Page 17 - Volume 15 Number 2

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.
Remedies
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
FEBRUARY 2021
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.
Preventive Measures
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 11 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.
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KING AIR MAGAZINE • 15