The fire you see isn't in the engine compartment.  

I've looked at several different videos of this one and it doesn't look like the fire is in the engine compartment. You EC-135 guys will have to educate me on what is in the compartment below the exhaust stacks, besides the tail boom attachment hardware, but whatever it is it needs ventilation. There are two screens on the left side, and you see the fire through the larger and aft most screen. I was a Bell medium type and, on the Huey/205 that is where the oil cooler is located.

Could an engine fire burn through the deck and into that compartment that quickly? The flight was around 3 minutes from takeoff to impact. Looking at the radar trace it looks like a pretty lazy turn back to the airport, and the pilot said he was returning to company parking, so it sounds like he'd delt with an earlier problem and was taking it back home. Did he even know there was a worse problem still ongoing?