1952 Cessna 170 Wing Spar Repair

[trafficinsight]

A week in the life of a little plane fixer

In the course of an annual inspection on a clean but well used old Cessna, we poked a mirror into the flap well and discovered a tiny indicator of big trouble... further inspection with a borescope revealed ouchies. Corrosion in the area of the joint between the inboard and outboard right wing aft spar. Very odd for this airplane as the interior was well coated with zinc chromate primer, though looking at the photo you can see that the lower edge of the spar was missed and this is likely the source of this corrosion issue.

Owner wants it fixed so the first step is to disassemble the wing enough to understand the scope of the problem! Removed the flap and aileron. Removed the flap well closeout from the outboard flap track to the end of the flap well to expose the damaged area. Since the outer aft spar extends all the way to the wing rib just outboard of the aileron bellcrank, we needed to remove the aileron auxiliary spar all the way to the wingtip. From there I determined that the best course of action was to splice the damaged inboard section end and replace the entire outboard section. This required removal of the lower aft skin, all the aft rib sections, and the aileron bellcrank structure. We didn't think there was much chance of the wing twisting but supported it with jacks in strategic places anyway.

after drilling out the ribs and outer spar, a very nerve racking cut into the inner spar freed the entire assembly. separating the two pieces showed the full extent of the corrosion.

Continued...

 

[trafficinsight]

After getting everything free I designed and fabricated the splice using aluminum angle, and two doubler sheets of 2024-T3, as well as a stub spar section, also of 2024-T3. Really dusted off those sheet metal bending skills and was pleased to see that it all fit perfectly!

After that I cleaned and primed all of the ribs, test fit everything and used near enough to every single cleco we had to pin everything back together, drilled about a thousand holes and the reassembly phase began.

 

[trafficinsight]

Careful attention had to be paid to the order of operations when putting this back together to avoid riveting ourselves into a corner. About a thousand rivets later it began to look like an airplane again. We added three inspection panels to the area as well to facilitate access for the final riveting and future inspection. Reinstallation and rigging of the flight controls, a lick of paint, paperwork, and this one flew away happy!

 

[ahw01]

Amazing

 

[Plata]

Thanks for the explanation and the photos. So many questions: no corrosion in the same place on the other wing? Was there a smoking gun that showed the cause of the corrosion? How did you maintain the factory twist in the wing (I’d guess that you used the same skin and ribs, so the holes lined up and held the twist)? How much “since we’re in here” additional work did you do?

Interesting shop: rag and tube, aluminum monocoque, composite airframes. Flat and round engines. You must be at a busy airport.

 

[trafficinsight]

Thanks for the explanation and the photos. So many questions: no corrosion in the same place on the other wing? Was there a smoking gun that showed the cause of the corrosion? How did you maintain the factory twist in the wing (I’d guess that you used the same skin and ribs, so the holes lined up and held the twist)? How much “since we’re in here” additional work did you do?

Interesting shop: rag and tube, aluminum monocoque, composite airframes. Flat and round engines. You must be at a busy airport.

Anytime I go to the trouble of gaining access everything gets cleaned and corrosion treated and painted.

Cause of this corrosion is likely moisture between the two pieces on the bare aluminum, only corrosion we found on the whole airframe believe it or not.

I leveled the airplane prior to taking it apart and checked certain spots to have a reference, I felt that the factory twist was not really in any danger as the main spar and majority of the ribs remained in place. Beyond that the aft spar had some pre drilled holes and I made sure those holes lined up with the corresponding ones in the original structure. I also used the jacks to lend more support where I felt it might be needed but reality is that it was probably not necessary. Some more checking at the "clecoing"
It together" stage showed that it hadn't moved at all.

It's a busy shop for sure, I dont work there very much but I get into interesting stuff when they have it.