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FYI: Common Oil Cooler Woes, early detection and prevention.
Text and photos by Wayne Thomas
Being employed in a position where several hundred as-removed aircraft oil coolers
pass within my view each and every month, year after year, very likely means that I’ve seen just about every form of deterioration, impairment, and catastrophic failure that could possibly occur to such a device.
Even though I haven’t compiled any hard statistical data on the subject of oil cooler mortality, I can tell you with great certainty that the most common causes of oil cooler damage, poor performance, and catastrophic failure are preventable.
Before continuing any further, I should point out that the number one, most
common reason for an aluminum aircraft oil cooler to become unrepairable, it
seems, is due to corrosion. Even if everything else that takes place during the
oil cooler’s operational lifetime goes exactly as-designed, the effects of time
and corrosion will eventually kill it. While not preventable, corrosion can be
slowed - and the oil cooler’s useful life extended, with periodic cleaning of
Rust may never sleep - but corrosion always wins. It is corrosion pitting as seen above that takes its toll on the front side of aluminum oil coolers. Eventually, pinhole leaks will develop, rendering the cooler un-repairable. Performing a weld repair anywhere near this type of corrosion is impossible due to the contaminating oxides present - and the varying metal thicknesses. An occasional cleaning of the oil cooler’s exterior surfaces - particularly the front side - can greatly slow the effects of corrosion and add considerably to the oil cooler’s useful life. While you’re at
it, removing the built up insect carcasses, bird feathers, vegetation, and the many other type of debris that gets trapped within the fin area will also improve the oil cooler’s performance.
THE ALUMINUM FOOTBALL
One of the more fascinating, costly, unrepairable - and yet highly avoidable
- forms of oil cooler damage is caused by over pressurization. Although a
somewhat rare occurrence (I see only about one oil cooler every couple of months
or so with this affliction) - this rarity is more than made up for by the
spectacular end result; a grossly deformed oil cooler. It is really amazing to
actually see evidence of just how much hydraulic force an engine’s lubrication
system is capable of generating - and how far ballooned-out most oil coolers can
get when the pressure races upward and out of control. Rectangular shaped oil
coolers can swell to a point where they start to resemble a football - and not
leak a drop of oil. Other times, an oil cooler will rupture before much swelling
takes place - depending on the type of cooler and the weight of the engine oil.
HOW THEY GET THAT WAY
There are several possible causes of oil cooler over pressurization, but the
most recurrent cause is operator error; operating the engine under cold climate
conditions without first pre-heating the engine and the oil cooler. According to
most engine manufacturers, when ambient temperatures are 20F (-7C) or below,
pre-heating the engine and oil cooler become necessary. It is recommended that
both the engine - and the oil cooler - be brought up to 60F (16C) by pre-heating
before operation in order to prevent damage to the oil cooler.
It is important to note that pre-heating only the engine - and not the oil
cooler, is of no value when attempting to prevent damage to the oil cooler - and
other components that make up an engine’s oil cooling system.
If only the engine is pre-heated, oil flow through the cooler will remain
severely restricted by the congealed oil that is inside. Continuing to operate
the engine when this condition exists, and then applying a power setting
required for flight, is when the most serious harm is likely to take place.
As soon as the oil temperature reaches its normal operating level, the
vernatherm closes off the oil by-pass route and attempts to force oil through
the oil cooler. At this point, however, the congealed oil inside the cooler
reveals its own agenda of staying-put. A pressure spike builds and eventually
causes the oil to force its way past the vernatherm’s spring loaded seat,
by-passing the oil cooler. Which is exactly how the system was designed to
The problem however, is that the system was not intended to deal effectively
with an oil cooler that is filled with cold oil. And while flying, the congealed
oil is likely to remain that way inside the oil cooler, shunting hot oil away
from itself, for a long period of time. During this time, the vernatherm and its
spring loaded seat undergo torture by way of rapid and repeated cycling. The
results are rapid wear and/or damage to the vernatherm and the oil control
seats, and very abrupt fluctuations - spikes - in oil pressure which may also
cause damage to the oil cooler. Even at normal oil pressures, these spikes
become like hammer-blows to the oil cooler and can cause it to deform and/or
Meanwhile, back in the cockpit, while the above described chaos is chattering
away inside the engine’s lubrication system, everything may appear to be just
fine; oil temperature and oil pressure both reading “in the green”. Spikes in
oil pressure that are capable of doing cooling system harm will never appear as
gauge readings - even if the pressure spike signals were to make it to the gauge
on the instrument panel, the gauge needle isn’t capable of moving quickly enough
to show them to you. …But don’t take your eyes off of the gauges for too long
because when cooling of the oil becomes absolutely necessary, a very sudden rise
in oil temperature and a simultaneous reduction in oil pressure may occur - so
long as the oil cooler remains filled with congealed oil.
The key to preventing this type of costly damage to an engine’s oil cooling
system is to always pre-heat both the engine and the oil cooler anytime
pre-heating is necessary. Also, avoid subscribing to the notion that pre-heating
only the engine and then operating the engine on the ground for an extended
period of time is an acceptable substitute for pre-heating of the oil cooler -
because it isn’t.
Photo at right: A severe rupture which opened up with such
force that both sides of the afflicted oil pass ripped wide open. Doesn’t this
oil cooler look like it has a mouth that is trying to tell us something?
Other potential causes of irreversible oil cooler damage from over
pressurization are mechanical in nature, as opposed to operator induced. Kinked
oil lines can not only cause the oil cooler to deform or rupture, but can also
be the cause of higher than normal oil temperatures. A kinked oil line leading
to the oil cooler is likely to cause an increase in oil temperature - while a
kink in the line that leads away from the cooler is capable of causing both a
ruptured oil cooler and high oil temperature - probably in that order and
immediately followed by a loss of oil pressure.
Oil coolers that are equipped with an integral thermostatic oil control valve
(ie; oil coolers which have a vernatherm screwed directly into them), have ports
that are clearly labeled “IN” and “OUT”, indicating the direction that oil must
flow through the cooler. Reversing the oil line connections on this type of oil
cooler can cause it to burst immediately upon engine start-up. Oil coolers which
do not have a thermal oil control valve integrated into their design - which
happens to be the case with most every aircraft that is powered by a "flat",
opposed-cylinder type of engine - can have oil flow through them in either direction.
Generally, radial engines and turbine engines nearly always use an oil cooler
with an integrated thermal control valve.
INSPECT FOR SIGNS OF INFLATION
Over pressurization damage to an oil cooler doesn’t necessarily happen on
just one occasion. The puffed up look that many over pressurized oil coolers
exhibit could be the cumulative results of any number of over pressurization
occurrences. Spot them early and remove them from service before the inevitable
The most obvious signs of an oil cooler having been over pressurized are
quite basic; parts of the cooler’s exterior which are supposed to be straight,
aren’t. Most aircraft oil coolers manufactured within the past forty to fifty
years are either square or rectangular in shape - and most of them have at least
two exterior surfaces that are (supposed to be) flat and straight. Any
detectable bulging should be investigated further, before continued operation.
This Schweizer helicopter oil cooler shows its symptoms of abuse very clearly - without (yet) leaking.
If possible, look through the oil cooler’s air-fin area with a bright
light-source on the opposite side. Any area where you see no light coming
through the fins is a typical indicator of bulged oil passes that have expanded
outward, crushing the air-fin material. This is where signs of over
pressurization damage often make their first appearance - before other, more
obvious bulges appear on the oil cooler’s exterior surfaces.
Looking through an oil cooler’s air fins with a light
source on the opposite side can help detect early stages of over pressurization
damage. Here, light is blocked from getting through the uppermost row of air
fins because the oil pass above it has bulged-out near the center part of the
Engine-mounted oil coolers such as those used on most every Continental
aircraft engine seem to be the most frequent victim of over pressurization and
the damage it causes - particularly the ones that attach to the front side of
the TCM 470 and 520 engine series. One reason for this could be due to the fact
that there are no oil lines used between the cooler and the engine - which may
be capable of providing a degree of pressure spike protection to the oil cooler.
Fortunately, this type of oil cooler also happens to be the most easily viewed
without any engine cowling removal. Therefore, having a glance at the oil cooler
during every pre-flight of your TCM 470 or 520 powered airplane - looking for
any hint of deformation of the cooler, has got to be a good idea.
Any evidence of over pressurization damage should warrant an investigation as
to its cause, with measures taken that will reduce or eliminate its chances of
recurring. The oil cooler should also be immediately removed from service and
condemned (over pressurization damage is never repairable). An inspection of the
vernatherm and the oil control seats for damage should also be performed.
DAMAGED THREAD ISSUES
This is another area where trouble is encountered over and over again -
instead of being avoided; damage to threads in aluminum oil coolers.
Boogering-up the threads on most any aluminum aircraft oil cooler is enough to
render it beyond economical repair (BER). Even on the rare occasion when the
thread damage is minor and was able to be corrected by chasing them with a tap,
the oil cooler must then be overhauled to remove any metal contamination.
Any threaded hole in an aluminum oil cooler is a place where trouble is
looking to happen - especially the ones where the fittings screw into the
cooler. Typically, this is a 3/8” tapered pipe thread which, as we all know,
requires a slight interference type of fit between the threads in the cooler and
the threads on the fitting in order to achieve an oil-tight seal. For this
reason, NEVER-EVER attempt to thread a fitting into these holes without first
applying some type of anti-seize compound - or even plain old Teflon tape to the
Also avoid re-use of aluminum fittings. In fact, throw all aluminum fittings
as far away as you can from the work you are doing - and use steel fittings
instead. Steel fittings, when installed with some type of lubricant on the
threads, will almost never gall, bind up, or damage the threads of an aluminum
oil cooler. In addition, steel fittings will un-screw cleanly from the cooler -
even many decades later. None of these things can be truthfully said about
Steel is the fitting material of choice if you want to avoid damaging the threads of an aluminum oil
cooler. Additionally, steel fittings will un-screw from the oil cooler cleanly and with no thread damage, even many years later. No matter what they’re made of, never ever screw fittings into an aluminum oil cooler without first applying some form of thread lubrication - and use aluminum fittings only as a last-ditch resort.
If you must use aluminum fittings for some reason, make sure that the threads are clean and defect-free, use Teflon tape or an anti-seize lubricant made for use on threads, and work slowly and carefully as you begin screwing the fitting into the oil cooler (and cross your fingers, also).
When pipe-threads are involved, be sure that the fitting is going to snug down and achieve a seal before the shoulders above the threaded portion of the fitting come in contact with the top of the boss on the cooler. When the shoulders on the fitting touch - or come anywhere close to - the top of the oil cooler’s boss before a seal is made, it means that the threads in the cooler have lost their taper and the cooler must be replaced.
IF IT AIN’T BROKE, DON’T BREAK IT
Avoid all un-necessary movement of fittings once they’re installed in the
cooler in order to prevent premature failure of tapered threads. Regardless of
the type of material that the fittings are made of, do not remove them from the
oil cooler when sending it out for overhaul. The best advise is to leave
fittings undisturbed unless absolutely necessary to do otherwise.
WHEN BACKUP IS CALLED FOR
One other extremely common - and entirely avoidable - form of oil cooler
injury is caused by not using a back-up wrench when dealing with fittings and
oil line connections. Each oil cooler manufacturer has their own uniquely-shaped
boss for the threaded holes that the fittings screw into. Most of them are hex
or square in shape so that it is possible to place a wrench on them. Except for
Harrison oil coolers, which have a round boss that requires the use of pliers or
a pipe wrench. In every case, failing to steady the boss on the oil cooler when
attempting to connect - or disconnect - the oil lines or the fittings from the
cooler, will allow the cooler’s threaded boss to bend from side to side and
cause the oil cooler to crack and leak.
Above: The “other” three brands of oil coolers in use today are, from left to right: Harrison, AERO-Classics, and NDM Kintex. Note the variety of oil cooler boss shapes. Use
of a back-up wrench or - in the case of the Harrison oil cooler - pliers or a
pipe wrench, is absolutely essential in order to keep from causing a crack to develop between those ridges on the cooler's tank area - and leaking. Though Harrison hasn’t made an aviation oil cooler in more than thirty years, there are still many thousands of them in use today.
This is what can happen when a back-up wrench is not used.
As with most other things, when it comes to aircraft oil coolers the commonly
made mistakes are also the easiest ones to avoid - if you know what to look for
and what to look-out for. Remember, a fluid ounce of prevention is always better
than eleven quarts of cure.
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