It’s a tribute to the King Air brand that so many are still flying today after 30 or even 40 years since manufactured; but the longer an airplane is “alive and kicking” the greater the opportunity for development of corrosion. When assessing the condition of any King Air, I’ve got the possibility of corrosion uppermost in my mind. In fact, I’m packing my bags right now to visit a King Air in Florida with known corrosion issues.
Atmospheric conditions have a corrosive effect on all metals. Aluminum, the primary material long used in aircraft construction due to its light weight, is susceptible to corrosion from air pollutants and is particularly sensitive to the salt-laden environments found in coastal communities. But it is not alone. The components in your engines are made of much tougher material since they are subjected to extremes of heat, fuel and exhaust. These sophisticated alloys are equally at risk for corrosion. Even fiberglass and composites are vulnerable to the corrosive effect of sunlight and air pollutants.
True story: An owner/operator parked his King Air on the ramp of his home airport, located less than 10 miles from the ocean. He always tailed it into the wind. He hoped this would keep the onshore breezes away from his engine intakes and minimize the adverse effects of the ocean air. However, when it came time for borescope inspections, his theory, and his bank account, were blown to bits. Corrosion was severe and widespread inside both engines. Many components required expensive repair and others had to be replaced (with no core credit).
A Tale of Two King Airs
Years ago, I had a couple of King Air 90s coming into my shop, each of them for the first time. One, a C90, was being ferried in from Hawaii. The other, an older E90, had been in the California desert for a while.
Months of preparation went into getting the C90 over to the mainland, and during that time I worried myself sick over the prospect of finding corrosion everywhere. I dreaded having to deliver the bad news. The E90, on the other hand, was of less concern. I’m very familiar with how King Airs fare in a desert climate; extensive corrosion was the last thing I expected to find.
To my surprise, the Hawaiian C90 was essentially corrosion-free! The engines looked like new inside, and we were hard-pressed to find one speck of corrosion on the airframe. How could this be? The maintenance manual has several sections directed toward King Airs operating in highly corrosive environments. The operator of this C90 had followed those instructions to a “T” and with great results. I might add, this C90 was built in 1980, so given its age, this corrosion-free condition was even more impressive.
In the other case, the E90 fared very poorly in the corrosion department. We found it at every turn. The spar caps were corroded, as well as the skin joints that meet the spar caps. We found it inside the wing. The props were coming off for overhaul, but even before we removed them, corrosion was visible all over the hubs. The inside surface of the nacelle tank cover panels were riddled with it. (Take note, this is a particularly vulnerable spot for corrosion on any King Air.) The more we inspected, the more corrosion we found. Everything I expected to find on the C90 from Hawaii was what I found on this E90 after being out in the desert. How could this be? Well, the logbooks and maintenance records eventually told the story. At one point this aircraft had been a bank repo. It reportedly sat outside, somewhere in Georgia, and nobody touched it for two years. That took a serious toll on it.
The spar cap corrosion was a huge red flag on the E90. I immediately called in an NDT specialist for an eddy current inspection to assess the condition of the wing spars. Fortunately for the owner, all damage found was within limits and could be treated. We were able to peel the skins back and take out the corroded panels. We then removed the corrosion, treated those areas as directed by the maintenance manual and the accepted standards of the industry. Before reinstallation, everything was sealed with zinc phosphate (once known as zinc chromate – that ubiquitous yellow-green paint found everywhere behind an aircraft’s cosmetic surfaces).
Corrosion is like cancer. Once it starts, if not attended to, it will spread and become more destructive. Moving the E90 out of Georgia to the desert may have slowed down the spread of corrosion, but it didn’t stop it. Like fungus or mold, corrosion grows into the material. It must be removed by sanding or scraping; if not thoroughly removed it will grow back. After removal, the affected area must be treated. Alodine is one example. It is used on aluminum to prevent corrosion.
Sometimes corrosion is so severe that the only option is akin to surgery – the affected areas are cut out and replaced. The treatment of corrosion, done properly, is labor-intensive and therefore expensive, but it has to be done. Paint shops find and treat corrosion all the time once the old paint has been stripped. Corrosion lurks unnoticed beneath paint. It has to get pretty bad before it disturbs the paint above it. When this happens, you’ll see bubbly patches or clusters of tiny craters in the paint. Just remember that paint is not a remedy for corrosion. It only offers additional protection to a corrosion-free surface that has been properly treated and sealed.
If you operate your King Air regularly in an environment that promotes corrosion, I will assume your shop is following the maintenance manual guidelines for operation in such conditions. But if they only see your aircraft once a year during Phase Inspections, there are a few things you could do in the interim. For example, if you are based on the coast and your sphere of operation is in that local area, consider compressor turbine washes at the end of every day you fly. You might do it every day or once per week, depending on your usage of the aircraft.
There is a compressor wash kit that can be installed on King Airs to make daily compressor washes convenient. Charter operators on the coast do this all the time. It may sound expensive and time consuming now, but when hot section inspections or overhauls come due, you will be singing a much happier tune. At minimum, a good clean water wash (where permitted) will do wonders. If de-ionized water is available, that’s even better.
Lastly, don’t forget the ACF-50. Every King Air owner should keep ACF-50 close at hand. Whenever you clean the aircraft or wipe anything down, spray every moving joint with ACF-50. This is a well-known, anti-corrosive agent. If I were made of aluminum and lived on the beach, I would bathe in the stuff. Continual use of ACF-50 is probably what preserved the Hawaiian C90 so well. I would definitely recommend using it on the aforementioned panels that actually seal the nacelle tanks (underneath the panels in the wing) as they are particularly corrosion-prone. On model 200s, 300s and 350s, the aux tanks have the same design and the same corrosion vulnerability.
A King Air doesn’t have to hang out on the beach to suffer the effects of a salt-laden environment. It might be many miles inland but still subject to salty air, heat and humidity, depending on the prevailing winds or local weather patterns. A King Air that lives in the desert but regularly flies to the beach is likewise vulnerable. Corrosion concerns everyone. There are no exemptions.
Frankly, if you operate regularly in a challenging environment, get with your shop and have them print out the pertinent sections of the maintenance manual that address the conditions in which you operate. Review that information and do everything you can, in between scheduled maintenance appointments, to augment the preventative measures regularly taken by your shop.
A well-maintained King Air is a beauty to behold and to fly.