Walk around nearly any airplane, and you’ll see a few spots of corrosion here and there if you look closely enough. If you’re willing to spend a little bit of time and money on corrosion prevention, you’ll save loads of time and money on corrosion repair.
One of the most significant factors in the corrosion equation is the environment where your airplane lives. An airplane tied down on an open ramp in Baton Rouge, Louisiana, will develop corrosion much faster than an airplane stored in a hangar in St. George, Utah. Unfortunately, a hangar isn’t always an option, but even if your airplane spends the majority of its life tied down on the ramp, there are some things you can do to minimize the potential for corrosion.
AC 43-4B (available as a free PDF download from the FAA’s website) recommends that a “crew familiar with corrosion problems and treatment” perform corrosion checks every 15 days for an aircraft operating in a severe environment (see map), 45 days in a moderate environment, and 90 days in a mild environment. Frequent inspections, emphasizing a few corrosion-prone areas, will go a long way toward keeping your airplane squeaky clean.
The entire underside of an airplane takes a real beating and should be cleaned on a regular basis. Exhaust, oils, fuel, and whatever was scattered, spilled or spewed onto the runways and taxiways is thrown at a high velocity in the direction of the belly, and much of it stays there. Engine exhaust, whether it’s from a turbine or reciprocating engine, quite literally creates corrosion hot spots. Because hot air holds more moisture than cold air, exhaust gas provides a steady stream of moist, hot air combined with all the contaminants of the outgas and byproducts of combustion directly onto everything behind it.
All of that nasty stuff streaks down the belly and sticks to the paint, supplying an adhesive coating that keeps corrosive elements lovingly sandwiched between a layer of scum and your aluminum. Thankfully, all it takes to get rid of all that crud is a little bit of quality time wiping the belly down with a good aluminum-safe degreaser from your favorite aviation supply store. Keeping a good quality paint job clean and dry is the next best thing you can do for your airplane to storing it in a climate-controlled hangar. If moisture and air can’t get to bare metal, there’s no chance of corrosion.
The only part of an airplane that takes a bigger beating than the belly is the landing gear, especially if it’s retractable. The gear is exposed to the same abuse as the belly, but there are more cracks, crevices, moving parts, hydraulic fluid, and grease that make the undercarriage more difficult to keep clean and protected from corrosion. It takes more time to clean the gear than it does the belly, but time spent degreasing, cleaning and thoroughly re-greasing all those moving parts will help ensure that you’ll have three reliable legs under you every time you move the gear lever.
Some other areas that deserve a good look are under and around batteries and lavatories. Batteries are literally buckets full of corrosive material in the form of electrolyte that allow the chemical process of electricity to occur any time you turn your master switch on. Lead acid batteries contain sulfuric acid, and Nickel-Cadmium (NiCad) batteries contain Potassium Chloride, both of which make quick work of eating metal.
Lead acid batteries are typically sealed, so there’s a lower chance of a leak, but that chance is never zero unless the battery is empty. NiCad batteries, more commonly found in turbine airplanes, require maintenance, so they’re not sealed and are more prone to spillage. It doesn’t take a lot of electrolyte to eat away primer or corrosion inhibitor on its way to bare aluminum. Spills should be taken care of as soon as they’re spotted. Baking soda and water will neutralize lead acid spills, and a 10:1 mixture of water to boric acid will neutralize NiCad spills. In both cases, you should flush the area diligently with clean water to ensure all the contamination is gone.
If you’re operating an airplane with a chemical lavatory, special care should be taken. Keep an eye on the skin, cables, wire bundles, etc. under the lav because blue juice, while it doesn’t contain corrosive salts, does contain moisture and other contaminants that will promote corrosion over time. The lav sits over a portion of the airplane that only gets looked at during periodic inspections, typically every 12 to 18 months. If an inspector was less than thorough for even one inspection cycle, you could be sitting on a festering pile of neon blue-colored corrosion. Any shop worth their salt (no pun intended), will be happy to provide you with photos of the area under the lav since all the interior and floor panels must be removed for the shop to get a completely unobstructed view of every square millimeter of the belly.
There are quite a few inexpensive but very effective borescopes on the market now, and there’s really no downside to owning one. For a few hundred dollars, I bought a borescope from Depstech that connects via Bluetooth to my iPad or iPhone and provides a clear picture from which I’m able to record video and take still photos. If you suspect you have a leak, it’s much easier to gain enough access to get a borescope under the floor panels than it is to be able to take a firsthand look.
If you’re walking around your Baron for the hundredth time and you happen to notice some paint bubbling on the RH wingtip around a fastener, do you ground the airplane? Of course not. You’ve looked at this place a hundred times before, and the paint has only just started bubbling up. Your best move is to make a mental note or better yet take a photo to document the anomaly. If you really want to make your mechanic happy, take the picture with a ruler lying on the skin beside the bubbling to show its exact size, orientation, and shape. Make a note to add the trouble spot to your work order when your airplane goes down for its next annual or progressive inspection, and you can rest easy knowing that it’ll be taken care of. It’s far better to spend a little bit of money now to take care of a small spot than it is to “just keep an eye on it” and spend a whole lot more down the road replacing ribs and skin after the corrosion has eaten away enough of the material thickness to make it unairworthy. An ounce of prevention and all that.
Treating corrosion can be a tricky task and shouldn’t be taken lightly. You can easily turn a “quick fix” into a major repair. If you grew up helping your grandparents work on tractors, your first instinct might be to grab a wire brush and rattle-can primer from the National Aviation Parts Association – or, as non-aviation types like to call it, NAPA – and get to work. But that’s not the best way to go about it. You can perform some corrosion repairs yourself, but unless you have technical guidance from either AC 43-4B or your airplane’s structural repair manual, and experience performing such repairs, you’re probably better off leaving it up to your mechanic or repair shop. If you’re cleaning your airplane and see a spot that gets your attention, clean it and get some photos, preferably with a ruler beside the damage – again, us maintenance folks love pictures with rulers – and get in touch with your favorite mechanic or shop. Many manufacturers have a structures department that will help you and your mechanic assess the damage and formulate a plan to take care of it if there’s any uncertainty about its severity.
If you decide to take care of a few spots yourself, make sure you know what kind of material you’re dealing with. If you’re working on a piece of sheet aluminum, there’s a good chance that it’s a sandwiched, corrosion-resistant material called Alclad. Alclad is an aluminum alloy with very thin sheets – about 5 percent of the total thickness – of pure aluminum on the outer surfaces. If you’re using Scotchbrite or sandpaper to remove corrosion, it won’t take more than a couple of swipes to dig down through that protective surface sheet and into the unprotected alloy, which is far more susceptible to corrosion.
It isn’t always easy to identify what kind of metal you’re working on, either. Unless you can see the interior portion of the sheet you’re working on – and if it’s not primed and still has the identifying markings visible – I’d leave it up to the pros. You don’t want a little surface corrosion to turn into a skin repair.
Corrosion is one of the biggest challenges to keeping an airplane airworthy. Gone undetected, it can reduce an airplane to a pile of dust. The good news is that with proper care and a little bit of attention, corrosion can be, at most, an occasional inconvenience instead of a major show-stopper.
