If you encounter significant corrosion during an annual or pre-buy, fixing it may be an option. However, as Coy Jacob reported in The Aviation Consumer, you'll need to exercise care in estimating the repair costs.
February 24, 2002
When we're airplane shopping, not many of us buy new models so we're stuck
making our picks from an increasingly older fleet. You'd think that a
30-year-old airframe flown a couple of hundred hours a year would be tapped out,
but most aren't.
Yet many are corroded almost to the point of being unrepairable.
Increasingly, in doing annuals and pre-buy inspections, we're finding instances
of serious corrosion undiscovered in previous inspections and, occasionally,
hidden beneath a quick fix of primer and paint. This is becoming tricky business
for buyers. You might very well negotiate a sweet purchase price on a nice
airframe with reasonable time and decent paint and avionics only to discover
months later that it needs $20,000 worth of corrosion repairs.
Better to find that out before closing the deal than after. And if you do
find it, tread very carefully in negotiating the price downward to pay for
fixing the rust. We often find that in disassembling structures for corrosion
repair, the damage is worse than we expected.
Flying is Better
Things You Hope You Never See
It's no exaggeration to say that an airframe's maintenance history is
written in any corrosion you find on a pre-buy. Properly inspected
airframes should never suffer the damage seen in these photos.
[Click on any photo for a larger image.]
In photo 1, the floorboards of this aircraft are severely damaged by
exfoliation, a result of intergranular corrosion that follows a laminar
pattern along the alloy grain boundaries. This didn't happen between
annuals but was simply missed or ignored. It's cause to walk away from the
airplane, hands down.
An example of pitting is shown in photo 2; it's caused by corrosive
alkalis or chlorides trapped on the skin. In this example-rather common,
actually-it was found inside an aileron. If limited to just a few
surfaces, it's not always a deal breaker. But the airplane needs to be
cleaned up and treated with anti-corrosion compound.
Photo 3 shows an owner's worst nightmare: intergranular corrosion on a
spar. This is the sort of damage that can render an otherwise decent
airframe into uneconomic junk and, once again, an automatic reject on a
Intergranular corrosion on structural components is one reason you seek
out and pay for competent inspections, not friendly pencil whips. Caught
early and treated, this damage can be halted.
The accepted wisdom has always been that airplanes wear out more from sitting
than from flying and judging by the number of corroded ramp queens we've seen,
we would have to agree. At least flying the airplane airs it out, driving the
accumulated moisture out of the places where it can do the most damage. Regular
flying is also good at preventing another kind of expensive corrosion, the sort
that ruins an engine.
According to the FAA's current fleet census, the average GA aircraft is
expected to remain in service some 43 years and, when you examine production
figures, the bulk of the current owner-flown fleet was produced prior to 1982.
While many designs may have had alodine, zinc chromate or similar treatments to
stave off corrosion, long-term corrosion resistance has not been a top design
Aircraft are designed with certain fudge factors in mind structurally.
Typically, that means most individual components are overbuilt by a minimum of
10 to 15 percent, and often much more.
Corrosion comes in several varieties but, generally, FAR 43.13 says that when
corrosion eats into 10 percent of the original thickness of the metal component
involved, it must be repaired or replaced.
Frequently, this is difficult for shops to determine because they don't have
the equipment necessary to measure the extent of corrosion penetration. So they
take the easy if not cheap way out; the affected component is simply replaced.
However, besides doing an FAA-approved field repair, another approved
procedure is to return the aircraft to service via OEM data — if it exists —
have an FAA designated engineering representative report outlining an
alternative method of repair or replacement.
The inspecting A&P/AI determines if the specific corrosion damage is at
or exceeds 10 percent of the original thickness. If it's greater, besides
arresting the corrosion itself, he must ultimately determine what replacement or
repair procedure should be done. If, for example, a spar cap or web is corroded,
an internal patch or repair may be practical. If a skin corrodes, say around a
rivet or lap joint, the only practical solution may be outright replacement of
the entire skin.
As the fleet ages, we see more total losses due to corrosion alone. When an
aircraft is declared a total loss, typically after an accident, the usual rule
of thumb with insurance adjusters appears to be to total an airframe whose
repair costs come to 60 to 70 percent of the insured hull value. In such cases,
the insurance company figures it's better to pay you off in full and then sell
the airframe for salvage.
Usually, when selling the salvage aircraft to boneyards, a good rule of thumb
is that the value of GA aircraft hovers around the street value of the installed
avionics, cockpit instruments and accessories that can be easily removed for
The remainder of the airframe as well as the engine and its components
becomes clear profit for the salvage yard operators as they part it out over
time. Using these numbers, it makes sense to scrap an airplane when the cost of
corrosion repairs approaches 50 percent of the airframe value.
While this decision may be painful, it may make sense economically because
your airframe won't have suffered collision damage and will thus have plenty of
serviceable parts to sell.
Another phenomenon we see with resourceful-perhaps desperate-owners is buying
another similar scrap aircraft and making two into one. While this may seem
expensive and time consuming, it may make good economic sense, especially if the
purchase price for the scrapped airplane is relatively low, as it should be,
since you need neither the radios nor the engine.
Before considering anything other than minor corrosion repairs, you should
carry your ownership timeline forward. You need to think realistically about
While a simple and FAA-legal external or internal corrosion patch will do, long
term, it may better support the resale value to simply replace the part in
question, if that's possible. Besides the obvious appearance problem, future
buyers are nearly always hesitant to consider aircraft that have patch repairs
on major structural components such as spars and primary fuselage structures, as
well they should. Every smart buyer's pre-buy inspection will mean a thorough
examination of the affected areas, including dissecting the workmanship and
materials used, as well as what was done to arrest the corrosion elsewhere.
Once an airframe is diagnosed with serious structural corrosion, many buyers
will shun it no matter how well the repairs are done, unless outright
replacement of all the damaged components has been documented and OEM guidelines
were followed. Again, this is just savvy buying strategy. You might get a good
deal on an airframe with corrosion repairs but it's just as likely you'll buy
into a disaster.
Obviously, aircraft stored outdoors and in warmer, humid areas are at greater
risk of corrosion than those hangared or kept in arid areas. Doubly at risk are
aircraft that tend to sit outdoors in high-humidity bottomlands, at seaside
environments or those stored near heavily-industrialized areas in the northeast
and midwest. If you think aircraft operating in Arizona or Utah are totally
safe, you'd be wrong. Our shop recently inspected a 1992 Mooney which had
exfoliation in its 7606 T-6 spar cap so severe that it required the wing to be
removed and disassembled. A new spar cap was installed to the tune of some
$28,000. If this airplane was 20 years older, it would have been a total loss.
Although it was based in the arid southwest and always hangared, exhaust
fumes had been allowed to enter the belly area, which provided the catalyst for
the corrosion on the spar. Further, it had never been fogged with any form of
corrosion proofing. This illuminates something that many owners don't
understand: Not all corrosion is caused by solely moisture. A major factor in
many spot cases of severe corrosion is bird and vermin infestation. We've seen
several aircraft which otherwise looked showroom perfect totaled or nearly so
once we uncovered mice or birds setting up housekeeping somewhere in the
airframe. Both leave behind deposits which eat away aluminum at an alarming rate and
can quickly penetrate the 10 percent thickness barrier.
Components made of magnesium — such as control surfaces on Beechcraft
products — are far more susceptible to corrosion than ordinary aluminum. Once
corrosion starts on such parts, you can all but watch them disappear into dust.
Aggressive, regular preventive maintenance is a must. Generally, corrosion on
skins, lap joints and areas under and around rivets are areas that trap moisture
and contaminants and thus corrode first.
· With a careful
inspection, corrosion can be caught before it gets expensive.
· If you own the
airplane, don't scrimp on corrosion repairs. It will cost more in the long
compounds — Corrosion X and ACF-50 — absolutely arrest corrosion if used
· On a pre-buy, specify
that major components such as spars get special attention. Missing
structural corrosion can be disastrous.
· Walk away from an
airframe requiring major corrosion repairs. There are too many good ones
to pick from.
When most of today's airframes were built, there were few products available
to prevent corrosion. Sometimes simple paint itself was the only practical
barrier available but paint can-and does-tend to conceal corrosion and can even
allow it to flourish undetected underneath.
Tremendous strides have been made in
practical compounds which can be applied to an airframe to prevent corrosion.
These are widely marketed so it's easy for shops to obtain and apply them. All
the owner has to do is give the nod and pay the relatively small invoice.
Anti-corrosion fogging makes so much sense that unless your zip code takes in
the Mojave Desert, there's simply no excuse for not doing it on a regular basis.
What product you use is your and your technician's choice, but there are several
to choose from. We've tested them all and find that all
In some ways, corrosion is like cancer and, as with cancer, the key to
protecting against it is twofold: early detection and aggressive preventive
maintenance. Once corrosion takes a foothold, it can spread rapidly without your
suspecting anything is wrong.
For the sake of aircraft value enhancement, we strongly recommend that you
make sure the shop doing the work provides log entries of all anti-corrosion
measures so a wary buyer can be put at ease.
If you encounter corrosion in your airframe, don't automatically opt for a
patch repair. It may be cheap but more costly in the long run, if it scares away
potential buyers or drives down resale price.
Last, when considering an airplane for purchase yourself, make sure the
pre-buy includes a review of the logbooks for a corrosion prevention program.
Not too surprisingly, many airplanes won't have had any treatment at all.
That doesn't mean they're shot through with rust, but they're candidates for a
careful inspection, especially of major structural components such as spars,
spar caps, wing and control skins and the like. Evidence of any but the most
minor corrosion is grounds to walk away from the deal.
And if the airplane turns up clean and you deem it to be a good buy, you know
what to do: Keep it that way with anti-corrosion treatments.