Here is a situation that took place many years ago and it involved a group of US Army H-34s that were on training mission in Europe. I do not believe that any of the pilots were instrument rated. They were in a trailing formation flying in a valley when the two lead helicopters ran into a fog bank. The flight leader contacted the trailing members telling them to come to a hover and the last one in line would reverse course and fly out of the valley followed by the next and so on. Meanwhile the leader and the man behind him were totally enveloped with no visual clues as to where they were relative to the ground or the sides of the valley. In a hover, the H-34 (as well as most Sikorsky helicopters) will be tail low by three degrees and left wheel down by seven degrees (explanation to follow). The attitude indicator indicated that the helicopter was in a slight climb and in a bank to the left. The airspeed indicated zero and the VSI indicated no climb or dive. The Pilots, it was assumed turned on their pitot heat but there was no change and with this they lost spatial orientation and crashed.
The reason the most Sikorsky helicopters hang tail low in a hover is that the transmission is tilted forward by three degrees and the head will align itself with the local horizon. The reason they hang left wheel down by seven degrees or thereabouts is that the mixing unit has a built in bias to compensate for tail rotor translation tilting the blade disc seven degrees to the left. The interlock from the blades will cause the helicopter to tilt as well.
Any comments Nick?