Hang Glider wing: How to make a rib's curve tangent to the intersection curve made by a wing's tube penetrating rib's plane?

You can "cheat" a little bit with "spline on surface" (in a 3D-Sketch) which are connected on the rib plane. The spline should be a closed loop, this will give you a ellipsis. on this sketch can you connect now the rib-sketch.
In the zip-file is my experiment.

  1. Step 1: