Jan Hovat, our reader from Norway, first brought the issue of spar carrythrough corrosion to our attention. We were able to get his problem resolved without replacement of the spar carrythrough, but in the process we learned of a potential 'causing factor' which deserves attention across the fleet.
At the end of this page, owner Tom Connor explains how he redid his air ducts to avoid any issues in the future.
This issue should be high on the list of details that should be looked into at your next annual. If you find clear loss of material and pitting of significant depth on your spar carrythrough, you should review this page for information on the specifications for assuring your spar is airworthy.
Update: By now most of the Cardinals have had their spar carrythroughs inspected. The large magority found that their spars were protected by a golden coating at the factory, and no issues were found. A few aircraft required redressing of the vent tubing to avoid direct contact with the spar. And many owners chose to replace the cat tubing with scat as preventive maintenance.
At this point only one spar carrythrough has clearly needed to be replaced. That spar exhibited intergranualar corrosion, a thickening and flaking of the spar caused by factors internal to the material at time of manufacture.
However this has proven to be a singular case and owners have instead found reason for preventive maintenance but no serious issue. In other words, we've caught this problem before it became serious.
For example, the story below starts out sounding ominous. However, upon review of all the materials and discussions with Cessna the subject aircraft of this story was cheerfully signed off, returned to service and is still flying with the original spar carrythrough.
There have been a few maintenance shops who have insisted on replacing the spar carrythrough without a clear airworthyness issue. Those members report being told stories which don't match the known documentation, and by all accounts their spars were in airworthy condition when replaced. If a shop recommends replacement of a spar carrythrough and can't show you a clear loss of material, we recommend at least a second opinion from someone who has seen many Cardinals and doesn't stand to benefit from a big project.
In summary, CFO now considered this to be an appropriate point of inspection and maintenance, but not a special issue or risk item. It should be part of the normal maintenance like all aircraft receive, similar to inspecting one's engine mount for cracks and corrosion. Certainly the incidence of actual issues has proven to be slight.
Here is Jan's story:
From: "Jan Hovet" (hovetco@online.no)Subject: C177 Severe corrosion detected in wing spar
My mechanic found some severe (my own description; not technical) corrosion on the wing carrythrough spar today, by the holes where the air vent hoses go. Our guess is that the thin steel wires in this hose has been rubbing against the aluminium. It is so bad that the aircraft was initiall considered no longer airworthy. Stricken by panic, I feared that the whole spar would need to be replaced, and that the cost would be too high for the aircraft.
What Jan had found was that the spar had substantial corrosion in the area near the hole where the air vent hoses passed through it. The hoses on the right side of the airplane can be seen in the above picture.
Then came a later note from Jan..
From: "Jan Hovet"
Subject: Wing carrythrough spar corrosion
About a week ago, I reported severe corrosion on my wing
carrythrough spar. This corrosion was starting from the air
vent tubes which are attached to the spar carrythrough, and
in some cases pass through it. As the second picture shows, there are more tubes on the left side of the airplane that also pass through the spar.
The problem with these tubes is that they are the lowest grade of tubing of this type, called Cat tubing. They are not impervious to moisture, so when the rain comes in it tends to soak the hoses and create a point of corrosion between the steel spiral wire and the aluminum spar carrythrough structure.
The picture below shows a place where the Cat tubing is actually tied up to the spar, causing a potential wear and corrosion point. In this installation there is a pad between the parts, but other owners have reported no such pad and in some cases substantial corrosion.
More from Jan as his saga continues:
From: "Jan Hovet" Subject: Got a new spar!
I am happy to report that I now have a spare spar carrythrough, which came off 88WC. I have not been able to inspect it yet, but the immediate impression is that it is in better condition than my existing one. I'll have to put it through some non-destructive testing. And I would never have found one without the great help I got from Keith Peterson, who has invested considerable time and effort in finding this one for me. Thank you so much, Keith, and if you do come over here, Keith, we'll go flying in the mountains together!
The solution that most Cardinal Flyers have gone to was the replacement of the Cat tubing with Scat tubing, a version with a much better resistance to moisture. With some improvements in the attachment methods and better padding between aluminum and tubing they felt confident that their spar would not suffer any problems.
Jan's work continued for a while, and he ultimately benefited from a letter written by Cessna detailing a method to determine the airworthiness of his spar carrythough. It turned out to be well within specs. The spar was cleaned, alodined, sprayed with Zinc Chromate paint and returned to service. He has since reported having a spare spar carrythough for sale.
Here's a followup from another reader who checked his recently:
From: Rick Wayne (fewayne@facstaff.wisc.edu)
Subject: redbird's annual
Finished up my
part of our 1975 177B's annual last night, I wound up
taking four days off work to do it but it was worth it,
as usual. I took the list's recommendation to drop the
headliner in order to get a good, unobstructed look at
the spar carrythrough and such, and was very glad that I
had. Turned out there was a tiny bit of surface corrosion
on the spar CT; not enough to worry anyone, but
definitely enough to clean off and treat. It may have saved
us BIG bucks a few years down the line.Ref the duct replacement, it took me five hours for the first side (left), and about three for the second on my 1968 Cardinal. (Steep learning curve, less guessing and fewer scrounging forrays). I left the headliner in when I did the left air tubes, and damaged it, so it came completely off for the right side. (Washed the headliner with Simple Green in the bathtub. It was filthy where you couldn't see it, impeding repairs).
The wing root fairings must also come off.
I think the four 2" tubes in the roof are either 30" or 36" long, and the two 3" tubes in the wings are 18-24" long. If I can find the old ones or my notes I'll measure and get back to you.
The 2" tubes above the headliner in my '68 were a piece of cake: It took longer to clean the old tubing remnants off the fittings than to install the new stuff. I suggest hosing the ends with WD-40 to soften the tubing's grip on the metal fittings. I also suggest pulling the plastic air box off the ceiling so you can push the tubing onto the fitting from inside, because the fitting aims upward, and the tubing comes in from the side, and there isn't much room. A machinist scribe with a 90 degree tip helped a bunch with both scraping and reinstallation. The back seat fittings have plastic 90 degree elles that line right up with the tube, so they are simple.
I got rid of the original metal bands and used double tie wraps to reinstall. I also tied the tubes to the headliner support brackets so there is an up-slope to encourage water to drain back to the air box rather than shower you in flight.
The 3" tubing in the wing was a pain. I've had far more fun in the dentist chair, and I'm convinced Cessna set the tube on a saw horse and built the plane around it. I originally tried to replace it thru the courtesy light hole. Fat chance - er - arms. Extra inspection holes at each end of the hose would be welcome. (I noticed enviously that a friend's C-210 has holes right where I needed them.)
Access requires removing the control cables from the air valves and removing the air distribution box from the wing. (Ninety degree ratcheting screwdrivers are the hot setup here). Remove the old tubing from the air box. (Ref the WD40 note above: You REALLY want the next end to come off in one fell swoop). Then, working by Braille, reach thru the wing and pop the metal band off the other end of old CAT and pray the duct comes off the leading edge fitting in one piece, or prepare to scrape.
One might judiciously apply WD40 thru the leading edge fitting, soaking the material in an effort to get a clean release, weeks in advance. With any luck it might fall off in the air blast of flight. This is what I did on the second wing, and it slid right off. However, the end had duct tape on it, so someone else fought the battle before me. (I still cursed him for not replacing the old, brittle tubing with new silicone SCAT).
Take your Doans pills ahead of time. This is a lot of time spent in a half crouch, and is why I didn't do both wings at once.
I also used wire ties in the leading edge wing fitting, which was a mistake. The first one I replaced popped off in the subsequent 50 hours, so I get to do this all over again on one side. I double wire tied everything in the second wing, and used much better quality ties. Time will tell if it works. Hose clamps seem a better solution, but I wonder about reaching in there to tighten them.
Last winter the air ducts clogged up with snow in IMC near Champaign IL, then created a filthy mess when it let loose, flushing out the old corroded tubing. My AP suggested sandwiching a screen between the wing skin and the inlet by drilling out the rivets in the leading edge air fitting, then reinstalling with screws. This would have facilitated installing the hose with a clamp. I ignored his suggestion, and it looks like I will pay . . .
Good luck!
Tom Connor
Thanks, Tom! Webmaster