NBC4 chopper's final moments: a blaring alarm, loss of altitude and panic. Why did it crash?

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Set us as preferred NBC4’s helicopter was one of several news aircraft flying over the scene of a deadly bus crash in Chatsworth Tuesday evening at sunset.It’s common for multiple news choppers to arrive at the scene of breaking news, beaming live images for newscasts and streaming.But around 7 p.m., something went horribly wrong and in a matter of seconds the NBC4 chopper dropped out of the sky, crashing into shipping containers and vehicles outside a warehouse a few blocks away, bursting into flames.Two station journalists, reporter Eliana Moreno and pilot George Marciniw, were killed along with a bystander on the ground.Federal investigators are now trying to figure out what went wrong.

The Times interviewed several experts, who reviewed recordings of the chopper’s final moments.They offered insights into the helicopter itself, its movements that afternoon and what is known about how its mechanics work.Here’s what they had to say:A video stream from the helicopter, as it was covering the bus crash, appears to show the helicopter hovering just before it came crashing down.

An alarm can be heard blaring as the helicopter appears to be quickly losing altitude.That alarm would be triggered by a potential engine failure, causing low propeller rotation, or a problem with the hydraulic system, said Jack Cress, a former helicopter pilot for the U.S.Marine Corps and a helicopter accident investigation instructor at USC’s Aviation Safety and Security Program.If the helicopter suddenly loses RPMs, there is a safety maneuver known as autorotation, where the pilot can try to gain enough momentum and upward flow on the rotor blades as it descends and moves laterally to increase rotation and control its landing.

But Cress said the fact that the helicopter appeared to be hovering in the air, and at a low altitude, would make it...