Lubrication
Engine components rely on proper lubrication to reduce friction, control temperature, and prevent excessive wear. Understanding aircraft lubrication systems helps pilots recognize how oil circulates through the engine to support reliable performance and longevity.
Introduction to Lubrication
- Lubrication systems are responsible for lubrication, heat management, and collecting debris that result from operation of the powerplant/engine.
- Wet and Dry oil/lubrication systems keep aircraft well-oiled while maintaining proper operating temperature.
- Depending on the aircraft, oil also powers other systems and accessories.
- When you feel you have a solid understanding of aviation oil/lubrication systems, compare your knowledge against the oil/lubrication related airman certification standards.
- Once satisfied, close out with the topic summary and prepare for your next lesson.
Lubrication Key Highlights
- Aircraft lubrication systems reduce friction, dissipate heat, and protect engine components from excessive wear and corrosion.
- Engine oil circulates through internal engine components to provide lubrication, cooling, cleaning, and sealing functions.
- Wet sump and dry sump lubrication systems are commonly used in aircraft engine designs.
- Proper oil pressure and oil temperature are critical indicators of engine lubrication system health and performance.
- Oil filters and screens help remove contaminants that could damage engine components during operation.
- Low oil quantity, contamination, overheating, or pressure loss can lead to severe engine damage or engine failure.
- Pilots should monitor engine instruments carefully for abnormal oil pressure or temperature indications during flight.
- Aircraft manufacturers specify approved oil grades and servicing procedures based on engine design and operating conditions.
- Regular maintenance inspections help identify leaks, contamination, and lubrication system wear before operational problems occur.
- Understanding lubrication systems improves engine management proficiency, systems knowledge, and overall flight safety.
Oi/Lubrication System Design
- Lubrication systems consist of either a wet-sump or a dry-sump system.
- To differentiate between the two systems, think about what happens when the engine is off.
- Wet sump systems maintain oil in reservoirs/oil pans (sumps) integral to the engine while dry sumps do not, leaving the sump "dry."
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Wet-Sump Oil/Lubrication Systems:
- In a wet-sump system, an integral reservoir stores oil. [Figure 1]
- The main component is the oil pump, which pumps oil from the sump and routes it to the engine.
- After the oil circulates throughout the engine, it returns to the sump.
- In some engines, additional lubrication occurs by the rotating crankshaft, which splashes oil (splash lubrication) onto various engine components.
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Dry-Sump Oil/Lubrication Systems:
- Oil is contained in a separate tank and circulated through the engine by pumps.
- These tanks are always larger than the oil it is meant to contain to compensate for thermal expansion.
- An oil pump also supplies oil pressure in a dry-sump system, but the source of the oil is located externally to the engine in a separate oil tank.
- After oil circulates through the engine, it is pumped from the various locations in the engine back to the oil tank by scavenge pumps.
- Dry-sump systems allow for a greater volume of oil to be supplied to the engine, which makes them more suitable for very large reciprocating engines.
- Most jet engines will consist of a dry sump design.
- When checking the oil level on a dry-sump engine that has been sitting idle, readings may be inaccurate unless rotated (or "burped").
- Refer to your POH.
Lubrication System Components
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Oil Filter:
- The oil filter is responsible for screening the oil for foreign debris.
- Oil filters accomplish this by pushing oil through a one or more screens or paper filters.
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Oil Screens:
- There are two types of oil screens:
- Pressure Screens, or;
- Suction Screens.
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Suction Screens:
- Suction screens are screens placed before the oil pump in the lubrication system , filtering out the largest contaminents before they enter delicate system components.
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Pressure Screens:
- Pressure screens filter finer particulates, down to 60 microns.
- There are two types of oil screens:
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Paper Oil Filters:
- Paper oil filters are a newer technology in which paper completes the functions of an oil screen.
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Oil Filler Cap/Dipstick:
- The oil filler cap/dipstick used to measure oil quantity is usually accessible through a panel in the engine cowling. [Figure 2]
- If the quantity does not meet the manufacturer's recommended operating levels, oil should be added.
- If oil is unexpectedly low, the source of oil burn/leak should be investigated.
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Oil:
- The type of oil required may vary on numerous atmospheric and operation conditions, as stipulated by the aircraft operations manual. [Figure 3]
- The AFM/POH or placards near the access panel provide information about the correct oil type and weight, as well as the minimum and maximum oil quantity.
- The loss of engine oil pressure would lead to engine vibrations, RPM would decrease, and the engine would eventually seize.
- Viscosity: the ability of a liquid to resist flow.
Heat Management
- A critical purpose of engine oil is to help manage cooling.
- This occurs by cool oil moving through warm areas, picking up the heat, and dissipating it through a radiator.
Oil System Gauges
- System is monitored through pressure and temperature gauges [Figure 4]
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Oil Pressure Gauge:
- The oil pressure gauge provides a direct indication of the oil system operation. [Figure 4]
- It ensures the pressure in pounds per square inch (psi) of the oil supplied to the engine.
- Green indicates the normal operating range, while red indicates the minimum and maximum pressures.
- There should be an indication of oil pressure during engine start.
- Refer to the AFM/POH for manufacturer limitations.
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Oil Temperature Gauge:
- The oil temperature gauge measures the temperature of oil. [Figure 4]
- It is usually measured after passing through the oil cooler.
- A green area shows the normal operating range and the red line indicates the maximum allowable temperature.
- Unlike oil pressure, changes in oil temperature occur more slowly.
- This is particularly noticeable after starting a cold engine, when it may take several minutes or longer for the gauge to show any increase in oil temperature.
- Check oil temperature periodically during flight especially when operating in high or low ambient air temperature.
- High oil temperature indications may signal:
- Plugged oil line or cooler.
- low oil quantity (possible engine failure).
- Defective temperature gauge.
- High oil temperatures can lead to metal on metal contact as viscosity decreases.
- Low oil temperature indications may signal improper oil viscosity during cold weather operations.
Oil/Lubrication Malunfctions/Emergencies
- Oil consumption depends primarily upon the efficiency of the seals.
- Oil can be lost through internal leakage, and, in some engines, by malfunctioning of the pressurizing or venting system.
- Increases in oil temperature are not always associated with a drop in oil pressure, nor a rise in CHTs.
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Low Oil Pressure:
- Low oil pressure can be caused by an oil leak which leads to lack of oil in the system, or an ineffective oil pump.
- These emergencies can be particularly detrimental when flying an aircraft utilizing a constant-speed propeller.
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Low Oil Pressure Primary Indications:
- Oil pressure will indicate low.
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Low Oil Pressure Secondary Indications:
- Rising Cylinder Head Temperatures (CHT).
- Oil temperature may rise (if the pressure drops rapidly then it is less likely you will have a corresponding temperature indication.
- Rough engine indications.
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Low Oil Temperature:
Primary Indications:
- Oil temperature will indicate low.
Secondary Indications:
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High Oil Temperature:
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High Oil Temperature Primary Indications:
- Oil temperature will indicate high.
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High Oil Temperature Secondary Indications:
- Other temperatures will indicate high.
- Possible smoke.
- Low oil pressure.
- High RPM.
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High Oil Temperature Considerations:
- Open cowl flaps, if equipped.
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Oil Leaks:
- Oil on windscreen may come from engine or propeller.
Oil System Inspections
- Inspecting the oil system is more than a routine check
- It is an opportunity for a detailed review of aircraft engine health.
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Oil Inspection:
- Pilots should inspect the oil as part of the pre-flight checklist.
- Inspecting the oil is critical in determining the quantity and condition of oil.
- Oil should never go below the Pilot Operating Handbook specified minimum quanty.
- The color of the oil is a reflection of oil age and engine health.
- New, clean oil should be light in color.
- Dark oil is generally the result of contaminates and oxidation after many hours of operation.
- Too dark is a judgement call based on the amount of hours of use.
- Dark oil may be due to bad piston seals.
- Larger oil sumps tend to absorb contaminents better as they hold more in suspension.
- Smaller sumps should therefore be changed/drained more often.
- Note that while wet-sump systems can be checked by simply checking the dip stick before flight, a dry-sump system may require the engine be turned a few times to circulate residual oil in the engine back to the sump.
- This may be referred to as "burping."
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Oil Filter Inspection:
- It is never required, but pilots who own their airplane may elect to inspect the oil filter for tell-tale signs of wear.
- Oil filters are designed to bypass, if clogged, but are only meant to last for the life of the oil.
- Oil filters should be changed with every oil change.
- Changing an oil filter often also allows for consistent engine health trend analysis.
Aviation Oil/Lubrication System Conclusion
- Oil systems reduce friction on moving parts, create better seals, reduce and remove heat, carry away contaminants and in some cases, run other systems.
- Oil can be sent off for analysis which checks small particles not caught by filter, but a filter will hide big issues.
- Many systems have pressurized sumps and a pressurized oil tank to ensure a constant head pressure to the lubrication pump to prevent pup cavitation at high altitudes.
- Changing oil is permitted as preventative maintenance and provides an indication of engine health.
- Oil consumption is relatively low in a gas turbine engine compared to a piston-type engine.
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