By Scott Sherer
Cessna Owner Organization

Editor’s Note: This article references an FAA corrosion document. You can download it here.

Pictures referenced in this article appear in the September 2018 digital issue. Read it here.

The good news for me is that my airplane is now in excellent condition, and since I moved to Burlington Municipal Airport (KBUU) about four years ago, my annual inspection cost has come down dramatically. This, however, wasn’t always the case.

Having owned six different aircraft in the past, I’ve owned my current airplane, a 1977 Seneca II, for more than 14 years. From the start, my plan was to purchase a “project” airplane for a low cost and restore it to mostly new condition. The Seneca proved to be the right plane at the right price.

Although it had been based in Norway for most of its life, I purchased the plane from a location in Venice, Florida, where it had resided for about four years before I rescued it. However, those four years in Florida, near the Gulf of Mexico, really took their toll.

For starters, the engines were run out. Amazingly, both Continental TSIO-360Es had 2,400 hours on a 1,400-hour TBO! How the previous owners, a flight school, had gotten away with that I have no idea.

At any rate, my first order of business was to have both engines removed and sent to Airmark Overhaul ( in Fort Lauderdale, Florida, for a complete overhaul. My experience with them was A+ and I would recommend them to everyone.

When the engines were removed for shipment and the engine mounts (along with everything else north of the firewalls) were removed for inspection, we found that the engine mounts were filled with rainwater. Yuck! This was my first indication that I would have to be very careful in the restoration process.

My then-mechanic wisely recommended spraying the wing interiors with a corrosion inhibitor called ACF-50. ACF-50, by the way, is available from a variety of vendors, including Aircraft Spruce. In fact, here’s a rather apropos quote from the Aircraft Spruce website describing the product:

“ACF-50 kills the corrosion process with just one application and will last for 24 months. Microswitches, cannon plugs, and relays will remain corrosion-free. Plus, ACF-50 will easily free seized nuts, screws, and bolts. ACF-50 holds written OEM approvals from: Gulfstream, Bombardier, ATR Regional Transport, Douglas/Boeing Helicopter, Bell/Textron, Robinson, Enstrom, Sikorsky, MD Helicopters, Schweitzer, Hiller, British Aerospace, Concorde Battery, Raytheon, Piper, Cessna, Pilatus, Beech, Van’s, Extra, Air Tractor, Lake, Rolls-Royce, Britten-Norman, Learjet, McDonnell Douglas, and Canadair. Try ACF-50. You will truly be amazed at the results!”

Having gone this route, I felt pretty confident that my airplane didn’t have any serious corrosion issues, and for the next 10 years there weren’t any. Later, when I moved to BUU and it came time for my first annual, they determined there was a lot of “catch up” work needed.

From then on, each year they would dig a little deeper into the airframe and engines during the annual inspection. For each of the first three years, my inspections were expensive — nothing serious, just lots of little deferred maintenance issues resulting in lots of labor hours and some parts.

Over time the “little” things were addressed and my inspection costs dropped off by nearly two-thirds. Obviously, I’m pleased to know that my airplane is safe, but I’m also pleased that the cost is now under control! However, while they were catching up and digging deeper, they discovered corrosion in four places. This article is about what they found.

Finding Hidden Corrosion

I’m very lucky in that, for the last four years, I’ve been using the Cessna Owner Organization’s A&P, Erich Rempert. Erich represents and implements the maintenance philosophies of BUU shop owner Jacob Remington. Those philosophies stem from one basic directive: “Safety and Quality First” period.

From time to time, I may moan and gripe about how long something is taking, but at the end of the day I know that this firm isn’t taking any shortcuts on my airplane. I truly hope that your shop shares the same philosophy.

As I said in the opening paragraph, the good news is that my airplane is in very good shape. The bad news, however, is that I have two small areas with corrosion that need to be addressed this year. Fortunately, neither are airworthiness issues, nor are they particularly expensive.

At my request, Erich was going to re-skin the nose gear doors on my Seneca, as there was some skin damage that I caused with my new aircraft tug. When he went to unrivet the skin, the inside of the nose gear doors had corrosion — and much more than just a little.

We also removed the center console between the two front seats to replace two electrical switches with new engraved switches, and we found surface corrosion there. A bit of Scotchbrite, some Alodyne, and an hour’s worth of elbow grease took care of that. The best part of that project was that I did the repair myself – under the watchful eye of Erich, of course!

A little over a year ago the news during my annual inspection wasn’t all that good. Hearing the word “corrosion” from your A&P is similar to hearing the word “cancer” from your physician. They’re both scary, but we’re only talking corrosion in this article.

As I mentioned, I’ve owned and have been restoring this airplane for the last 14 years. I got it in questionable condition, but it was as close to free as any 3,000-hour twin could be.

Upon purchasing the airplane back in 2004, I gutted the interior and addressed a little bit of corrosion around and under the windows. When the engines were out for overhaul, everything firewall forward (and behind) was also addressed.

Later I replaced all of the glass so that there were no leaks, and the landing gear was overhauled and treated. Of course I also had ACF-50 sprayed into the wings to arrest any corrosion that may have been there. While the ACF-50 wept from the wings for several years and was quite annoying, it certainly was the right thing to do.

Basically, the airplane was in good shape with no corrosion that could be found — until recently. The two locations where there was corrosion were dissimilar and unrelated.

FAA Enlightenment

Erich recommended that I download and read the Advisory Circular AC-43-4A entitled “Corrosion Control for Aircraft,” available at no charge from the FAA website, I retrieved it and had an epiphany while reading the document. You should retrieve this and read it, too.

According to the document, there are seven forms of corrosion that occur on airframes. Seven! As depressing as that sounds, I took comfort in my airplane only having two small areas containing two forms of corrosion. So without further ado, here are the seven types.

  • A. Uniform Etch Corrosion
  • B. Pitting Corrosion
  • C. Galvanic Corrosion
  • D. Concentration Cell Corrosion
  • E. Intergranular Corrosion
  • F. Exfoliation Corrosion
  • G. Filiform Corrosion

Rather than try and quote the FAA publication on what each of these are, I recommend that you download the document and look on page 14. The descriptions are accompanied by photographs to show you what each kind of corrosion looks like. The publication also includes complete, detailed descriptions on how to remove and repair the corrosion should you find any. Now, back to my airplane.

Playing It Safe

The first place that corrosion was found was on my vertical stabilizer, between the stabilizer and the rudder (see Picture 1 at the beginning of this article). Once the rudder was removed, Erich removed the rudder attach hinges and, lo and behold, underneath the hinges was pitting corrosion. Picture 2 is a close-up of one of these two areas. At first glance you see that it is shiny and clean. Well, it is shiny and clean, as Erich cleaned up the area — but you’ll notice that above the large hole is a round area that looks slightly bumpy and not as shiny as the other cleaned-up area. I thought that Erich would just clean it up, perhaps treat it with an anti-corrosion treatment like chromate primer, put the hinge back on, reattach the rudder, and on to the next project.

Unfortunately, however, it never seems to work out that way. Instead Erich took a caliper and measured the thickness of the good area versus the area with the pitting corrosion. What he discovered was that more than 10 percent of the thickness of the aluminum plate had been eroded away. He said that it was not a safe practice to just treat the area and reassemble it once more than 10 percent of the aluminum was gone. Of course I’m thinking, “How much is this going to cost?” Erich is thinking safety.

At this point I’m also thinking that I’ll impress him with my 47 years of airplane ownership and give him the answer before he could tell me what he was thinking. So I said, “throw on a doubler!” while feeling quite proud of myself. He slowly shook his head “no.” Can’t do that, apparently. Instead we have to order a new plate; drill out all of the rivets; prime, paint, and install the new plate; and only then put the rudder back. “Well, it’s only money,” I thought and told him to proceed. Picture 3 is the new rudder vertical spar after replacement — nice and green and new. And safe! Picture 4 is a close-up of that spar.

A Near Miss

A couple of days later I was back to see the progress on my plane, and I got called over to the cabin area. Erich had been searching the entire plane for corrosion, and not to be denied, he found some. He had removed the back two seats and the carpeting underneath them to check on the control cables and pulleys. If you look at Picture 5 you’ll see a steel angle bracket riveted to two pieces of aluminum. The green chromate can be ignored, as can the glue that was holding some insulation and carpet down. It is obvious that the steel plate has a significant amount of rust on it, and 6 inches away is another bracket holding another two pieces of aluminum together — that bracket is rusted, too.

I figured that Erich would get some Scotchbrite, clean it up, re-chromate it, and move on. Of course, no! Instead, Erich drilled out the rivets and removed the two brackets. You can see what he found in Picture 6. Underneath the bracket on Picture 6 and on Picture 7, the aluminum turned to dust. Erich caught it in time to prevent the deterioration from spreading to other areas. Now all we had to do was order new brackets and the appropriate aluminum parts. And of course, pay for it. Again, it’s just money — I didn’t want to leave my kids any, anyway! If the pictures don’t specifically mean anything to you, try this on for size. The elevator, pitch trim, rudder, and rudder trim cables are right there, supported by pulleys and a shaft through them. The corrosion came within ¼ inch of the shaft when the corrosion was found. This could have resulted in a very fatal day had Erich not found the corrosion.

On the interior, apparently there was a water leak at some point long ago (at least 10 to 15 years ago, at best guess). The water pooled under the rear seat and started the corrosion, which festered for at least a decade and was missed by all of my prior mechanics. Unfortunately, at the very least the corrosion cost a couple of months in the shop and some money. At worst, it could have progressed to something unthinkable, as mentioned previously. I don’t even want to go there.

Pictures 8, 9, and 10 show corroded parts which were removed from the airplane. You can look at these pics and come up with your own words. Make a mental note to have your mechanic dig very deeply in your plane while doing the annual. No shortcuts to save a few bucks! Picture 11 shows the area cleaned up, with the old parts removed. I noticed that there were two signs taped to the airplane: one by the front door and one by the rear door. I asked my corrosion expert, Erich, why. He said, “the parts removed were structural. If someone gets in the plane while these parts are removed, you could bend the fuselage.” Oh, that’s just wonderful! You can add 2+2 yourself. The corroded parts were structural. Oh boy! Looking at Picture 12, you’ll see that the entire floor was cleaned and coated with two coats of primer. I can’t print every picture that I took of the process, but my expert, Erich, removed many more aluminum pieces from the area, inspected, cleaned, primed, and reinstalled them. You see two new aluminum spars and steel brackets installed. Almost done!

Lesson Learned

Another item that I’ve learned: First, what you’re looking at in Picture 12 is the floor. Actually, I mean the real floor. This is the only piece of metal between you and a great view of the ground below. There isn’t a second layer anywhere to be found. The structural integrity becomes ever more obvious when you think about it in these terms. Second, there are actually several tiny drain holes in the aluminum skin that you’re looking at. Any pooled water in this area should be allowed to drain out. Obviously it didn’t. Why? My mechanic told me that my carpet throughout the airplane had been glued down with adhesive and that he was troubled by that.

With the carpeting glued down, none of my mechanics of the last 10 years were able to inspect the area without damaging the carpeting — so they didn’t. As I personally installed the upholstery kit that I got from Airtex just over 13 years ago, I began to wonder if I didn’t do it correctly. In case you’ve never installed your own upholstery from Airtex; these are wonderful high-quality custom kits that you install yourself and save the labor of someone else installing them. Airtex kits don’t come with any instructions whatsoever. At least they didn’t back then. Their support is excellent, and they will answer any installation question that you may have. However, nowhere that I can find does it say not to install carpeting with adhesive. Since I’ve now done four airplanes with Airtex interiors and have glued all of the carpeting down, I’m wondering what other purchasers are doing?

I asked Erich what to do and he took me over to a Beech King Air. He said to do what the expensive business aircraft do: use either Velcro or snaps. That way a mechanic can remove and re-install carpeting in just a few moments. Water will find its way to the drain holes, should there be any, and mechanics (and owners) can check for corrosion at any time. Thanks, Erich!

So I ordered new carpeting for the rear of my airplane from Airtex and installed it, with Velcro. I got my plane back together and back in the air.

At the end of the day this was the extent of the corrosion damage. With a couple of down months and a significant bill comes peace of mind. I have a safe, reliable airplane that’s aging gracefully and safely. It pays to have a quality team taking care of your airplane, and I feel like I have the best. I hope you do, too. If you have questions about your airplane or feel like your mechanic isn’t digging deep enough during its inspection, talk to him. You have an expectation of quality and safety in your flying machine. If you’re worried about that quality level, don’t ignore it. It’s your life!

Safe flying,

Scott Sherer

Read this article in the September 2018 issue.