A rigging failure caused the air conditioning unit being lifted into place at 261 Madison Ave. in Manhattan to fall to the street 28 stories below on May 31, according to an investigator on the scene.

The massive unit hit the step-backed building as it fell, raining debris on Madison Ave. between 38th and 39th streets and injuring eight people in the area and two construction workers.

There were some reports that the unit fell as it was being put into place or that the floor cracked or gave way as the unit was being moved into place. “None of those are true,” said the investigator, who spoke on the condition of anonymity.

The floors are solid concrete, and there are no cracks in the floor, the investigator said.

A study of crane accidents by Haag Engineering found that lost or dropped load and rigging failure were the third and fourth most common types of accidents when a load was on the hook.

The study noted that dropped loads were directly related to a lack of understanding of the center of gravity of the load. That is frequently the case in a marine setting when crates are loaded that do not indicate the location of the load. Similar problems can occur when a large heating/cooling unit is loaded that does not have lifting instructions indicating the location of the compressor within the housing, the study found.

The roughly 10-ton air conditioner was being lifted by a mobile crane with a telescopic boom of 168 ft with a 207 ft lattice luffing jib rented from Bay Crane and operated by Skylift Contractor Corp. of Brooklyn, N.Y.

Investigators are looking at rigging issues. Preliminary indications are that human error was not involved in the accident.

The New York Dept. of Buildings is investigating the accident.

Buildings Commissioner Rick D. Chandler, speaking at a press conference shortly after the incident, said the crane was in good shape and that the site has had active construction permits since February.

His office said the accident could have been much worse, but the impact was mitigated by the fact that the permit limited the crane operation to the weekend, safety requirements required the lower floors to be empty prior to the lift, and four lanes of traffic were closed prior to the lift.

Office of the Mayor, NYC
Aftermath of dropped load accident on Madison Ave. in Manhattan.