The third seat mod is a floor beef-up that allows you to put up 180lbs (limit) in that area. Very few people actually use the mod for a third person. Good luck getting out of that area if you are on fire or sinking....
Key word experimental. An stc is permission to do something to a certified airplane. When an stc is issued there has to be something done to justify the issuance. It is my understanding that the small diagonal is for the purpose of justifying a gross weight increase. While it may stiffen the fuselage, it is doubtful that it makes a great deal of difference. I would not hesitate operating a Cub without these two diagonals. I'll give an example of fuselage stiffness. Have you ever flown a Citabria on floats in rough water? The fuselage twists and wiggles all over the place. It is a wonder that the door doesn't pop open. A Cub doesn't do this without the small diagonal.... but experimental. ......... I doubt I will ever get 180lbs back there but it would be nice to have that capability. ....... So is the short diagonal (2000#) brace worth my time and passenger comfort. As I understand it it keeps the door frame from racking under the heavy loads. ......haven't seen a real stc and second curious as to the process.
Mtv, has explained the old guide set up pretty well, however one should remember none of the guides I
Knew ever
Used the factory seat backs, most of us just used a packboard(that is much narrower) so the guides
Legs would fit passed the "sports" sides, I have an old picture of a guide circa late 70s that is 6'2"
Stuffed into the 3rd seat ! His head is pushed down right into clients, very uncomfortable for them
But he never once complained, loved to fly and hunt!
The cross bar is supposed to be installed in flight. You can physically move the fuselage. I know people who fly without it but I wouldn't.