It’s “One Flew Over the Cuckoo’s Nest” on Wards Island.

For the past three months, staff at the massive Manhattan Psychiatric Center have been “nonchalantly” trudging dangerous inmates — including the “Butcher of Tompkins Square Park” — through the facility’s non-criminal side to get them some fresh air since a plumbing mishap conked out the facility’s elevators, insiders told The Post.

“The forensic patients love it,” said Dr. Catherine Mortiere, a psychologist who works at the hospital, which houses both criminally insane and private patients. “They get to go through the civil hospital.”

But there is growing concern among staffers that one of the unhinged criminal inmates could harm the others.

Already, two inmates have breached their wards since the elevators flooded — “not making i

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