There she is, sitting by the wash rack: the picture of health. You’ve just finished washing off the bugs and grime, vacuumed out the interior, and you can’t wait to head out on your next cross-country. One thing that we often don’t stop to consider however, is the question of whether or not the next aircraft you eagerly await flying looks as good on the inside as it does on the outside. I don’t mean the upholstery or avionics, either.
APPEARANCES CAN BE DECEIVING
As it was with Oscar Wilde’s Dorian Gray, old age and deceit can sometimes fool us. The average general aviation airplane is old enough to vote. Believe it or not, if you were to pick out any airplane from among those tied down at your local airport, there is a good chance that it was probably around before your senior prom. To be precise — according to the EAA, GAMA, and AOPA — the average age of factory-built general aviation aircraft is now more than a third of a century. Translation: By the year 2010, the average age of aircraft in our general aviation fleet may be near 40 years.
We can fix the faded paint, worn upholstery, and antiquated avionics. We can replace a run-out engine. But some things are a bit more pernicious. I am referring the big “C” … corrosion.
ASSAULT AND BATTERY
In a way, corrosion operates by the same electrochemical reactions that are responsible for tainting those bronze-plated baby shoes, or those within a common battery. How it works: Two substances (metals, usually) interact with help from a conducting fluid or electrolyte, which carries electrons from one metal to the other. As you might guess the fluid need be nothing other than water. Corrosion can even occur in the virtual absence of a conducting fluid. One example is two different metals in physical contact with each other. Most aluminum airplanes are exactly that, by the way: an alloy, where assemblages of atoms from one type of metal are intermingled with those of another. When corrosion occurs, the byproducts of these electrochemical reactions (such as oxides, in the case of iron, which we simply refer to as rust) begin to replace (and displace) the original metal. The ultimate result: decreased strength. The infamous Aloha Airlines cabin that turned into a convertible fifteen years ago, in which the top of the passenger cabin (which had been substantially corroded and then weakened by many pressurization cycles) suddenly peeled away in flight, sucking out one flight attendant in the process, is a memorable example.
There are various ways to prevent corrosion, such as plating the metal surface with a pure metal cladding, or simply painting it. Either one keeps the electrolyte (water) at bay. There are various types of corrosion, each having a different name. And you’ve probably seen some of them. Did you ever see, for example, a widespread area that looked sort of like bumpy cottage cheese that had been painted over? Actually, rather than a surreptitious cosmetic fix being involved, this was something of a special case. Certain late-1970s Cessnas had not been properly prepared prior to painting, and some material involved in the painting process acted as an electrolyte between the metal and the paint, which was later applied. The paint, rather than protecting the surface, served instead to promote and protect the chemical reaction over large areas. (That is known as filiform corrosion.)
Fumes from an aircraft battery can accelerate corrosion — as can exhaust gasses. Any pitted areas in a painted surface (or chips in one that is chrome-plated) are prime breeding grounds for corrosion. Other areas to watch are those within crevices, like the joints in wing skins. Here, water and airborne pollutants can remain protected and in contact with metal. Another type of corrosion occurs at the site of a welded joint (such as engine mounts). Stresses in areas that endure frequent and repeated load cycles accelerate the effects of corrosion. This is the case when two surfaces “fret“, or are in light contact. The wheel wells of retractable gear aircraft are another high-risk area, being frequently exposed to water, dirt, (and airborne salt, anywhere near the ocean). High-workload critical areas that routinely experience abrasion such as propellers should head the list of spots to check, and for constant-speed propeller installations, the hub and blade attach points, as well. High heat areas with multiple recesses, such as the cooling fins on engine cylinders, any areas around fuel tanks and bladders, piano-type door hinges, and of course, the battery box are all places to look. For that matter, if you live anywhere near the ocean shore, your whole airplane is up for grabs.
Corrosion often starts right out in the open, when a protective covering is abraded, scratched, flakes off, or is otherwise lost, thus exposing the alloy surface to moisture.
- A process called intergranular corrosion can begin at boundaries between the grains of some aluminum alloys. If allowed to progress, it can reach the stage where the byproducts of corrosion occupy more volume than the metal being displaced, resulting in a phenomenon called exfoliation. In this case, layers of the material expand upward — much as in the case of weathering rocks — and break loose in sheets, somewhat like fine phyllo in the popular Greek dessert baklava (or in the case of your airplane, layers of aluminum foil).
- Stress corrosion cracking also follows grain boundaries and can occur in areas of sustained tensile stress, or load-bearing structures.
- Another common form of corrosion on aluminum alloys is pitting corrosion, first noticeable as a white or gray powdery deposit, similar to dust. When cleaned away, tiny pits or holes can be seen in the surface.
WE ARE NOT ALONE
I must hastily note however, that interceding between the floor of our aircraft and eternity, are A&P mechanics. These gallant souls usually come early to our rescue. The signs of corrosion are well known to those who maintain our aircraft, and inspections to mitigate it are very specifically delineated. You can have a look for yourself in publications such as FAA Advisory Circular 43-4A, Corrosion Control for Aircraft, which describes the identification and treatment of corrosive attack on aircraft structure and engine materials in detail. (That’s what all those access panels on your aircraft’s skin are there for: knowledgeable probing with gooseneck flashlights, mirrors and the occasional borescope.)
Where to look, and what to look for:
- propellers, propeller hubs
- whitish-gray powder on aluminum
- trademark red rust on steel
- bumps or blisters beneath painted surfaces
- trailing edges of control surfaces
- inside wheel wells for retractable-gear aircraft
DEFENSE: WHAT YOUR A&P WILL DO
For areas of minor surface corrosion, what is usually involved is removal with abrasive action down to a level underneath where it exists, followed by a protective coating. For corrosion severe enough to have done away with significant amounts of your airplane, patching of the component — or replacement in its entirety — may be required. Most service manuals provide guidance in determining the extent of repair necessary.
Some locales are worse than others. The “Boneyard” at Davis-Monthan Air Force Base out in Arizona wasn’t chosen just because real estate out there doesn’t go by the square foot. It’s pretty parched country, and corrosion isn’t real high on the list of things that make airplanes get old (ultraviolet light, yeah … but water and salt, no way). The aridity of the air keeps electrochemical reactions from making much headway. As I already mentioned, and as you probably already knew, coastal areas (especially those near major population centers having higher levels of pollution) are the absolute worst. Salty air makes for a great galvanic cocktail party, and your airplane is the hors d’oeuvre. Take a nice salty fog, mix in some exhaust fumes, and you can get a pH approaching a value of 2. Good paint and frequent inspections, and lots of washing and waxing are obviously more than idle activity there. (When it comes to airplanes, the US Navy is the expert on the subject of fighting corrosion with penetrants, primers, plating, and other manner of coatings.) Applications of anti-corrosion treatments, such as the corrosion inhibiting compound ACF-50, are worth the investment. (This compound prevents future corrosion as well as helping to stop corrosion already in progress.) Being attentive to something as simple as keeping drain holes unclogged can be important. Obviously, a hangar becomes a more important investment (especially if you live in places like Hawaii, the coast of Maine, or Hilton Head Island).
THE BOTTOM LINE: Although the cost of repairing corroded aircraft is estimated as being 100 times the cost of preventive maintenance, the degree and extent to which corrosion might concern you depend greatly on the relative dangers in your particular climate and operating environment. The amount and rates of potential corrosion are also a function of the type and frequency of preventive maintenance you provide. May there always be less of the former, and more of the latter.