have an indoor flight project planned and this would be a disaster in that environment.  Appreciate whatever guidance or ideas you might have.
Having people offer their guesses won't solve a flight mystery.
Looking at the recorded flight data is what's needed to see what actually happened.

To get to your flight data, go to: https://www.phantomhelp.com/LogViewer/Upload/
... where you'll find instructions to upload the data to that site and it will give a report
For help interpreting the data you can post a link for the report here.
Or just post the .txt file.

Probably it was a loss go GPS and then it caught up and want to get back to where it thought it should be,  perhaps post your log - http://phantomhelp.com/logviewer/

xagoras Posted at 9-29 14:51
It probably got into ATTI mode when it lost GPS. If you could upload the flight data, would shed some light on what happened.

Thanks. I will get the flight log and post.

Labroides Posted at 9-29 17:17
I need to figure this out because I actually have an indoor flight project planned and this would be a disaster in that environment.  Appreciate whatever guidance or ideas you might have.
Having people offer their guesses won't solve a flight mystery.
Looking at the recorded flight data is what's needed to see what actually happened.

Thanks for the flight log info.

I agree with the others.  DJI drones utilize 2 systems to understand where they are in space.  If both of them are unavailable, then it will drop into ATTI mode.  GPS is the first system.  It of course utilizes signals from satellites in space to be able to triangulate a location.  The system then uses that to understand if it is moving and in what direction.  The second system is the visual positioning system (VPS).  This system utilizes downward facing cameras that look from features underneath the drone to help it identify if it is moving or not.  If the VPS system can't see anything underneath it, then it can't help with orienting the drone.

In your case, once you get far enough into the tunnel, the GPS system won't be able to receive signals from the satellites and it will likely be dark enough that the VPS system won't be able to function.  When both of these systems aren't able to work, then it drops into ATTI mode.

ATTI is short for attitude.  In this mode, the drone tries to stay stabilized in height, but it won't be able to brake or position itself.  

Tripod mode is very much like having a car that is only in 1st gear.  You can have the throttle floored, but it will only go so fast. Once the GPS signal went away and it got too dark for the VPS system to function, its essentially like your limiter went away and it immediately jumped to full speed.

I've seen this exact behavior from other users that were in tripod mode down in a canyon.  When the GPS signal dropped out due to the steep canyon walls, it leaped forward suddenly at full speed into a rock wall.  Same behavior as you describe.

Hi. I am sorry for the trouble this has caused. With regards to this concern. Flying the drone in a tunnel may affect the performance of the GPS and VPS during flight. Please fly the drone in a location with strong GPS and better lighting to ensure stability and safety during flight. Thank you.

KlooGee Posted at 9-30 01:55
I agree with the others.  DJI drones utilize 2 systems to understand where they are in space.  If both of them are unavailable, then it will drop into ATTI mode.  GPS is the first system.  It of course utilizes signals from satellites in space to be able to triangulate a location.  The system then uses that to understand if it is moving and in what direction.  The second system is the visual positioning system (VPS).  This system utilizes downward facing cameras that look from features underneath the drone to help it identify if it is moving or not.  If the VPS system can't see anything underneath it, then it can't help with orienting the drone.

In your case, once you get far enough into the tunnel, the GPS system won't be able to receive signals from the satellites and it will likely be dark enough that the VPS system won't be able to function.  When both of these systems aren't able to work, then it drops into ATTI mode.

Thanks. this was a great response.  Super helpful.