Hi niko,

> Why is that method not working here?

It's because your wing part is not just a single surface, it is made up of 4 surfaces joined together.

Note that when you select it, the object type indicator (circled in red below) says that it is a "Joined srf", and not just a "Surface" like it would if it was just 1 surface.



And similarly the fuselage is also made up of 2 surfaces joined together.

You could still use surface/surface filleting by breaking things down into individual surfaces by selecting them and running Edit > Separate.

But that surface/surface filleting will usually be something that you would try if the edge-based one is unable to figure things out.

It can be harder to deal with the surface/surface one when there are multiple pieces involved because you may need to do things like trim the fillet pieces manually and stuff like that.

- Michael