SIGNATURE RESTORATIONS  

When it came to the sheet metal structure of the airplanes, Midwest’s plans changed as they dove deeper. “We didn’t really plan to take the sheet metal work as far as we did. When we first started the restoration of the three airplanes, our intent was to just replace sheet metal as necessary. While we had the structures open, we’d clean them up and spray epoxy primer inside. Once we removed the wings of the first airplane and took a hard look at the extruded stringers, we realized that there was a bigger problem. In the wing root, where it attaches to the center section, we found that much of the extruded ‘bulb’ stringer had started to rapidly corrode. We went and looked at a number of other wings we had in inventory or that were on other flying airplanes. We even looked at my personal airplane that was being disassembled for restoration. What we found was disturbing. After doing some research, we came to believe that these problems were not caused by the environment that the airplane had operated in, but by the inherent problems in the metal. Granted, a high corrosive environment is going to accelerate the intergranular corrosion problem, but we have found some measure of it in each airplane.”

While Midwest could have ordered new old-stock stringer material and just replaced the bad sections. They determined a better fix was necessary. “We initially just planned to order the NOS stringer and replace the damaged sections per the Structural Repair Manual. As our team had discussed it, we decided that we needed to develop a long term fix. The NOS material would likely be subject to the same production abnormalities that were inherent in the material installed in the aircraft. We couldn’t really see going through the entire process of rebuilding wings and then replacing components with items we were already suspicious of. We decided to invest in having new stringer extruded and heat treated to T3511 versus the original T4. This will provide much better corrosion resistance than utilizing NOS stringer. In order to do this, we needed to obtain FAA approval. That required an extensive report prepared by a Designated Engineering Representative.” << 2 / 4 >>