It's actually very thin steel - I am just so used to typing aluminum that's all I know how to write.
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On fire - with a three-quarters cowl (the lower third is cut away, exposing the engine), fire is going to wind up exiting the compartment and along the bottom of the fuselage if it gets going. That's one of the reasons I opted for a wooden floor (as in the original) versus running fabric all the way up, as a lot of folks do with these replicas.