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Technology update
Ensuring good oil quality in aircraft engines
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Aviation oil performs many critical functions in reciprocating aircraft engines. It must prevent wear by lubricating engine parts, keep abrasives and deposit-forming materials in suspension, dissipate heat, and seal off blow-by of combustion chamber gases into the crankcase - and it must perform these tasks even under the most severe operating conditions. Therefore, ensuring that quality oil is flowing through the aircraft's engine becomes important for reducing operating, maintenance, and replacement costs. Stephen Sunseri, Aviation Piston Engine Oil Product Line Manager at Exxon, has highlighted several areas often overlooked in maintaining aircraft piston engines.
Viscosity - Selecting the right viscosity of oil for the engine is important. Selecting a lubricant that is too thin will result in insufficient lubricant film. This can lead to increased wear and potential metal-to-metal contact in engine parts such as journal bearings, which require hydrodynamic lubrication. However, selecting a lubricant that is too thick results in increased fluid friction or "drag," increased power requirements, and adversely affects fuel economy. Fluid friction also results in mechanical overheating of engine parts and increased oil temperatures, which can accelerate oxidation and reduce a lubricant's life. Engine OEMs recommend oil viscosity based on a particular engine's ambient operating temperature.
Oil changes - If the engine has full-flow filtration, Sunseri recommends that the oil should be changed every 50 h or four months, whichever comes first. Without full-flow filtration, the oil should be changed every 25 h. Because water and contaminants settle at the bottom of the pan when the oil is cold, it should be changed when hot.
Oil temperature - Sunseri recommends that oil temperature be maintained in the 180-185°F range during flight. This allows moisture that has accumulated in the oil on the ramp to boil away during flight. Moisture, which can lead to the rusting of engine components, is caused by condensation in the engine when it cools down at night.
In turbocharged engines, high temperatures can be a concern. If the oil temperature is significantly above 180°F, check the baffles to make sure that there is good airflow to cool the cylinders. Also monitor CHT and exhaust gas temperature (EGT), and adjust the engine appropriately.
Consumption - Some oil consumption is good because it indicates that the oil is providing a seal at the compression ring. Low oil consumption may indicate that sealing is inadequate, leading to blow-by, power loss, and increased cylinder bore wear. If oil consumption exceeds the consumption test limits defined by the OEM, it is too high, indicating that there might be an unseated or broken ring.
According to Sunseri, the average maximum rate of oil consumption for all Textron Lycoming engines during run-in averages about one quart per hour. Following break-in, oil consumption should stabilize at some lower rate. The lower limit is more difficult to establish because there are many variables affecting oil consumption. Unless there is a problem, oil consumption should stabilize and remain between 0.08 quarts per hour on the low end and the oil consumption test limit as defined by the OEM on the high end.
Oil analysis - As aircraft engine oil lubricates, cools, and cleans the engine, its composition changes as it accumulates contaminants, such as combustion byproducts, dirt, corrosion articles, and metallic wear particles. Analyzing the content of this used oil can help determine engine problems early on.
Additives - Supplementary additives are sometimes necessary to ensure good oil performance. When using top quality engine oils (SAE J1899), additional products are not needed because the manufacturer has already ensured that the oil will perform all the necessary functions required by the engine. According to Sunseri, a notable exception is the Lycoming anti-scuffing/anti-wear additive LW-16702, which is required by the manufacturer on its O-320-H, O-360-E, LO-360-E, TO-360-E, LTO-360-E, TIO, and TIGO-541 engines.
In addition to Sunseri's recommendations, Exxon has recently published a brochure answering some of the commonly asked questions about aviation lubrication maintenance practices and about aircraft piston engine oil. Along with making several aviation oil products, the company provides technical assistance at its Tiger Resource Center located in Houston, TX.
Information provided by Stephen Sunseri, Exxon Company.
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