Walk Away or Fix?

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.

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When we’re airplane shopping, not many of us buy new models so we’re stuckmaking our picks from an increasingly older fleet. You’d think that a30-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 instancesof serious corrosion undiscovered in previous inspections and, occasionally,hidden beneath a quick fix of primer and paint. This is becoming tricky businessfor buyers. You might very well negotiate a sweet purchase price on a niceairframe with reasonable time and decent paint and avionics only to discovermonths later that it needs $20,000 worth of corrosion repairs.

Better to find that out before closing the deal than after. And if you dofind it, tread very carefully in negotiating the price downward to pay forfixing the rust. We often find that in disassembling structures for corrosionrepair, 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.

Exfoliated Floorboard
[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.

Aileron Pitting

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. 

Intergrannular Spar Corrosion

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 pre-buy.

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 sittingthan 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 theaccumulated moisture out of the places where it can do the most damage. Regularflying is also good at preventing another kind of expensive corrosion, the sortthat ruins an engine.

According to the FAA’s current fleet census, the average GA aircraft isexpected to remain in service some 43 years and, when you examine productionfigures, 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 tostave off corrosion, long-term corrosion resistance has not been a top designpriority.

Aircraft are designed with certain fudge factors in mind structurally.Typically, that means most individual components are overbuilt by a minimum of10 to 15 percent, and often much more.

Corrosion comes in several varieties but, generally, FAR 43.13 says that whencorrosion eats into 10 percent of the original thickness of the metal componentinvolved, it must be repaired or replaced.

Frequently, this is difficult for shops to determine because they don’t havethe equipment necessary to measure the extent of corrosion penetration. So theytake the easy if not cheap way out; the affected component is simply replaced.

However, besides doing an FAA-approved field repair, another approvedprocedure is to return the aircraft to service via OEM data — if it exists —or tohave an FAA designated engineering representative report outlining analternative method of repair or replacement.

The inspecting A&P/AI determines if the specific corrosion damage is ator exceeds 10 percent of the original thickness. If it’s greater, besidesarresting the corrosion itself, he must ultimately determine what replacement orrepair 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 arivet or lap joint, the only practical solution may be outright replacement ofthe entire skin.

Total Losses

As the fleet ages, we see more total losses due to corrosion alone. When anaircraft is declared a total loss, typically after an accident, the usual ruleof thumb with insurance adjusters appears to be to total an airframe whoserepair 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 sellthe airframe for salvage.

Usually, when selling the salvage aircraft to boneyards, a good rule of thumbis that the value of GA aircraft hovers around the street value of the installedavionics, cockpit instruments and accessories that can be easily removed forquick cash.

The remainder of the airframe as well as the engine and its componentsbecomes clear profit for the salvage yard operators as they part it out overtime. Using these numbers, it makes sense to scrap an airplane when the cost ofcorrosion repairs approaches 50 percent of the airframe value.

While this decision may be painful, it may make sense economically becauseyour airframe won’t have suffered collision damage and will thus have plenty ofserviceable parts to sell.

Another phenomenon we see with resourceful-perhaps desperate-owners is buyinganother similar scrap aircraft and making two into one. While this may seemexpensive and time consuming, it may make good economic sense, especially if thepurchase price for the scrapped airplane is relatively low, as it should be,since you need neither the radios nor the engine.

Look Ahead

Before considering anything other than minor corrosion repairs, you shouldcarry your ownership timeline forward. You need to think realistically aboutresale value.

While a simple and FAA-legal external or internal corrosion patch will do, longterm, it may better support the resale value to simply replace the part inquestion, if that’s possible. Besides the obvious appearance problem, futurebuyers are nearly always hesitant to consider aircraft that have patch repairson major structural components such as spars and primary fuselage structures, aswell they should. Every smart buyer’s pre-buy inspection will mean a thoroughexamination of the affected areas, including dissecting the workmanship andmaterials used, as well as what was done to arrest the corrosion elsewhere.

Once an airframe is diagnosed with serious structural corrosion, many buyerswill shun it no matter how well the repairs are done, unless outrightreplacement of all the damaged components has been documented and OEM guidelineswere followed. Again, this is just savvy buying strategy. You might get a gooddeal on an airframe with corrosion repairs but it’s just as likely you’ll buyinto a disaster.

Desert Creampuffs

Obviously, aircraft stored outdoors and in warmer, humid areas are at greaterrisk of corrosion than those hangared or kept in arid areas. Doubly at risk areaircraft that tend to sit outdoors in high-humidity bottomlands, at seasideenvironments or those stored near heavily-industrialized areas in the northeastand midwest. If you think aircraft operating in Arizona or Utah are totallysafe, you’d be wrong. Our shop recently inspected a 1992 Mooney which hadexfoliation in its 7606 T-6 spar cap so severe that it required the wing to beremoved 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, exhaustfumes had been allowed to enter the belly area, which provided the catalyst forthe corrosion on the spar. Further, it had never been fogged with any form ofcorrosion proofing. This illuminates something that many owners don’tunderstand: Not all corrosion is caused by solely moisture. A major factor inmany spot cases of severe corrosion is bird and vermin infestation. We’ve seenseveral aircraft which otherwise looked showroom perfect totaled or nearly soonce we uncovered mice or birds setting up housekeeping somewhere in theairframe. Both leave behind deposits which eat away aluminum at an alarming rate andcan quickly penetrate the 10 percent thickness barrier.

Components made of magnesium — such as control surfaces on Beechcraftproducts — are far more susceptible to corrosion than ordinary aluminum. Oncecorrosion starts on such parts, you can all but watch them disappear into dust.Aggressive, regular preventive maintenance is a must. Generally, corrosion onskins, lap joints and areas under and around rivets are areas that trap moistureand contaminants and thus corrode first.

Prevention

Checklist

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 run.

Anti-corrosion compounds — Corrosion X and ACF-50 — absolutely arrest corrosion if used correctly.

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 availableto prevent corrosion. Sometimes simple paint itself was the only practicalbarrier available but paint can-and does-tend to conceal corrosion and can evenallow it to flourish undetected underneath.

Tremendous strides have been made inpractical 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. Allthe 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 inthe 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 severalto choose from. We’ve tested them all and find that allare effective.

In some ways, corrosion is like cancer and, as with cancer, the key toprotecting against it is twofold: early detection and aggressive preventivemaintenance. Once corrosion takes a foothold, it can spread rapidly without yoursuspecting anything is wrong.

For the sake of aircraft value enhancement, we strongly recommend that youmake sure the shop doing the work provides log entries of all anti-corrosionmeasures so a wary buyer can be put at ease.

If you encounter corrosion in your airframe, don’t automatically opt for apatch repair. It may be cheap but more costly in the long run, if it scares awaypotential buyers or drives down resale price.

Last, when considering an airplane for purchase yourself, make sure thepre-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 acareful inspection, especially of major structural components such as spars,spar caps, wing and control skins and the like. Evidence of any but the mostminor 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 knowwhat to do: Keep it that way with anti-corrosion treatments.

Addresses

ACF-50
Lear Chemical Research
P.O. Box 1040, Station B
Mississauga, Ontario, Canada
800-256-2548
http://www.learchem.com
Boeshield T-9
PMS Products Inc.
76 Veterans Dr. #110
Holland, Michigan  49423
800-962-1732
http://www.boeshield.com
CorrosionX
Corrosion Technologies Corp.
P.O. Box 551625
Dallas, TX 75355-1625
800-638-7361
http://www.corrosionx.com
LPS-3
LPS Laboratories
4647 Hugh Howell Road
Tucker, GA  30085-5052
800-241-8334
http://www.lpslabs.com

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