Typically, a plane is rather low on fuel when preparing to land which means it's tanks had more fuel vapor than liquid fuel. Vapors will generate a massive fireball when ignited. That said, I've seen posts that this same helicopter had encounters with up to three other aircraft that night and beelined towards the AA plane so ... weird, huh? Also, his altitude fluctuated a lot and he was too high. Why was the helicopter in a known flight path for a commercial airport anyway? I know they use the Potomac as a flight path but still.