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Aircraft Engine Oil

Engine Lubrication, Part VI

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 oil 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.

Before flight the pilot checks the oil level and adds any if needed. During the flight he (or she) must pay close attention to lubricant temperature and pressure.

Operational Aspects

Engine oil systems are usually very reliable but the daily (and in between flights) checks of the oil level can not be forgotten as aircraft engines will use a little bit of oil during operation. During preflight the pilot should check the cooler for obvious blockages by foreign matter and leaks (under the engine on the ground). Make sure to check under the cowling and fuselage for stains, it could indicate a minor leak from the sump or oil lines.


When topping up, make sure not to add too much oil. For example: the PA-28-180 runs perfectly on 6 quarts but will throw out anything above that (minimum is 2 quarts), a Rotax should be kept at maximum level for optimum cooling. Make sure to top-up with the correct quantity, type and viscosity grade of oil.

Oil pressure

After engine start the first and most important item to check is the oil pressure, it must register within 30 seconds (60 seconds when in cold to freezing conditions). Check your POH / engine manual for precise details.

Oil system malfunctions

These system faults are rare and usually are related to pressure and or temperature, make sure that you are familiar with the normal indications for your engine. Again, the engine (maintenance) manual will be your guide to follow here.

Fluctuating oil pressure

Engine OIL Pressure Indicator

This can be an indication that the oil level is getting low and the pump is drawing air from either the in- or external sump. A failing scavenge pump may cause oil not being transferred to the external sump. On the Rotax 9 series engine the pressure sensor is a resistive type mounted on the engine near the oil pump and due to vibrations from the engine the sensor will eventually fail while indicating increasing fluctuating pressures. Newer engines from Rotax use a different type of sensor which does not fail so quickly.

High pressure

Usually caused by a faulty pressure relief valve or failing oil pressure sensor (more likely). An oil pressure which is too high may cause seals to blow out resulting in a heavy loss of oil.

Low pressure

May be caused by a low level, loss of pressure by a failing pump, broken oil line or relief valve or even a faulty pressure gauge or oil pressure sensor may cause a low pressure indication. Keep in mind that a high temperature will cause oil viscosity to be lower and that pressure will drop slightly. You see this happen too during a cold start and runup in the winter.

High oil temperature

Aircraft Engine Oil Temperature

Extended climbs in high OAT will cause temperature to rise and pressure to drop slightly. If temperature rises with a large pressure loss then a oil leak might be expected and you should act accordingly.

High power settings combined with low airspeeds (extended climb) will increase the pressure due to a higher RPM and more heat developed in the engine. If combined with a low or reducing oil pressure this may indicate an oil leak with a resulting engine failure closeby.

Keep an eye on the oil pressure and temperature as these are indications of general engine health, if in doubt: land a.s.a.p. at the nearest suitable airport and consult your or any aircraft engineer familiar with the engine.

Written by EAI.

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