Car seat question - threading seat belt
 

Perhaps I am missing something obvious here...

We have a normal convertible car seat (britax advocate). Forward facing, the seatbelt just threads through the back, between the cushion and the frame. However, on an airplane, the buckle ends up right in the middle of the cushion.
What are you supposed to do about this, besides tie a knot in one side of the belt to shorten it?

Car seat question - threading seat belt
 

I just put one of the folded airline blankets in the seat behind my child's back.

This is a good idea. Along the same lines - I "borrowed" a small US Airways pillow years ago and have been using that ever since.