Tail Cross Tube Support Modification


Some builders have reported making this modification. It allows easier tightening of a loosened tail support cross tube AN3 Philips head bolts used to secure the tail cross tube support. It seems, in some cases, the nuts that complete the bolted cross tube connection slightly crushes/bends the 1" down tube leaving some slack in the connection which can result in up and down movement in the tail assembly.


Because this loosening occurs in a small number of Challengers, we can only speculate as to why it develops on some planes and not on others. This loosening is probably due to rough handling such as repeatedly dropping the tail hard when getting out of the plane, or frequently allowing the tail to contact the ground when taxiing too rapidly over a rough surface. Another possibility may be when a Challenger is tied-down outdoors with the tail wheel on the ground in high wind conditions. This tends to apply high loads to the cross tube. Also, a landing in which the tail wheel contacts the runway either before or at the same time the main gear touches down will place higher than normal loads on the joint connections in question.

While the following option for strengthening the crush resistance of the down tubes at the cross tube supports is not normally required, it may prevent the looseness from developing in those instances where the plane operates in unusual environments. Our suggestion is to be considerate of the plane. Try to avoid dropping the tail, or otherwise unnecessarily loading the tail. The plane is tough but not indestructible. The following remedy may help to prevent this.



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This modification to the tail cross tube support/down tubes is a very simple one to make "if" you have not covered your fuselage yet. If you have or are making this modification to an existing built Challenger, then it will be far more difficult due to its location on the frame and the close quarters of the enclosed fuselage interior. The tail cross tube support (2FB-9) is located at the rear down tube at the firewall, where the down tube meets the rear-most six-tube junction plate. The AN3 bolt and nut in question here is the AN3, Phillips head bolt securing the tail cross tube support to the rear-most down tube. The tools needed in an uncovered fuselage will be a #2 Phillips screwdriver and a 3/8" box wrench or a box wrench and a 3/8" nut driver. If your plane is already covered and the fuel tank installed you will need a short-coupled ratchet driver with a #2 Phillips tip and 3/8" box wrench to remove the bolt and nut.


You will need to order the following parts. 2 AN3-16A bolts (undrilled) 1-3/4" long, 4 AN960-10 flat washers and 2 AN365-1032A nyloc nuts. You will then need to fabricate 4, 1/3rd rounds (approximate) 3/4"x2" as additional tube supports to be placed over the bolt on the nut side of the down tube. Use two 2 of the 1/3rd rounds, or saddles, per bolt to double their strength. The 1/3rd rounds should be made from the same size tubing. If using larger tubing, the pieces will need to be bent slightly to make a smaller curvature as I did here. These pieces were made from a piece of 1-1/2" Dia. tubing removed from the leading spar of the ailerons when they were cut to fit a wing with fiberglass tip, then hammer formed to create the same size curvature of the 1" down tube. Cut the tubing to about 3/4" length with a Dremel tool with a metal cutoff wheel or any other suitable cutting tool. Measure and drill these aluminum saddles to accommodate the AN3 size bolt, as seen in the photo. NOTE: The tube rounds only need to be square, not as tall as shown here.

         
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Aluminum tube standoffs are also available commercially, as well as from the factory, and are a better choice than making the tube rounds (saddles) like shown. The tube standoffs you can buy are flat on the nut/washer side and curved to fit the tube on the other end. This provides a more even distribution of loads across the tube as well as across the nut/washer face. If used, 2 AN3-20A bolts (undrilled) 2" long are required instead of the 2 AN3-16A bolts.

         
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Do one side at a time. Replace the AN3 Phillips head bolt used to bolt the tail cross tube support to the down tube. Reassemble with the AN3-16A hex headed bolt in the following order. Slip the AN3-16A bolt, head toward the fuselage nose, through the AN960-10 flat washer, then slip the bolt with washer through the tail cross tube support and the down tube. Slip 2 of the rounds (saddles) onto the rear facing threaded end of the bolt, assemble a AN960-10 flat washer and the AN365-103A nyloc nut. Using the appropriate wrenches tighten the nyloc nut to about 100 125 in/lbs.See following photos for completed modification.



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