Hi,
It happened to my phantom 2 (with zenmuse 3D and Hero 3 Black Edition) twice this week.
The symptoms were identical even though the flight situations were different.
The 1st time I was shooting a video at 300 feet up then started a straight descent. Everything went well till it was 10 feet above ground with a battery still having roughly 40 % left. then the drone suddenly fluttered downwards like a dead leaf with reduced power but rotors still spinning. I tried to immediately increase the power by pushing the left stick but the Phantom did not respond at all, fell and bounced back on the ground. Luckily no damage. After which i was able to regain control and make it land smoothly. I thought maybe the descent was too quick (even though the latest firmware is supposed to prevent fluttering by limiting descent at 2m/s max).
But the 2nd time (next day with a fully rechraged battery), the fatal one for the drone (fell into sea water while hovering over a sea harbor), happened with the same symptoms even though this time i was flying at low speed and level. the drone suddenly lost power (but still with the rotors spinning like in the 1st time) even though the battery had 33% left.
Note : in both cases the RC switches were both pushed forward, the Hero 3 wifi was off, there was no rotor guards mounted (as this can sometimes cause instability in case of wind), there was no high power line or anything strongly "magnetic" in the area, there was bright sunshine but no super heat which could interfere with the GPS (I had a permanent 8-9 satellites connection), moderate wind (5-6 knots max).
Question : what can cause a sudden reduction (but not total loss) of power (and I suppose voltage even though I cannot remember the exact value at the time of crash) with an active battery above 30 % charge left?