Other than changing the engine’s oil and watching for drips of it on the hangar floor, you may not pay much attention to the engine’s lubrication system as a whole. It is a major system with critical hoses, fittings, oil cooler and pump, to name a few components.
While you might leave the maintenance of those items up to your shop or mechanic, there are specific things you should do and understand about the engine’s oil circulation system. Here’s a technician’s overview.
Oil System Basics
The oil system on most light aircraft engines is simple and mostly effective. Most systems are of a wet sump, low-pressure design. Typically, the oil reservoir (or sump) is located within or attached to the lower part of the crankcase and oil is continuously returned by gravity to the sump after the oil has done its job of lubricating and cooling.
The oil pump cavity is fed by gravity and/or suction through a filter screen, and then the gear-type pump pressurizes the oil manifold or pressure lines. Some engines (the original Continental O-470 series comes to mind) also employ an oil pressure screen (as opposed to a filter on later models), which is intended to trap large particles before supplying the oil cooler and engine. More on what you should know about filter technology in a minute.
In addition to the pressure screen is a pressure relief valve designed to prevent excess pressure from damaging engine parts and the oil cooler.
Farther down the line is the oil temperature bulb, which sends the oil temperature reading to the oil temperature gauge (upstream of the oil cooler) just prior to supplying oil to the engine’s main bearings.
Oil temperature is typically controlled by a thermally operated valve—or vernatherm—which either bypasses the oil around the cooler, or routes it though the cooler passages. Some early Lycomings instead employ a viscosity valve, which is arguably a less effective method to control oil temperature, in concert with a healthy oil cooler.
Along with the oil temperature sender unit is the main oil pressure fitting. The fitting is directly attached to the oil pressure galley. This fitting, as in most oil pressure fittings, has a restrictor in it to limit pressure surges and to prevent large-scale leaks from occurring should the plumbing to the gauge fail.
What you probably didn’t know is that in addition to the restrictor in the fitting, the oil in the line connected to the pressure gauge is normally a lighter grade of oil than regular engine oil. This provides faster responses to changes in oil pressure, especially during cold-weather operations. This oil doesn’t circulate, but instead is contained inside the line transferring the oil pressure to the gauge.
While engines may have slightly different oil systems, all have some type of screen or filter in the suction side to avoid contaminating the pump. As for measuring temperature, engine manufacturers have specific locations for locating the sensor.
We’ve seen some installers take shortcuts when installing aftermarket engine monitoring systems, deviating from these locations when installing the temperature probe, which often results in inaccurate oil temperature readings. If you’ve been using an analog gauge, you may never recognize the difference after the upgrade.
Some early Lycoming engines that have oil coolers do not have a thermostatic valve, so oil is routed continuously through the oil cooler. These are known as continuous cool systems and require the use of a winterization plate during cold-weather operations. The plate is attached externally to the air supply of the cooler to block off cooling air to the unit.
Oil Cooler Upkeep
While aircraft engine oil coolers resemble those of automotive radiators, the cooling fins of an oil cooler are much smaller—usually .006-inch thickness—and are made of weldable aluminum. What you need to know about aircraft oil coolers is that the fine passages of the internal tubes are susceptible to clogging and the buildup of carbon deposits. This definitely has a negative effect on cooling efficiency. Cylinder blowby and neglecting the oil change intervals add to the problem.
On larger Continental engines, the oil cooler is mounted directly to the engine, while Lycoming engines connect to remote coolers. The obvious advantage to the direct-mount Continental design is its lack of external oil transfer hoses, a source of potential leakage. Still, engine-mounted coolers live harder and shorter lives because they sustain more engine vibration than firewall-mounted remote models.
Common failure modes include stress cracks and corrosion-induced leakage. Remote coolers can last a long time, but only if serviced regularly and properly. According to repair facilities and manufacturers we spoke with, the primary reason for premature oil cooler failure is the simple lack of preventive maintenance.
We’re told that oil coolers are designed to last as long as the engine, but only if they’re kept clean. When new, the TIG-welded cooler is chemically cleaned and treated with a corrosion preventative and then pressure-tested.
Topping the chart of improper oil system care is mating a contaminated oil cooler with a freshly overhauled engine. Instead, while you’re waiting for the engine, send the cooler to an approved shop that’s experienced with overhauling and repairing them. If it’s badly damaged, replacement may be the only option.
There are mixed views on overhauling oil coolers. Some believe that no matter how extensive the cleaning process is, you simply can’t wash out all of the containments from the cooler’s nooks and crannies. That may be the case with field cleaning, but established shops with years of experience have the process down.
Pacific Oil Cooler Service in La Verne, California (a sister company to manufacturer Aero-Classics) has a time-consuming overhaul/ultrasonic cleaning process. Pacific’s Wayne Thomas told us the company sends coolers through three different flushing machines. Additionally, the key to effectively cleaning a cooler is sending high volumes of high-pressure Stoddard solvent flush (reversing the flow direction every 60 seconds) followed by flushing it using a 10-micron filter screen. Thomas said oil coolers it overhauls spend two hours on each flushing machine. The first machine targets oil and internal sludge, the next machine focuses on carbon deposits and the last removes any residual cleaning material.
The cooler is then inspected and any remaining paint, anodyne and corrosion is stripped off using a special proprietary cleaner after either a dye penetrant or magnaflux inspection is employed. If the unit passes that inspection it is then pressure/leak tested. If it passes, the unit is re-coated with a protective chemical coating that will prevent oxidation and corrosion. It’s then returned to the customer with an FAA 8130-3 airworthiness form.
A major concern with oil coolers is blockage. By nature of its design, the oil cooler makes for an effective secondary filtration system, catching carbon, metal and other engine contaminants. Simple flush cleaning probably won’t do it much good.
But you might also have to deal with physical damage, including stress cracks and leakage. In general, remote-mounted oil coolers tend to live longer than engine-mounted coolers, but there are things you can do to keep any cooler in top shape.
Start with a visual inspection, while looking for corrosion. Check for obstructions in the air passages and clean if necessary. While you’re at it, check for cracks and oil leaks over the entire oil cooler, especially around mounting brackets and oil fitting attachment areas. Clean the fins with compressed air in a reverse flow if possible. See any bent cooling fins? There are straightening tools designed to carefully bend the fins back into position.
Is the oil cooler even securely attached? We remember uncowling one airplane only to find the remote cooler missing two of the four attachment bolts. That must have been a Friday afternoon job.
While you’re at it, check that the proper spacers and washers are used in the remote oil cooler installation to prevent the squeezing or binding of the mounting bracket. This is a common cause for cracked coolers.
Check that oil fittings and lines are tight and are not leaking. When removing or installing oil fittings to oil lines always use two wrenches, one to hold the fitting and one to turn the “B” nut. Distorting or twisting the remote cooler during maintenance may cause stress or cracks to develop in the cooler or mounting brackets.
Last, have the cooler repaired or overhauled by professionals. It’s money well spent. We’ve seen plenty of shoddy field repairs made to oil coolers. Welding, brazing and epoxy may be a temporary fix for oil leakage, but these are probably not going to survive long-term use. In general, quality overhauls or exchanges for common oil cooler models start at around $800.
Heat Transfer Concerns
In hot climates and in summer operations, high oil temperature is as much of a concern as cold temperature is in the winter, but for different reasons. Assuming the temperature gauge is accurate, it’s generally considered that continuous oil temperature in excess of 220 F during cruise flight is too high and should be investigated (180-200 F is the preferred temperature).
Also, with extended operation, oil tends to break down while in operating temperatures above this range, so consider changing it more frequently. But excessive oil temperature might not be related to air temperature. Assuming that you’re using the proper grade of oil for the air temperature—and the correct quantity for the engine—there are things to check.
First, is the gauge reading correctly? Boiling water at sea level should prove accurate enough to measure 212 degrees F when put in contact with the temperature sensor. It’s a cheap test.
Beyond the sensor and the gauge, check that the oil cooler (both the cooling air inflow and outflow) is free of obstructions. This includes leaves, bird nests, pollen, dirt and any other substance that limits airflow. If that all looks good, focus on the engine cowling.
First, check that engine baffling is intact, that the rubber seals are sealing and that cooling air over and around the cylinders is not obstructed. Related to this are problems with the cowling bending or bulging in flight, allowing intended cooling air to escape through gaps created by distorted cowlings.
If the aircraft has cowl flaps, their proper rigging is essential for cooling efficiency—and for getting the most speed from the airframe. This is tough to check because while they might appear to be operating perfectly on the ground, this might not be the case in the air.
We used to fly a Piper Arrow that suffered from high oil temperature in the summer. Turns out that worn pivot bushings were causing the flap to close (we never could figure out how much, but it was enough to spike the oil temperature), restricting airflow.
When inspecting oil supply and return hoses, look for more than just leakage. There could be restrictions that you can’t see. The hoses should be soft and flexible. If the hoses are coming apart on the inside, it’s the perfect setup for an oil flow restriction—including a blocked oil cooler.
Sometimes it’s the engine itself that’s prone to high oil temperature simply because of its inefficiency. Worn or poorly maintained engine components place more heat transfer demand on the engine because it’s working hard. Think of the big picture. Engine compression, proper ignition timing, correct fuel mixture and the mentioned oil cooler play a role in generating heat.
Closing Words On Oil Filtration
Everyone knows that oil filters help keep engine oil clean by filtering metal, dirt and other contaminants. Oil filters for certified engines carry TSO and PMA approvals, and spin-on filters from Champion and Tempest are the most common brands.
When we last looked extensively at these two brands in a head-to-head shootout (March 2013 Aviation Consumer), we determined that both Tempest and Champion performed equally well, at least from a filtration standpoint. Consider that paper filters have an average porosity of 40 microns. That means that particles smaller than 40 microns can enter the engine—which is part of the reason we change engine oil at around 50 hours. For engines that have oil screens, that interval is reduced to 25 hours, on average.
Some oil filters have an internal filter bypass—a mechanical failsafe pressure valve that opens if oil flow through the filter element becomes restricted. This happens if it becomes clogged with contaminants or if cold oil is too thick to flow through the filter. The idea here is that dirty oil is better than no oil.
The bypass valve is really nothing more than a pressure valve-spring cage that, when open, allows oil to continue to flow toward the filter’s outlet. Since most Lycoming engines have a filter bypass somewhere on the engine, the Lycoming-specific 48110-series filters omit the bypass valve. What can go wrong with a bypass-equipped filter? Enough that Tempest designed its filters with additional failsafe.
Unique to the design is a bypass valve safety cap, which surrounds the valve in case it were to break loose from the securing spot welds, avoiding sending pieces of the metal valve into the engine.
Tempest oil filters also have a unique filtering ring magnet intended to catch fine particles of steel that could easily pass through the filtering paper. But the magnet is also handy for spotting critical metal during an oil change. More than one mechanic told us the filtering ring is the first place they look (second to looking for larger pieces of metal when cutting the filter open) when changing oil. Fine pieces of steel, which creates a fuzzy appearance on the surface of the ring magnet, could indicate valve and cam wear.
If you accomplish your own oil changes and dread the task of replacing the filter because it’s a bear to access (some might even change the oil, but—unwisely—not the filter), you might consider a remote oil filter kit.
Airwolf Filter Corporation (www.airwolf.com) has a wide-reaching STC for remote filter kits for Continental, Lycoming, Franklin and other engines. There are also filtration kits for engines with filter screens, which get rid of the screen while extending the oil change interval.
The remote filter kit weighs just shy of 5 pounds and puts the filter in a more accessible area. This means less time-consuming oil changes and less mess. Airwolf’s kit, which is anodized to eliminate corrosion, filters the oil before it enters the oil cooler and is STC’d with a Champion filter. The kit doesn’t come standard with hoses because not every installation is the same, but Airwolf offers TSO’d hoses it sources from a third party.
Airwolf’s remote filter kits are offered by numerous OEMs, including Aviat, American Champion and Maule, and is a popular accessory for aerobatic aircraft.
Airwolf told us that installation can generally be accomplished in one shop day and is best done during an oil change or while the aircraft is opened up for an inspection or engine swap. The typical kit is around $600. Airwolf also offers oil filter cutting tools, air/oil separators and oil filter chillers.
Like most systems on the aircraft, the engine lubrication system—and other systems that contribute to its efficiency—needs regular maintenance. Luckily, you can do a lot of it yourself.