Mayor de Blasio had a close call earlier this month when an NYPD helicopter that was shuttling him to Rikers Island suffered mechanical trouble upon landing, The Post has learned.

Witnesses said the aircraft let out a massive boom when it touched down on the island Sept. 1. The landing caused damage to the craft’s propeller blades, sources told The Post.

“It sounded like a military artillery shell going off,” said a Rikers correction officer.

Hizzoner, who was at the facility to speak with correction officers and announce security changes, had to fly back in a different helicopter, according to NYPD officials.

An NYPD spokesman said the mayor was never in danger. The bang happened when the chopper’s blades tilted forward and struck protective gear attached to the front, as the pilot landed on a slanted surface, the spokesman added.

The blades were replaced on site before the chopper took off later that day.