Bitmap mask instead of power window

I want to bring a shadow of spinning rotor blades (only!) on to the chassis of a helicopter. The shadow were composed and rendered as a green screen clip. Fusion failed in countless attempts to render the entire scene with all components, so I decided to go via a green screen clip.
In color tab I have to mask the shadow to the helicopter. Becaue I have a bitmap of it I wonder how I could use it as a mask instead of a power window. Any idea? May be there's a simple solution I didn't found yet.

Try importing your shadow as a matte clip, then in the node tree you can add an external matte.
(you may have to make a matte only version of your shadow first)

Thank you for your advice. I tried it out, but unfortunately there's a new problem. It was easy to compose the matte clip in Fusion and in preview all is fine but back in the timeline there's only a black screen, same in the rendered video. Next try I'll color the shadow, may be my green background disappears anywhere