he knew it was clear enough to fly on his route TO the fog bank. He also knew there was a ceiling of clouds I think at 2,500'... may be off on that but there WAS ceiling.
As he was flying the ground altitude was rising.. leaving less and less room between the ground and the cloud ceiling. Once he knew he couldn't continue reversing course and descending would have been the fastest way to get back into VFR, or Special VFR.... that's why I think he descended... to get back down 'under' the fog.
Hovering down... interesting. My guess is... he didn't know precisely where he was... he would not have turned into a box canyon if he did UNLESS if he had no choice, (mechanical failure). IOW... he wasn't aware he was in deep trouble that would have required a hovering descent.
