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 Savvy
Owner Notebook:
A Costly Lesson About Corrosion
Sam tried to do everything right when he purchased a
Cessna 340, but wound up having to major both engines just 30 hours
later, at a cost of nearly $70,000.
by Mike Busch (mike.busch@savvyaviator.com)
 I
recently received an email from an aircraft owner in
Seattle--let's call him Sam--who asked if I could recommend any
engine overhaul shops in the Pacific Northwest. I responded with
several specific shop recommendations, and a dialogue ensued during
which Sam told me the unfortunate circumstances that led to his
query.
Sam had recently purchased a 1980 Cessna 340A. He had
contacted one of the best-known twin Cessna brokers in the industry,
who led Sam to an aircraft that
appeared to be in pristine condition in a hangar in northern New
Jersey. The aircraft had excellent logs, mid-time engines (about 850
hours SMOH), and had received its last half-dozen annual inspections at a well-known Midwestern maintenance facility
that has a reputation for being one of the best twin Cessna shops in
the country. The airplane returned to New Jersey following its 2003 annual, whereupon its owner stopped flying (for whatever reason) and
put the aircraft up for sale.
By the time Sam found the airplane, it had been sitting
unflown in its northern New Jersey hangar for about 8 months. Sam
had the plane ferried back to the same well-known Midwest shop that
had performed the annual, this time for a pre-purchase inspection.
The airplane had flown only 7 hours since the annual 8
months earlier, almost all of that ferry time from the Midwest shop
to the New Jerse hangar and back. The shop cut open the oil
filters and didn't find any metal. It also borescoped the cylinders
and said they looked fine. The airframe was opened up and inspected
for corrosion, but nothing significant was found. Sam was satisfied,
purchased the airplane, and flew it home to Seattle.
Everything seemed fine until just 30 hours later when Sam
noticed that he had a problem keeping the props synchronized during
a flight from Seattle to San Diego. On the return flight to Seattle,
he lost a vacuum pump. Back in Seattle, Sam
brought his plane to a local maintenance shop and asked them to
replace the vaccum pump and check into the prop sync problem. While
inspecting the prop governor, a
mechanic noticed an unusual stain on the #5 cylinder,
and further investigation showed that this cylinder had developed a
nasty head crack. The shop contacted Sam, who authorized them to
replace the bad cylinder. Upon removing the cracked jug, the
mechanic noticed a considerable amount of rust pitting in the cylinder bore,
and took advantage of the cylinder being off to take a closer look
inside the crankcase. Shortly thereafter, Sam received a phone call
from the mechanic and heard the words that every aircraft owners
dreads: "Sam, I think
you better stop by the shop and have a look at this." Uh oh!
Sam was shocked by what he saw when he got to the shop.
The faces of the valve lifters were severely spalled (see photo).
Several cam lobes
were severely worn. Subsequent oil analysis showed the other engine was in
similar trouble. Clearly, both engines
were well down the road to self-destruction, and both would need to be torn down.
After consulting with me and others, Sam got quotes from several
leading engine shops. The total tab for overhauling both TSIO-520-NB
engines, including removal and
reinstallation, would be close to $70,000.
How Could This Happen?
The TSIO-520-NB is a durable powerplant used in the Cessna
340 and 414 and the Mooney Rocket conversion, and a close cousin to
the other permold-case TSIO-520s used in other twin Cessnas, turbo
Barons and turbo Bonanzas. TCM publishes a conservative 1600-hour
TBO for these engines, but savvy owners routinely get 1,800 to 2,000
hours from them, and commercial operators routinely take them to
2,400 hours before overhaul. These engines sometimes have mid-TBO
cylinder problems, but their bottom ends have a well-deserved
reputation for being nearly bulletproof. It's certainly sad when
these engines go bad
well before their time, as did the ones in Sam's airplane.
The culprit here is clear: corrosion due to 8 months of
disuse. Perhaps you can get away with this if your plane is based in
Tucson or Albuquerque, but you sure can't in New Jersey (even in a
hangar). Had the previous owner spent $200 to "pickle" the
engines when he realized the plane would not be flying for a month
or more, the premature demise of these engines could almost
certainly have been avoided. (Such "pickling" is a simple
procedure that uses special preservative oil and dessicant plugs to
protect the engine against corrosion damage.)
But why wasn't this problem picked up during the prebuy? The key is that when the prebuy was performed, the
engine (and oil) had only about 3 hours of operating time on it
since the corrosion occurred. (That's how long it took to fly from
the hangar in New Jersey to the shop in the Midwest.) Now, once the
cam lobes and lifter faces develop corrosion pits, their destruction
typically occurs quite quickly ... but not in 3 hours!
It's useful to think of a lifter or cam with corrosion pits as
being like a brick wall with a few missing bricks. Each missing
brick undermines several neighboring bricks which will ultimately
work lose; each of those loose bricks then undermine more
neighboring bricks, and before long the entire wall comes tumbling
down. In the case of a lifter face or cam lobe or cylinder wall, the
"bricks" are the crystalline structure of case-hardened
steel. Even microscopic corrosion pits disrupt that structure and
inevitably lead to disintegration of the surfaces.
After just three hours of operation, the spalling of the
lifters was probably still in its microscopic stages, and hadn't
progressed far enough to leave visible metal in the oil filter.
("Spalling" is a technical term that means the
disintegration of a smooth surface by chipping or flaking.) With the
benefit of 20-20 hindsight, it's clear that it might have been a
good idea to pull a few lifters during the prebuy and inspect them
under a magnifier for corrosion pits, but in fairness to the shop
this is by no means the sort of thing that is routinely done during
a prebuy or even an annual inspection.
Why didn't the prebuy borescope inspection
of the cylinders reveal rust damage to the cylinder bores?
Once again, three hours of operation was probably just enough to
scrape off most or all of the visible rust, and not nearly enough to
allow the microscopic rust pits on the cylinder walls to progress to
the point of visible spalling that could be seen with a borescope.
There were undoubtedly lots of rust particles in the oil filter, but
rust is nearly impossible to see during filter inspection unless the
filter media is inspected under a microscope. Had the
engines been on oil analysis (which they weren't), the report might
have revealed higher-than-usual iron concentration (although perhaps
not given that the oil only had 7 hours on it), but this is expected
after a period of disuse so the lab's recommendation would probably
have been to "resample after 25 hours."
Lessons Learned
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TCM TSIO-520-NB.
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There are a few important lessons we can learn from this sad
tale. The most important lesson for savvy owners is that a few
months of disuse can wreak havoc on your expensive piston aircraft
engine, even if the aircraft is hangared. Anytime you know that
your aircraft won't be flown for 30-60 days--because you're taking a
vacation in Europe, recovering from an illness, or temporarily lost
your medical, for example--it's absolutely essential to
"pickle" the engine to prevent corrosion damage. Both
Lycoming and TCM have service bulletins that spell out the
recommended preservation procedure. Basically, it involves draining
the oil and servicing the crankcase with special preservative oil,
removing the top spark plugs, spraying preservative oil into each
cylinder, placing dessicant plugs into the spark plug holes and
dessicant bags into the exhaust and induction pipes. The
procedure is simple enough that even an owner can do it (legally)
without requiring an A&P signoff. If you have your shop do it,
it shouldn't cost more than a couple of hundred bucks, and it's
probably the smartest money you could ever spend on maintenance.
For savvy buyers, there are a number of lessons to be learned
here. One is that you're not likely to learn very much by inspecting
an oil filter after just 3 or 7 hours of operation. A prebuy or
annual inspection on an aircraft known to have undergone a significant period of
disuse should include a visual inspection of the cam and lifters if
at all possible. This is not something that is routinely done unless
you specifically ask for it, but it can be done without cylinder
removal on engines with barrel-style lifters (i.e., all TCM engines and
Lycoming O-360-H2AD engines). On engines with mushroom-style lifters
(i.e., most Lycomings other than the -H2AD), inspecting the cam requires pulling one or more cylinders,
something the
seller might not agree to during a prebuy.
Another lesson for savvy buyers is that the prebuy should
always be done by a mechanic who has no prior connection with either
the aircraft or the seller. In this case, the prebuy was done by
a shop with a top-notch reputation, but it was unfortunately the
same shop that had performed the last half-dozen annual inspections,
including an annual just 7 hours prior to the prebuy. The A&Ps
at this shop naturally treated this airplane like an old friend, and
could not help but be predisposed to believe that it was in great
mechanical shape. At a different shop, one that had never set eyes
or hands on this aircraft before, the mechanics would have
undoubtedly approached the aircraft with an appropriate attitude of
skepticism, and that's precisely what you want during a prebuy.
Finally, buying an airplane with a mid-time engine always
involves a certain element of risk, since it's very hard to know
exactly how the engine has been operated or maintained, and
therefore to estimate how much useful time is left on the engine.
For twins, the risk is doubled. That's one of the reasons I'm a big
fan of buying airplanes with runout engines (at a suitably
discounted price, of course); you buy the airplane knowing that
you'll have to major the engine soon, and once you've done that you
know precisely what you've got.
| Do you have a maintenance-related
"war story" that you'd like to share with fellow
aircraft owners? If you do, I'd
love to hear from you. The most interesting stories
I receive each month will be rewarded with highly prized Savvy
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shipping address. Also be sure to let me know if you'd like
me to "change the names to protect the innocent"
when sharing your story. |
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