NYC officials probing Saturday’s ferry crash are zeroing in on the propulsion system that had failed the vessel at least once before — and also revealed that the assistant captain was at the helm at the time of the latest incident.

The propulsion system of the Andrew J. Barberi — the ferry infamous for a deadly dock crash because of pilot error in 2003 — is of particular interest because it failed in 1995, and initial reports indicate it was a factor in Saturday morning’s incident, a Department of Transportation source said.

“It’s not a totally unique thing,” the city official noted of the problem.

A DOT official said “mechanical error” is the official reason given for the apparent power failure that left the Barberi unable to reverse as the pilot tried to slip it into the St. George Terminal dock.

National Transportation Safety Board investigators yesterday interviewed the ship’s chief engineer and inspected its engine room and outer hull.

Investigators today are expected to talk to Barberi Capt. Donald Russell and Assistant Capt. Maqbool Ahmed, who was piloting the ferry at the time of the crash, city officials said.

The two front engines of the Barberi, which normally would have controlled its momentum as it approached the pier, shut down prior to the collision. But the two rear engines continued to operate, the NTSB said yesterday.

It was normal for Ahmed, a Staten Island ferry pilot since 2008, to have been at the helm at the time, the DOT said.

Ferry captains traditionally break up the piloting responsibilities, with captains in control en route to Manhattan and assistant captains taking over on return trips to Staten Island.

“Capt. Russell was right next to him in the pilot house,” said Capt. James DeSimone, chief operations officer for the Staten Island Ferries. “The two jobs are basically interchangeable.”

As part of their probe, investigators also want to meet with the propulsion system’s manufacturer.

“We’ve asked Voith Schneider Propeller, the manufacturers of the Barberi propulsion system, to meet with us,” DeSimone said.

“Any time we have an issue with city boats, we bring in the particular vendor.”

The hard landing the Barberi experienced Saturday would have been far worse if a quick-thinking bridge operator hadn’t lowered a ramp on the Staten Island side that prevented the ship from ramming straight into the concrete ferry slip, officials said.

Dock worker Ari Vidana was hailed as a hero by co-workers when he showed up for his regular shift yesterday. He realized something was wrong Saturday when he saw the “horror” on the pilot’s face as the Barberi tried to land, one worker said.

“He yelled to the other guy to hang on, and he got the ramp in place, and then he just braced himself,” said a colleague. “He went flying when the boat hit. He got hurt, but not badly.”

NTSB officials said no alarms had gone off in the ship’s engine room before the crash. The chief engineer reported no unusual activity, officials said.

But those findings did not affect the initial determination of equipment failure.

“If there was a mechanical error, you can’t assume an alarm should have gone off,” DOT spokesman Seth Solomonow said. “The boat is an incredibly complicated piece of machinery.”