We receive many calls regarding the ‘smoking rivets’ on the inboard bottom of Cessna 210 wings. More often than not, the customer/mechanic will also ask about similar black aluminum soot trails found inside the wing around the wing attach forgings and their attachment pins. This is a first indication of movement between the wing spar forgings and the spar caps. These forgings and pins can be seen by removing the inboard forward and aft wing inspection panels and looking at the spar caps. The 177 has 12 pins per fitting for a total of 48 per airplane and the 210 has 15 per fitting for a total of 60 per airplane. After inspecting these attach areas, here are a few questions to address:
Do you really have movement in the wing attachment forging area?
The black soot is an early indication of movement in the wing attachment structure. A squeaking or creaking sound from this area when the wing is flexed up and down is another indication of probable movement. If you can actually move any of the pins by hand or see/feel movement between the forgings when the wing is flexed up and down then you definitely have….. well…. movement.
Why do you have movement?
In a perfect world, the ideal situation is for all the pins to have a tight even fit with no free play or “wiggle room”. This minimizes movement between the parts and all the pins will carry an equal load. Of course this is not that perfect world. The factory cut all the holes to one size, so the largest diameter pin takes the load until its hole elongates enough for another pin to take some load. The holes in the spar web will elongate first because it is the thinnest part at 0.032”. At some point all of the pins are under a load but at the expense of elongating at least some of the holes. Eventually the flight cycles catch up and the parts begin to move against each other. After a few ‘runway slap-per’ landings and cumulonimbus close encounters, the moving parts will make enough aluminum filings to be seen by the attentive inspector. Add a routine treatment of Corrosion X to suck it out in the open and everyone will see it.
When should you take action?
If you’ve been inspecting this area and just now noticed some slight aluminum soot, then you have time to plan your course. If you made your first Corrosion X treatment within the last two years, I would recommend cleaning the area and inspecting it again at the next annual. However, if you can move any pin by hand then it is time to take action. If you hear that creaking sound when the wing tip is moved up and down then try this; place the end of a finger at a junction between a spar forging and the web then have someone move the wing tip up and down. Any movement between the parts at your finger tip means it is time to take some kind of mitigating action. No matter how small the movement, fretting corrosion has probably started on the mating surfaces and is best stopped sooner than later.
What action should you take?
The repair is fairly simple: remove the Huck pins, ream the hole for close tolerance or 1/64th” oversize hi-shear bolts, and install the new hi-shear bolts. One can say it in about five seconds, but a bit more effort is needed to make sure the repair is actually a cure. Plan for about a 12 hour set up and installation of the first set of pins. 10 hours should do the second set and 8 hours each for the last two sets. Take the time to make a job you’ll be proud of, even if no one will ever know but you. The 177 and the 210 use nearly identical wing designs and have similar attachment hardware. The earlier mentioned pins installed by the factory are 5/16th inch diameter high strength Huck pins that look like round head bolts without threads. An aluminum collar is pressed into groves on the end of the pins to hold it in place similar to the nut on a bolt. The pins have a fairly wide design tolerance in diameter which will aid in the simplicity of our repair. The holes in the wing are cut for an ‘interference’ fit which means the pins require a light tap with a small hammer to install. The diameter of the original pins and holes are just less than 5/16” by about the thickness of a piece of paper. The close tolerance bolts are typically less than 5/16” by only about half the thickness of that same piece of paper This minor increase in diameter is just enough to allow us to ream the old hole to a nice clean round condition.
The Cessna 177 and 210 Service Manuals allow substituting any standard hardware that meets or exceeds the specifications of the original pins. More data for approval and general information on the hardware can be found FAA AC 43.13-1B change 1, chapter 7 section 3 paragraphs 7-39, 7-40, and 7-44. Several different lock nuts may be used, so also pay close attention to table 7-1 for proper torque values. These bolts are for shear applications, so don’t be surprised at the low torque values. We use a close tolerance hi-shear NAS6605 bolt which meets all those requirements and can be installed with a standard lock nut instead of the pressed collar.
Let’s pursue a best case scenario first and assume that your mechanic noticed a black line around the edge of one of the forgings that did not exist at your previous inspection. Our repair begins by placing jacks un-der the wings with just enough pressure to relieve the shear load on the Huck pins. We’ll start with the lower forging and remove the retaining collars so the pins can be tapped out using a small ball peen hammer and punch. Apply more or less pressure with the wing jack to find the neutral position allowing the pins to move with the least effort. Remove no more than half of the pins at any one time to maintain alignment of all the parts during the work.
We use a selection of piloted reamers with cutting flutes approximately 3” long to ream the holes for the new bolts. Genuine Aircraft Hardware (888-247-2738) sells an excellent starting reamer in their catalog; p/n PPRL-.3105. We have similar reamers custom made in ½ thousandths inch increments up to the actual 5/16”. This allows for each hole to be custom fit to a particular bolts’ diameter. Ream the holes with the smallest reamer using plenty of cutting fluid to clean out the aluminum shavings. The holes are deep, so remove the reamer frequently for cleaning. When finished you will have a nice round smooth hole and the new close tolerance NAS bolt should fit in very nicely. Install the new bolts using a light tap with a small ham -mer. I use 4 or 5 layers of masking tape on the head of the new bolt to protect it from the hammer. If the new bolt fits too tight, try running the reamer through the hole several more times with cutting fluid. If it is still too tight, ream the hole with the next size up reamer and try the fit again. Mark each bolt so it can be reinstalled later in its’ respective hole. Getting all of the new bolts to have similar fit is important to achieve the even loading discussed help prevent the original cause of movement.
If reaming for the close tolerance bolts does not fully repair the elongated holes, ream to 1/64th inch over size and use an X value (1/64th oversize) close tolerance hi shear bolt. The process is the same, but you will need slightly larger reamers. It is also far more likely that fretting corrosion is an issue, so the forgings should be removed to fully examine for fretting. This will require removing the wing from the airplane, proving again why repairing sooner is better than later.
Install the first round of new bolts with non-locking nuts and flat washers then remove the remaining old pins. Now ream the holes for the remaining pins and install them with non-locking nuts and flat washers. Remember to mark these bolts as you did the first set for later reinstallation in their particular holes. With the wing jacks still keeping the load neutral, remove all of the new marked bolts. The forgings will now move slightly allowing for a good flush and clean of the area. Next, treat between the metals with a corrosion inhibitor like Dinitrol AV-8 or Boeshield T-9. They are thin enough to allow proper fit of the assembled parts and dry to a wax film. They are also more permanent than Corrosion X or ACF-50. Before corrosion inhibitors dry, use 3 or 4 old AN5 bolts to pull all the layers of parts together. This will keep the forging from flexing away from the spar as the new bolts are installed in their previously marked position using a thin washer under the head and thick washers under the nut as required.
That finishes the bolts one set of forgings. Proceed with the other three sets in the same manner. When the last set is complete, remove the jacks (and the jack pads), clean up the mess, and put up the tools. Always have someone else look inside the wing for forgotten tools/hardware/rags/etc. Then install the inspection panels. The job is now complete, except for the paper work of course.
No special approval or 337 form is required if you are able to solve the problem with close tolerance bolts. Your mechanic may want to do a bit more paperwork if the 1/64th oversized bolts must be installed. Plan the work with your mechanic and be sure he/she is comfortable cutting metal from the wing attachments. A steady hand and a crafts -man attitude for detail will make for a relatively simple repair.
I am still collecting information on this problem. If you would like to participate, please send photos of the inside or each wing showing attachment forgings and photos of the bottom of each wing root showing the most inboard rivets that attach the skins to the main spar. Include the aircraft model, serial number, airframe total time, and any significant damage history information. Be sure the photos are taken before any cleaning is done. I will create a data base and present it in a future writing with some analysis. You can send via e-mail to email@example.com .
So far, I have not seen a pattern that tells me to expect loose pins on any certain serial number group or anything related specifically to flight time. I have noted evidence of pin/forging movement on wings with as little as 3000 hours and have seen wings with no evidence of movement that have as much as 10,000 hours. I suspect it is mostly about inconsistencies in the fit of the original pins combined with extreme flight loads en -countered by the plane over a period of time. In any case, the damage occurs slowly so you have time to schedule a good plan of action.