A firefighter yesterday described in harrowing detail the frantic scene inside a Bronx inferno as he and five comrades became trapped by smoke and flames - forcing them to jump 40 feet to the pavement. The leap killed two of them.

"I saw flames from the floor to the ceiling. Brother talked over brother and it got chaotic," Jeffrey Cool, 41, told a Bronx Supreme Court jury, describing the deadly events of Jan. 23, 2005, which came to be called "Black Sunday."

"It was just real crazy. It was starting to get like hell in there," he said.

Equipped with thermal imaging, Cool could see how flames had cut them off from the front door and the fire escape.

"I said, 'I gotta go get these guys because they don't know the fire's behind them,' " Cool testified. And then he told one firefighter, "Hey brother, we got to go. We got to back up out of here. We got fire behind us."

"It was blackout conditions," he told a rapt jury.

Cool said he could barely see, but knew he was near the others because he could hear them breathing through their masks.

He said that in the ensuing chaotic seconds his tools, an ax and possibly his helmet, were knocked off when he banged into another blinded firefighter.

"There was Mayday after Mayday after Mayday going off. It started to get into a panic state," he said.

That's when he got on top of an air-conditioning unit outside. "Now I'm fleeing. It's time to go," he told jurors.

"I could feel fire coming up my leg. Fire is coming up my back, my neck. I'm thinking about my wife, my kids. I'm thinking somebody has got to get us."

He said he looked to his left and saw fellow firefighter Joe DiBernardo on another window ledge.

"I look at Joey. We're in a bad spot here," he said. "Joey's radioing maydays."