Phantom 3 out of control at 81 mph and no wind!

[Meta4]

It's the second time that this happens. I'm now to scared to even fly the dam thing. Any suggestions?

Start by looking at the flight data.
Go to https://www.phantomhelp.com/LogViewer/Upload/
Follow the instructions to upload your flight record.

Come back and post a link to the report it provides.

 

[flyNfrank]

I had the same problem. My battery was still at 58% and all of a sudden it tell me ( critical low voltage the craft is landing). It started to go down by it self over the sea and with great despair i just managed to land it on land but not with out a great fall. Lucky on soft land . It's the second time that this happens. I'm now to scared to even fly the dam thing. Any suggestions? Regards to all.

As Meta4 mentioned, uploading the flight log file is a start on looking over the data.

From experience, most likely your issue is related to a battery with many flights, or a weak cell. I fly a lot with a battery like what you probably have. I have to monitor the volts as I fly and keep them above the known critical point. To do that generally is a matter of letting up on the elevator stick so that the aircraft is not asking for so much power so to speak. It can get to the point of just having to fly with the stick near 1/2 way. You can set the app to display the volts in the upper right corner of the screen. In your case the cure will be to replace the battery, or do as I do and use a newer battery when you know you might be flying in a more critical flight, like out to sea.

Don't forget to upload the file and paste the link here.