A NYPD detective plunged from a helicopter — clinging to a rope that wasn’t secured — during training at the police academy in Queens Thursday afternoon, and “snapped his leg in half,” police said.

Video of the frightening fall, obtained by The Post, shows the police officer grab onto a rope to slide down from the chopper around 11:20 a.m.

But the rope either came loose or wasn’t attached to anything leaving sending the cop hurtling down to the ground from what appears to be 15 or 20 feet.

It wasn’t clear why the rope was unattached. Police sources said the officer’s leg was “snapped in two” in impact.

The detective, whose name wasn’t immediately released, was rushed to Jacobi Hospital to treat the broken leg, police said.

The demonstration was part of an NYPD Hostage Negotiating Team conference.