Aircraft Maintenance: Managing oil temps for a healthy engine
Oil is a multi-role actor in the cast of characters that keep an aircraft engine healthy and happy. It plays the leading role in lubricating critical parts, but also handles 25 percent of the heat produced by the engine, controls the propeller, carries away contaminants for filtration, and protects vulnerable metal parts from corrosion when the plane sits idle in between flights.
With all we ask of our engine oil, it is our responsibility as pilots and aircraft owners to make sure that it is getting the help it needs to do its job. That begins with clean oil of the correct type. I have done extensive research in the past 12 months with both Phillips 66 and AeroShell, including training in Houston at AeroShell's technical facility, and meetings with both companies on the science behind their oils. Based on this, and on my personal experience, I use Phillips 66 X/C 20W50 year-round, with Camguard additive for extra corrosion protection. The biggest surprise from my research was learning what type of oil does a better job protecting internal engine parts from corrosion when the airplane sits idle in between flights. Most mechanics believe that thicker, straight-weight oils are superior because they cling better to internal engine parts over time. Laboratory testing, however, has shown the opposite to be true; 20W50 multi-weight oil actually leaves a protective film on engine parts that lasts longer than straight weight oils. This means not only are multi-weight oils better for lubrication across various operating temperatures from engine start to cruise flight, they also do a better job protecting parts from corrosion over time.
Managing oil temperatures in an aircraft engine can be a challenge. In a liquid-cooled automobile, we have thermostatic valves to keep the coolant inside the engine until it heats up, radiators to cool it when it gets too hot, and supplemental fans to cool things down even more where there isn’t enough airflow. Aircraft oil systems are similar, but not as sophisticated, and they operate across much wider temperature and operating ranges than most cars.
For temperature control, the system relies on three things: the oil reservoir, the oil cooler, and a thermostatic control valve (also called a vernatherm) to regulate the system. The oil reservoir is more important than most pilots give it credit for. If the same “slug” of oil flowed continuously through the engine, it would heat up beyond the oil cooler’s ability to cool it back down. The oil reservoir (or sump) provides the critical mass and cooling capacity to the system, which tempers temperature swings and makes it all work efficiently. Your reserve oil is like a heat sink for your engine. This is why it is so important to maintain oil levels as high as your engine will tolerate without blowing excess out of the breather. The more oil in your sump, the cooler your engine will run.
When the engine heats up, the vernatherm expands to gradually close off the oil cooler bypass pathway, forcing more and more oil through the oil cooler until that is the only pathway available. At this point, the cooler is doing all it can to bring oil temperatures down.
The system relies on several factors to manage oil temperatures properly. The oil pressure must be set correctly so that oil is circulating as it should. Oil viscosity must allow the oil to flow properly (see my comment above about multi-weight oils). The vernatherm must divert oil to the cooler, and the oil cooler must be in top condition to exchange the oil heat with the cooling airflow.
Testing the vernatherm for proper operation is fairly simple, and the procedure can be found in every engine maintenance manual. Knowing that your oil cooler is working as it was designed to is a different story. An oil cooler is a heat exchanger. It is critical that all of its passages are clear and, importantly, that the oil passing through can quickly conduct the heat through the metal tubes to the fins and the cooling airflow. If you have ever seen the inside of an old engine, you know that oil tends to build up a varnish coating of old oil residue over time. This varnish acts as an insulator, preventing effective heat transfer between the hot oil and the cool air going over the fins. Additionally, contaminants in the oil can build up to block or reduce flow in the passages within the cooler. This can greatly diminish the oil cooler’s ability to function properly. The same is true for external damage in the oil cooler. The fins between the channels of the cooler are extremely fragile and can easily bend, closing off critical airflow.
The solution to this inevitable degradation is to have your oil cooler serviced proactively, and repaired or replaced if it is damaged. I routinely use Pacific Oil Cooler Service for oil cooler flushing and repairs, and Aero-Classics for new oil coolers, if needed. According to Pacific, most owners would be surprised at how much gets flushed out of a cooler that has been in service for 10 or 20 years or longer. With small passages that collect particulate fall out as the oil cools, an oil cooler tends to act like a secondary oil filter. Oil coolers constantly collect oil deposits, carbon, solids, metal, and other contaminants. Pacific says that a typical oil cooler flush can collect enough carbon and metal contamination to fill up half a cigar tube. The company supplies replacement vernatherms and associated parts.
Pacific's 25-step cleaning process dramatically improves efficiency with the use of two ultrasonic cleaners, multiple inspections, pressure tests, and a series of different flushing benches and machines. Technicians do what they can to straighten and repair bent and damaged cooling fins, which can restrict airflow and reduce cooling efficiency. If a cooler cannot be economically repaired, or if the customer simply desires a new cooler, Aero-Classics can usually meet the need, as it is the world’s largest manufacturer of FAA-approved oil coolers.
One last note for those flying in cold climates: Running an engine with oil that does not reach proper operating temperature (typically 180 degrees Fahrenheit) is detrimental to the health of your engine. It’s very important to get the oil hot enough to boil off moisture to reduce corrosion between flights. Many airframe manufacturers originally produced” winterization kits” for their aircraft that included plates that blocked off portions of the oil cooler to help maintain higher oil temperatures in winter. Unfortunately, these kits can be difficult to find. Although they are often referenced in the pilot’s operating handbook, much of the data and the parts availability seem lost to time. Your mechanic can use either an aluminum plate or aluminum tape to block off part of the oil cooler to achieve the same goal, but it must be done correctly as a minor alteration and documented properly.
Most important, any proposed changes to oil cooler airflow require proper understanding of the system. For example, in Continental IO-550 engines, the oil cooler has an integral vernatherm that circulates bypass oil in one part of the oil cooler even while the engine is cold. The best strategy for this situation is to reduce winter airflow over the bypass portion of the cooler, while leaving the hot side available for oil cooling as needed once the vernatherm heats up. On other engines, if no oil flows into the cooler until the oil reaches a preset temperature, there is little point in restricting airflow to the cooler. Every situation is different and only the manufacturer or a qualified mechanic can make an oil cooler airflow modification that is both safe and effective.
Our aircraft are magical traveling machines that can handle bitter cold or scorching heat, and transport us to all the fun places in between. Paying attention to oil temperature and maintaining a healthy system makes it all possible for years to come. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.
