The owner of an air charter service crash-landed his plane at Republic Airport in Farmingdale Monday afternoon when his landing gear and a key part of his aircraft’s warning system failed, the pilot said.

Ponderosa Air president Jan Nerud said he and a passenger pilot were not hurt but his 1978 twin-engine plane was “totaled” after it touched down about 6 p.m. at 125 mph and slid 2,000 feet on its belly.

The gear horn, an “excruciating,” repetitive blast, failed to sound the alarm when his wheels did not lower, he said.

“You’re flying the plane . . . and it’s sinking awful low,” Nerud said, “and then you hear the sound that nobody wants to hear, that’s tick tick tick tick tick — that’s the end of the propellers hitting the pavement.”

The Federal Aviation Administration is investigating why the Swearingen SW3 aircraft made a gear-up landing after flying from Westchester County Airport in White Plains to the company’s base in Farmingdale, an agency spokesman said. One runway was closed Monday as FAA investigators examined the plane.

Nerud said he plans to switch to all office work starting in September at his Republic-based business and was getting his passenger, another experienced pilot, familiarized with the small plane so he could take over some flights.

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They were ending their 8-minute flight from Westchester with what started as a “perfect landing” and going over landing procedures when they got a jolt.

Clouds of smoke billowed behind the plane from the friction with the tarmac.

A 17-year-old who works at Nassau Flyers, a flight training school, said he saw the crash and heard a screeching sound.

“You could definitely hear it, a loud scraping on the runway,” he said. “There were no sparks, just a little bit of dust.”

Nerud said he had recently replaced entire systems on the plane, including $150,000 for avionics, the electronic systems that run communications, navigation, the display and other functions.

Sounding philosophical about the hard landing, Nerud, 68, of Glen Cove said his plane’s gear horn and gear system had never failed him.

“It’s just one of those things,” Nerud said. “It was a great landing, except for one thing — the gear wasn’t down.

“We just took a little ride down the runway, that’s all.”