The massive air conditioner that fell 29 floors to Madison Avenue made the terrifying plunge when an unstable floor buckled under the unit’s enormous weight, causing the crane cables to break, police said Monday.

The crane operator was identified by sources as Christopher Cosban, 48, whose license was suspended in 2010 after he improperly secured a crane while working for Skylift Contractor Corp. and it crashed into a lower-Manhattan building.

Cosban, of Huntington, LI, didn’t directly cause that accident, but leaving the crane in a dangerously extended position was a factor, sources said at the time.

He was working for Skylift Sunday when the air-conditioning unit fell, injuring 10 people as glass and steel debris flew through the air.

The 23,000-pound unit was being hauled into a room on the 29th floor of 261 Madison Ave., but the floor supporting it gave way, creating a “weight shift” that caused the crane’s cables to snap at around 10:45 a.m., police said.

Cosban was allowed to continue operating the equipment, owned by Bay Crane, just after the incident to retract the rig, sources said. He refused to comment.

Fire Commissioner Daniel Nigro said Monday that the city “dodged a bullet.”

Several floors suffered significant damage from the unit tumbling down the side of the 30-story building, and a steel beam broke through the ninth floor and into the eighth.

The 11th floor was filled with debris Monday morning, and workers also had to repair a small water main break.

“There are certain areas that are still impacted by the damage,” said Bob Schmidt, a carpenter for Pacific Drywall and Construction.

Mayor de Blasio called the incident “quite aberrant” considering how many buildings are under construction in the city.

“If you think about the reality of this city constantly under construction — cranes, some of the highest cranes anywhere in the hemisphere, and yet there’s a very strong safety record, by and large,” he said.

Additional reporting by Yoav Gonen