Its not real clear in the manual, but if you look closely at the rib installation drawing it shows the stiffeners going the full length of the rib, so they actually butt up to the steel tubing on each end. I believe that is why the instructions say to put the stiffeners in AFTER gluing the ribs in place. Whether this is really necessary or not is probably open to opinion; however, my experience leads me to believe its a good idea. I did it on the horizontal stab but neglected to do this on the elevator. When heat shrinking the fabric (Polyfiber) to the final 350 degrees on the horizontal I had no problems, but when shrinking the elevator all the ribs that were not fully stiffened ended up "S" curved from the strong pull of the fabric. It was not too bad, but the small steel tubing in the rear was not real straight. Those 1/8" thick ribs have very little compressive strength anywhere they are not stiffened. Other results and opinions may vary.