Spar Carrythrough Corrosion

and Air Ducting

UPDATE 2/2023: As our spar carry-through has gotten more scrutiny in recent years, the issue described below has become less of a concern.

Those of us with engineering training have always considered these slight material losses in the middle of the spar flange to be inconsequential. After all, Cessna® drilled a 4 inch hole through the spar web, so it doesn't seem like critical material. And the simplest stress analysis tells us that this area is not critical.

Indeed, once the FAA and others looked at the spar, the focus turned to the bottom flange, which is where the tension is consentrated, and as such is the properly place to focus. Subsequent inspection requirements do not require inspection of the area described below.

It is still good to keep your spar clean and unmarred, so this tech page will remain, partly as a history document to share the concerns we were looking at around the year 2000.

Jan Hovat, a CFO member from Norway, first brought the issue of spar carrythrough corrosion to our attention. We were able to get his problem resolved without replacing the spar, but since then we have heard of a couple airplanes where the spar carrythrough twas replaced and a few others others that were close to having a problem.

While the issue is far from common in the Cardinal fleet, it is well work checking before purchase and a few moments of time for each owner to make sure their spar is not at risk for future damange.

Several mechanics have shared that this issue also exists on the Cessna® 210 Centurian, so there may be some useful knowledge to leverage across the fleet.

Click here to read Tom Connor's discussionof how he redid his air ducts to avoid such issues in the future.

If it's too late for you, see Peter Staeuble's spar replacement story.

This issue should be high on the list of details that should be looked into at your next annual. If you find corrosion on your spar carrythrough, you should review this page for information on the specifications for servicability.

Here is Jan's story:

From: "Jan Hovet" (hovetco@online.no)

Subject: C177 Severe corrosion detected in wing spar

My mechanic found some 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 is no longer considered airworthy. Stricken by panic, I suddenly foresee that the whole spar must be replaced, and that the cost will be too high for the aircraft.

Does anyone know approximately what cost I should typically be looking at, or what can be done in such a situation?

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. Cessna® has authorized scraping away the corrosion on the worst places, but my mechanic feels that there are too many of them, and that only changing the spar will really fix the problem in the long run. There is also some starting corrosion the side of the spar, where you cannot scrape it away without taking off the wing. And according to Cessna, these spars are no longer available, although previously made to order. Does anybody know where I could obtain a new or overhauled (or "experienced" as they call "used") unit?

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 to the right 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.

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.

There are a couple of key locations which are the most likely to have problems. You can find them by looking at the locations where the CAT tubing touches the spar. With experience one can know where that usually happens and check even if the CAT has been replaced.

This inspection can be done while looking through the flap cable access panel in the back seat ceiling, although proper cameras are a lot better than the mirror that many mechanics use for inspection.

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. Probably saved us BIG bucks a few years down the line.

For information on replacement of the duct tubing, so as to avoid this problem, check out this page.

These pages are a collection of the ideas and impressions of the Cardinal pilots who frequent this site. This information is anecdotal, informal and may not be completely accurate. The Cardinal Flyers are not certified mechanics and do not guarantee the accuracy of the contents of these pages. Please research and confirm anything that is referenced on these pages with the experts appropriate to your situation.

As always, the aircraft maintenance, operations and flight manuals, the FAA and the advice of a certified mechanic and flight instructor, should be your primary sources of information regarding safe maintenance and operation of your aircraft.

All trademarks remain the property of their respective owners.