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Buy - Sell - Exchange - Overhaul - Service - Repair - Fast Turnaround - Competitive Prices FYI: Common Oil Cooler Woes, early detection and prevention. Text and photos by Wayne Thomas
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 its exterior.
THE ALUMINUM FOOTBALL
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 operate. 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 rupture. 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.
OTHER CAUSES 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 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.
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.
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 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 threads. VERY IMPORTANT: 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
<-- This is what can happen when a back-up wrench is not used.
AND FINALLY |
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. 
HOW
THEY GET THAT WAY
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.