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Aircraft Engine Oil

Engine Lubrication, Part II

With the correct oil, friction losses in an engine are reduced to a minimum. This is done by taking into consideration circumstances as engine usage, ambient temperature, time of year and climate, location and engine design. The engine manufacturer usually recommends a certain type of oil to use taking these circumstances into account.

Lubrication plays an important part in the life of the engine and during maintenance it will be replaced, on certified aircraft the pilot can only replenish it. Without it the engine would fail within minutes, keeping a watchful eye during flight is therefore important.

Having a basic understanding of engine oil is a must for the professional and private pilot, here we can only scratch on the surface of a very interesting subject.

The oil system in an aircraft engine is very reliable and needs little maintenance, changing oil and filters in regular intervals plus visual inspection. The pilot must keep an eye on levels, pressures and temperatures during operation.

Oil system

A typical aircraft engine oil system has a dry or wet sump. A dry sump means that oil is collected in a separate tank and these are normally used in radial, aerobatic and the well known four stroke Rotax Bombardier engines (912, 914 series). A wet sump system as most Lycoming or Continental engine use have the oil in the sump underneath and attached to the engine.

Oil pump

Aircraft Engine Oil Pump

Oil is being kept in the sump and flows around through a cooler (sometimes with a thermostat), a filter and to the high pressure oil pump (with regulating valve) which pumps the oil through galeries to spray and splash the lubrication points inside the engine. Dry sump engines contain a scavenge pump to remove the oil from the engine to the separate tank. Oil pumps are usually two gears (gerotor) driven by the camshaft.

Pressure & temperature

A pressure gauge is connected in the high pressure line after the pump as an indicator for the pilot and a temperature gauge shows temperature after being cooled en before entering the engine again for another cooling and lubrication cycle.

Oil Pressure Sender

Some pressure sensors are equipped with an extra switch which closes at the moment correct pressure is build up by the pump. This can be used by a light on the instrument panel indicating oil pressure in addition to a proper pressure indicator. The indicator shown to the right has two connections, one for the indicator and the other for the light. This is the preferred model.

Screens and filters

A screen is used in the sump to act as a coarse filter and a screw-on type external oil filter is used as the main filter. This filter sometimes contains a pressure relieve valve to let oil through should the filter become clogged and this valve makes sure that the oil keeps flowing, without filtration.

Engine Oil Cooler

Oil cooler

A radiator type cooler, is basically an air/oil heat exchanger. Outside air is led through the cooler and the cool air picks up the heat from the tubes and fins in the cooler. These items can be equiped with an thermostat with bypass valve should the cooler become blocked for some reason.


Those models equipped with a thermostat (either internally or as add-on) keep the oil on a preset temperature, regardless of the outside air temperature, which really helps getting the engine to operating temperature that much sooner after a cold start.

This is much better than having to keep the engine warm with a winterization kit which blocks part of the air flow into the cowling. The engine will reach operating temperatures much quicker after a cold winter start and maintain it during a long descent, so you will not have to worry about shock cooling all that much. Remember that the cylinder heads remain air cooled requiring you to keep them warm during these long descents.

Preflight inspection

For the pilot there is not much he/she can do than to perform a thorough preflight looking for any oil inside the bottom cowling and on the hanger floor and leaks near the engine and or propeller (if it is hydraulic constant speed model). Any oil leak detected warrants an investigation into its cause; as flying over inhospitable terrain is not the place to see the oil pressure drop to zero!

Written by EAI.

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