Swapping out that Center Section II
You made it. The old section is out now, how do you get the new one in?
12.After removing the old center section, clean any areas that were not accessible and paint them with a suitable primer to retard corrosion. Self-etching primer dries more quickly than zinc phosphate, but is less durable. Once you have those areas cleaned and primed, reverse the process to install the replacement center section. You will probably need four people to raise the center section into place and one person standing in the middle to guide it into place and install the bolts. Angle the center section in from the rear until the front spar is sitting on the brackets attached under the forward belly skin, then raise the rear spar so it is skimming under the aft belly skin. Once the rear spar clears Bulkhead E, force it up into the fuselage. You may need paint scrapers to help start it and will definitely need to have your helpers pushing and aligning it as you maneuver it into place. Once the rear spar is in place, the front spar will be easy to maneuver into position. Loosely thread the nuts and bolts attaching the rear spar to the bulkhead and then proceed to do the same with the bolts on the main spar at Bulkhead C. Try to align the rivet holes in the rear main spar with the bulkhead. (If the holes will not align, you may need to drill them to match and use doublers and larger rivets.) After the center section is in place, start securing it by progressively tightening the bolts to draw everything together. Complete the installation by riveting the rear spar to the bulkhead.
Once the center section is in place, support the aircraft by either replacing the main landing gear or supporting the main spar. We were still working on the landing gear, so we placed tires on pallets under each side of the spar and lowered the engine hoist until the airplane was resting on them.
There are several joggled angle stiffeners that need to be removed from the old skins and riveted to both the new upper and lower skins before riveting the skins in place on the center section. The rivets holding these in place can be drilled out either while the old center section is in the airplane, or after it is removed.
Once the airplane is resting on its main gear again, you can start putting things back together. It will be much easier to install electrical wiring, brake lines, and pitot/static lines while the skins are off the wings.
Re-install the lower window tracks to the bulkhead and main spar at this time
You will need to joggle the leading and trailing edges of the new skins before riveting them into place. We used a bead roller with dies to do this. There is a triangular doubler plate that rivets into the forward outboard corner of the upper wing skin. Fit that into place first. The leading edge of the upper skin fits under the lip of main spar cap. Attach the upper skins with Clecos, making sure everything lines up. Line up the triangular doubler plate and use Clecos to hold it in place. If you did not pre-rivet them prior to installing the skin, do not worry about installing the other angle bracket doublers until the skin is attached to the ribs. Rivet from the center out along the ribs to the edges. Save the inside rib for last; this is where the center fillet will lay on top of the wing skin and you will be riveting through two sheets of aluminum into the inboard rib. This may require a longer rivet. Once the skin is attached to the ribs, you can install the angle bracket doublers and rivet the skins to the walkway. If you need doublers in the channels under the walkway, cut them out of thin strips of .020" aluminum, have a friend help Cleco them in place, and then rivet the center, then the inboard and outboard along the walkway. As you can see, installing the doublers under the walkway is child's play!
You will need an assistant in the cockpit with a handful of small bucking bars to help rivet the fillet and skin to the inner rib. This is the hardest part since the person with the bucking bar will be working upside down.
Attaching the lower skins is a bit more difficult since you will be working with bucking bars through the lightening holes. You should pre-install the angle brackets since you it will be difficult to get to them with the upper and lower skins in place. Also, if you are installing the inspection plates per SB-32, you should rivet the re-inforcing rings into place before installing the lower skins. Again, Cleco first, then work from the center out to the edges. You may remove the Clecos and pull the skin down from the edges to get a bucking bar onto the ribs in the middle of the center section. Leave the inside ribs for last-- once again, you will be going through two sheets, the wing and the belly skin. Don't forget about the lower trailing edge fillets. If you removed them completely, they need to be Clecoed into place and the front edge of the fillet triangle will be riveted on with the back edge of the lower skin.
Once you have riveted the lower skin in place except for the inboard edge, you should install the belly skin. Again, it may be easier to pre-install the long parallel angle bracket doublers on the belly, but you can get to these from inside the cockpit. Decide if you need to comply with SB-18 for Belly Skin reinforcement at this time. Cleco it in place, have a helper with bucking bars in the cockpit, work from the center out, and finish up with the where the outer belly skin edges meet the inner lower wing skin edges.
Finally, you should realize that you will not be able to get a bucking bar on all the rivets. Try to use standard rivets as much as possible, but when you get to the point that you just can't access them, use Cherrymax or similar blind rivets.
Once you have the skins installed, the center section swap is mostly complete! You can start re-installing wiring, pitot/static lines, and flight controls.
If you are going to this much trouble, here are some words of advice-- 1. Organize your tools and fasteners. As you remove bolts and screws, bag and tag them with the area they were removed from so you can identify them later. Don't scrimp on fasteners. You can re-use screws and bolts, but don't re-use self-locking nuts and cotter pins.
2. In anything looks questionable (wiring, pitot lines, etc.) REPLACE IT NOW! It's much easier to do with the airplane apart than have a problem and do it later.
3. Pay attention to corrosion and trouble spots where corrosion may develop. If you see surface corrosion, remove it using a bead-blaster, wire brush, or grinding stone. Acid etch and Alodyne treat every piece of aluminum, even your skins. Use primer before putting parts together, the another coat after assembly. Self-etching primer works OK if you don't expect any wear or contact, but it can chip and flake off easily. Zinc Phosphate primer takes forever to dry, but is generally more durable. If time is a problem, you can use the quicker drying self-etching primer as a base coat, assemble the parts, then top coat with the heavier Zinc Phosphate primer. You MUST use a dust mask and eye protection when you are spraying Zinc Phosphate primer, especially in an enclosed area!
4. Take pictures and document everything as you go! This is a lengthy process, and you will forget things from the time you start until you finish.