Loss of ADI
Canadian Beaufort Sea in December 1979.
Blacker than the inside of a duck in ice fog and darkness.
Aircraft was an Sperry IFR Bell 212.
We picked up off the rig deck into a hover and turned into wind (away from the rig lights).
I pulled collective as the FO called 20', 40', rotate.
As I rotated into the blackness my ADI slowly tumbled with NO FLAG.
I looked at the ASI, VSI and compass. All normal.
I looked at the standby horizon and it was normal too.
I said to my (low time) FO, "I'm losing my ADI get ready to take control if I ask".
This whole thing took maybe 5 seconds.
We flew the a/c IFR for a week with a duff captain's ADI.
Times were different then.
I had a similar thing happen (December again) with a Sikorsky 61 night IFR slinging north of Rea Point, Nunavut (Lougheed Island) at about N 78.
Same result.
FWIW, I am also current on glass cockpits in fixed wing.