Why High Oil Temperature Might Indicate Issues with Your Aircraft Engine
Engine Oil Has Six Essential Functions, High Oil Temperature May Signal an Unsolved Issue
Engine oil does more than lubricate—it cools, cleans, seals, actuates, and preserves. Since cooling, cleaning, and preserving are just as important as lubrication, any changes in these areas should be investigated promptly.
This topic arose during a recent talk at Flugring Austria in Wiener Neustadt. A participant asked why their aircraft consistently experienced high oil temperatures, not only in summer but also during winter. They mentioned replacing the oil radiator, which slightly improved the situation but didn’t resolve it. Interestingly, the issue had worsened over the years, though it wasn’t a problem initially.
What Could Cause High Oil Temperature?
Oil plays a critical role in cooling an engine internally. It circulates through the engine, cooling components such as pistons and generators. High oil temperature may indicate a problem with the oil system or elevated temperatures within the engine itself. Possible causes include inefficiencies in the oil or air cooling systems, ignition issues, or an improperly adjusted fuel system.
Oil System
Inadequate oil cooling can result from design flaws, but this is unlikely in a certified aircraft that performed well in the past. A more plausible explanation is a change in the oil system, such as blockages in the hoses or radiator. Obstructions, whether on the oil or air side, can restrict flow, reduce heat exchange, and compromise cooling efficiency.
If the oil cooling system hasn’t changed, elevated temperatures might stem from the engine itself. For example, the cylinder, cylinder heads, and pistons may be overheating due to issues with the air cooling, ignition, or fuel systems.
Elevated Blow-By
Excessive blow-by gases can increase oil temperature. However, the participants didn’t report abnormal values during differential pressure checks, nor did they notice unusually dirty oil.
(Air) Cooling System
Inefficient (air) cooling can raise temperatures in the cylinders, cylinder heads, and pistons, which subsequently elevates oil temperature. Cooling inefficiencies might result from design flaws or damage, such as leaking engine baffles.
Baffles and seals create a pressure difference that directs air over the engine for cooling. Any gaps, leaks, or obstructions, such as bird nests, can disrupt airflow and reduce cooling performance. Regular inspections can help identify and address these issues.
Ignition System
Improper ignition timing, particularly if too advanced, can lead to high oil temperatures. Advanced timing increases cylinder pressure and stress, raising cylinder head and oil temperatures as the engine’s internal components overheat.
Fuel System
A poorly adjusted fuel system can cause a lean mixture, leading to inadequate internal cooling. This issue is especially critical during high-power applications like glider towing. Aftermarket modifications, such as new cylinders, may exacerbate the problem.
Fuel system misalignment often develops gradually, making it difficult to detect. Over time, pilots may become accustomed to rising cylinder head temperatures and decreasing fuel flow. Engine condition monitoring with fuel flow data would help identify such trends early.
Conclusion
The gradual rise in cylinder head and oil temperatures suggests the fuel system is likely the culprit. I recommend inspecting the fuel system for misalignments or modifications, particularly aftermarket parts. Maintenance teams should verify fuel flow and check the cooling and ignition systems for proper function.
Additionally, installing engine condition monitoring with fuel flow indication can help track performance trends. Regular borescope inspections and oil analysis should also be part of an ongoing maintenance plan. I look forward to hearing back if these steps solve the puzzle.
About Quest Aeronautics
Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.
About Author
Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.