Arduplane Crash at sea unknown problem

Hi there fellow enthusiasts

I have a few questions regarding a crash recently with my Pixhawk4 and Hobby King Skyeye plane. I have had several very successful flights with this plane and absolutely no problems to speak of. Yesterday however, the plane was on a mission and it completed the first few way-points successfully. it then went off to do a rather long waypoint of 3Km. After around 1Km the GCS reported a loss of telemetry with the plane although it also reported a signal strength of 79%!

I change into RTL and the plane spiral dived into the sea, hitting it at 79 KPH just over 1Km out. I am asking if anyone can help me to look through the Mission planner log to see if they can find out why?

I tried downloading the dataflash log from the Pixhawk 4 but it is lost so I only have the GCS log.

Many regards

2016-05-31 15-48-29.tlog (703.5 KB)