Back in 2010 I started some research into the movement at the attach points on the Cessna cantilever winged singles. That would cover all the 177 Cardinal series (fixed gear and retractable) as well as the 210 series beginning with the 1967 year model. I wrote an article published in the September 2010 issue of this magazine if you’d like to refresh your information. Of all the planes measured to date, almost none are within the original manufacture tolerance. The lack of in service tolerances means the decision on when to take some action may be left to your mechanic.
As I mentioned in the previous article, this movement between the wing and the cabin mustn’t be too foreboding, as we don’t have evidence of cantilever wings failing at the attachment dowel pins. That said, we don’t have an understanding of why this excess movement is so prevalent in the fleet. All the steel wing attachment dowel pins we’ve measured to this point are within the manufacture tolerance, although almost all are at the minimum diameter.
Most of the movement found on planes we’ve checked to date are within about ten to twenty percent of the maximum. But once in a while a real doozy shows up. We recently had a Silver Eagle converted P210 arrive that the owner’s mechanic discovered movement more than twice the manufacturing new part tolerance. This is the worst we’ve seen and bad enough that the cause needed to be determined and a repair definition created.
The Investigation
The first and easiest thing to do was to remove the wing attach pins for inspection and measurement. The lower pins on the 210 are one inch diameter and the top pins are seven-eighths inch diameter. The 177 series lower pin is the same diameter as the top pin on the 210 and the top 177 pin is three-fourths inch diameter. We found all the pins on this airplane right at the minimum diameter but absolutely no indication of any wear or damage. A little clean up and they looked brand new.
The wing pins showing the “as removed” on the left and the cleaned versions on the right. All measured at the minimum manufacturing diameter tolerance
The bushing pressed into the wing attach forgings and the cabin wing spar carry thru were another matter. There was evidence of deep rust in all the bushings that was mechanically removed. The method of removal was so coarse that all the plating was removed and even some of the steel. As a result, the inside diameter of all the bushings exceeded the maximum manufacturing limits and caused the problem wing movement.
If the wings of your plane have never been removed, then it’s very unlikely the bushings will have any signs of rust. However, we often find evidence of wing change on airplanes with no written history of any wing work at all. The bushings are standard 4130 chromoly steel with cadmium plating. They last a long time when protected from the elements, as when installed on a 177 or 210, but not as well when left outside in the rain.
These bushings are not shown in the parts manual for the wings or the cabin wing spar carry thru. To find out about the bushings, one must contact Cessna engineering for a parts break down of the wing attach fittings and the carry thru. The part numbers aren’t a secret; it probably just never occurred to anyone at Cessna, back in the day, that bushing replacement would ever be performed in the field.
Like the parts manual, the service manual is silent on even the existence of the bushings. The closest it comes to them is the removal and installation of the wing attach dowel pins. There is no procedure, or any mention, of the wing attach pin bushings in the service manual. That means a proper procedure needed to be developed and approved or at least accepted for the removal and replacement of the bushings.
The Plan
A call to Cessna engineering revealed that they don’t have record that this bushing replacement has been performed before. They are pretty sure it has, but Cessna wasn’t involved enough to generate any procedure or documentation of the event. As it turns out, similar bushings are used on many current production Cessnas and a procedure for installation is in the engineering archives.
The procedure for installation of the bushings is to refrigerate the bushing to a minimum of -100 degree Fahrenheit, coat with molybdenum disulfide powder, coat the fitting with the same molybdenum disulfide powder, then insert the bushing in the fitting. Nothing to it!
The refrigerator in the shop break room only gets to about 32 degrees so something more drastic had to be found. I remember those slightly dangerous days In High School chemistry class when we froze bananas in liquid nitrogen and used them for hammers. This job calls for a little more adult supervision. Liquid nitrogen boils at about 320 fahrenheit which should do the trick. Of course, we couldn’t install the new bushings until the old ones were removed.
It’s important to note that at the moment, none of the wing fittings or carry thru spar are available at Cessna. Cessna being Cessna, I’m sure they’d entertain building replacement parts. Keep in mind, they haven’t built these parts in decades, so there will be logistics to sort out to get them built. A supplier must be found that is willing to produce the part and get FAA approval as a parts builder and supplier. All that cost to set up will be spread out over the one part, not thousands of parts like a car factory. These parts are produced with current day overhead, not thirty year old overhead. If you need one, prepare yourself for a financial jolt.
Tooling Required
These bushings are an integral part of the very primary structural wing attachment and main spar assembly. Extreme caution is the word of the day. Excessive force or side loads should be avoided or minimized as much as possible. In other words, driving these bushings out with an eight pound sledge hammer and a drift punch is not in the plan.
New bushings showing the original smooth clean surfaces.
New bushings showing the original smooth clean surfaces.
Since a sledge hammer was off the menu, the next best thing was a press of some sort. The normal shop floor press would work great if we could hold everything in place to use it. A simple tool could be set up using a strong C clamp and a couple sockets. One socket just a fraction smaller than the bushing on one side and another whose inside diameter is larger than the bushing placed on the other side; then press the two together with the C clamp. Unfortunately, the amount of pressure needed will imprint the thin shape of the large socket into the forging. That would be a bad thing.
I designed a simple but effective tool that applies enough force to extract the bushings while providing a large surface area of support for the fittings. Self centering plugs were machined for each different diameter bushing. A Acme thread screw was used to push the plug into the bushing. I was amazed at how much force was required to get the bushing moving. That first movement created a bang that sounded like a gunshot, but it came out just the same.
Installing the new bushings called for some sort of tool that would hold the part and act as the driver as well. I machined six different tools to handle the three different diameter bushings. The business end of the tool has an outside diameter just under the inside diameter of the bushing. It also has a shoulder to push the bushing without damaging or deforming it in the driving process. Some of the driver tools have a shoulder that stop the tool when it reaches the face of the fittings. Other tools were machined to fit through one of the cabin carry thru outer lobes to reach the center lobe.
R & R
Fortunately, I have lots of 210 wing cores to experiment on to allow the process to be fully vetted without worry of ruining someone’s airplane. The down side is that all the core wing bushings are rusted to some extent due to being stored outdoors. As the first test bushing was removed, there were obvious lateral drag marks left on the fittings. We discovered the rust caused some jagged edges on the bushings, which cut a small grove as they slid out of the fitting. Another set of tests removing and replacing the bushings in our 210 wing jig showed that clean bushings didn’t leave damage as they came out. Now we were ready for prime time.
After extracting the bushings we have excellent access to the aluminum fittings to examine them for any discrepancies. Eddy Current checks were performed on all the fittings with detailed examination of the holes where the bushings mount. No cracks were found, so installation of the new bushings was good to go.
The aluminum wing and cabin fittings were thoroughly cleaned and deoxidized, then chem film coated with Alodine. A primer was applied very thinly because the fit between the wing and cabin fittings can be very tight. Prior to wing reinstallation, all these parts were coated with a particular grease called out in the service manual which helps with corrosion prevention as well. The new bushings and the holes they will be installed in were coated with molybdenum disulfide powder per Cessna’s recommendations, and all was ready to fit some new parts.
Then we got to have some high school chemistry lab fun. A good bit of research was done to be sure we were properly equipped and educated on the safe use of liquid nitrogen. A quick Google search revealed lots of wrong things to do with it. As a result, we were properly attired with gloves, goggles, face mask and apron. We are also the proud owners of a dewar, which is basically a fancy thermos to keep liquids cold long enough to get the job done.
We have a couple of short videos on our website of our trial runs removing bushings from a core wing and installing bushings in our wing jig. We’ve reworked the tools since this video, but you’ll get the idea. Visit www.tennesseeaircraft.net and click on “videos” on the menu across the top of the page.
We submerged our driving tool and the new bushing into the dewar full of liquid nitrogen and waited for the boiling to stop. At that point the parts were as cold as possible and the bushings diameter as small as we can get it. Mounting the frozen bushing on the likewise frozen tool also helped keep everything as small as possible as it was driven into the warm aluminum fittings.
I expected the new frozen bushings to just drop in the fittings with no fuss at all, I was sorely mistaken. The relatively narrow bushings in the cabin carry thru went in with a few square shots with a heavy mallet. The wing bushings are the widest and took some pretty serious persuasion with a four pound hammer, but they finally yielded. Just for a final test we slid the dowel pins through the fittings and ensured a great fit.
All back together with the large area retaining washers in place capping off each end of the wing dowel pins
A Plan Comes Together
I don’t know if it takes a village to keep our legacy planes flying, but it certainly takes commitment and cooperation by many people. The good folks at Cessna really came through on this project to support a thirty plus year old machine.
Yes, I know Cessna parts prices can be high, but at least you can usually get them. Of course, CPA is a great source of information, but there are times when the only one holding the data needed is the one who designed it in the first place. In this case, the Cessna support techs were able to dig into their archives and find the specs for the parts and method of installation. Their parts system was willing to find a current subcontractor to build the new bushings and make them available to us. It’s a good thing, too, because these bushing were not found anywhere else in the world.
I don’t usually call out names because I know there are many layers of people involved in supporting our planes. For this job in particular, however, I just feel compelled to give a shout out to a few that spent a good bit of time on this effort via phone and email: Thanks to Carol Gooch in parts, Ron Tramel in product support and Jerry Balman in engineering. I’m sure they, and those they work with, think they were just doing their jobs, but their effort made this project a success.
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