I echo what Roger A states above…
This wasn’t a case of “Inadvertent IFR”; this was a case of “Advertent IFR”: any time you’re mucking around in reduced visibility, there’s always a possibility that you could lose all visual reference. Especially in fog. (Been there; done that)
There’s a saying in the rotary wing world: “When you go down, you slow down”. I think that’s the main error here: the pilot did not manage his speed to compensate for his visual conditions; had he done so, it is possible he could have “winkled” through to destination. (Maybe not a smart thing to do, but in a situation where it’s an absolute necessity…) At a minimum, there would have been sufficient reaction time to land or do a “one eighty” and return to better conditions.
If you’re going to ‘muck about’ in low viz conditions, you have to have a plan - something in your hip pocket on which to rely.
Something like: Upon losing visual reference;
Go on the dials
Note your heading
Note the reciprocal and commence a level turn toward the low ground
Maintain altitude
Fly back to better conditions
Now, if you’re surrounded by hills as in the Kobe accident, you better have intimate knowledge of the lay of the land or be equipped with synthetic vision. (A real boon, BTW)
Given the general area of operations and historical weather conditions, I can’t believe that this was the pilot’s first encounter with reduced visibility operations; why he didn’t slow down will always remain a mystery.
(I post as a dual qualified pilot: rotary- and fixed-wing)