BAE 146-200 British Airways 1:48

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Hi Gary!
Firstly thanks so much for putting this here free to download. Models like yours helped me enormously in the pandemic and other life challenges and made me very happy.
I am confused about the wing spar. I can see there’s a box section for the inner and outer spar. But there is also a 2-part ‘flat’ section which runs the length of the 2 box sections. I am struggling to know where it goes.
At first I thought on top would be logical to help prevent sagging, but realised the ribs aren’t as tall as the box and flat combined, so then thought actually it would strengthen the box section better if the flat was placed on the front or rear faces of the box.
Please can you advise?
Hope you can help!
Kind regards,
Alan Newman