Story going around the Paris Air Show when the Mi 12 was there.
US and Russian pilots talking about it and the question was raised on how its certification was going. The reply was that one thing that was remaining was the landing without engines.
The Boeing pilots were amazed. "You've got four engines and you're going to dead stick it?"
"Yes, our government has got stupid rules as well."
Whether this ever happened is uncertain but the vibration harmonics was the problem that killed it off.