All About Aircraft Oil :For Piston Engines

All About Aircraft Oil

For Piston Engines

By Stephen M. Sunseri, ExxonMobil

March 2000

A great deal is published in aviation magazines about "best maintenance practices." And there is no doubt that preventive maintenance is the key to keeping an aircraft ready while also reducing operating, maintenance and replacement costs for your customer.

One of the areas involving preventive maintenance that is a "given" is oil changes. Yet, as simple as this seems, there are often misunderstandings about what types of oils should be used, how often the engine should be serviced and other important elements related to engine oil. Following is a review of engine oil basics, as well as some important points to consider related to engine oils.

Selecting the right viscosity or "thickness" of oil for a particular engine is very important. We often hear people say "The heavier the engine oil, the better it will protect the engine." Indeed, viscosity is the single most important physical property of an engine oil because one of its primary functions is to separate moving metal parts within the engine.

However, there are trade-offs when choosing oil viscosity. Selecting a lubricant that's too thin will result in insufficient lubricant film, which can lead to increased wear of engine parts and potential metal-to-metal contact in engine parts such as journal bearings, which require hydrodynamic lubrication (separation by lubricant film).

Conversely, selecting a lubricant that's too thick will result in increased fluid friction or "drag," increased power requirements and will adversely effect fuel economy. Fluid friction will also result 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. Make sure you're using the correct oil viscosity for your customer's engine and operating conditions.

Oil Changes
Oils are designed to get dirty. That's how they keep an engine clean. This only works if you change the oil when you're supposed to. If an engine has full-flow filtration, you should change the oil every 50 hours or every four months, whichever comes first. Without full-flow filtration, you should change the oil every 25 hours.

Since water and contaminants settle to the bottom of cold oil, you should change the oil when it's hot. When the engine is fully warmed before it is drained, a higher percentage of contaminants is removed with the oil. If you drain it when it's cold, some contaminants will remain in the oil pan and will compromise the fresh oil.

Oil Filters
Always change the oil filter when you change the oil. Otherwise, you could leave behind a quart or so of dirty oil that is mixed with the fresh oil. Additionally, filter life has been reduced by the first drain interval and may plug or fail during the next drain interval, leaving the engine unprotected.

Oil Temperature
Maintain oil temperature in the 180 to 185 F range during flight. This range will allow moisture that has accumulated in the oil on the ramp to boil away during flight. When the aircraft sits on the ramp or in the hangar, the engine heats up during the day and cools down at night. This cooling process condenses water vapor in the engine, forming moisture, which drains into the oil. This can lead to rust on engine components.

Oil temperatures that are "in the green" are not necessarily hot enough to boil moisture away, so check the aircraft's engine temperature gauge for accuracy. Some gauges are marked with actual numbers. Some are not. If the gauge is marked, it should read approximately 212 F when the probe is placed in boiling water. If your gauge is not marked, a good practice is to mark your oil temperature gauge with a reference mark at 180 F.

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