Several months ago, my friend Paul McBride wrote about the proper way to check the oil level in your aircraft engine. I have received several questions about not only how to check your oil, but also at what oil level to operate a certified aircraft engine.
Now at first, that may seem like a dumb question, but in reality, it is a very good question.
When an aircraft engine is certified, determining the crankcase capacity is a relatively complex process.
First the manufacturer must determine the minimum oil level at which the engine can be operated with no damage to the engine. In addition, they need to determine the maximum oil consumption allowable for that engine. Then they combine all of this information to determine the crankcase capacity.
To give you an example, say a large six-cylinder engine needs at least six quarts of oil, including the one in the filter. The maximum allowable oil consumption for that engine has been set at a quart per hour.
If the aircraft carries enough fuel for operating six hours, the crankcase capacity for this engine would be set at the six quart minimum required, plus an additional six quarts for possible consumption for a total of 12 quarts.
One of the problems here is that there usually is not enough room to put in a larger oil pan. That means there is excess oil flying around in the crankcase. A lot of it can be pumped out the intake guides or swamp the oil rings capacity to remove oil from the cylinder walls before the combustion process. The result is high oil consumption.
For example, we were testing several aircraft in the past and found that if we ran the engine at the 12 quart level, the oil consumption was two to three hours per quart.
But when we ran the engines at the 11 quart level, the consumption went down to four to six hours per quart and six to eight hours per quart at the 10-quart level.
So should everyone just operate at two quarts below recommended level? No, because there are engines like the Lycoming O-320H that need to be operated at or near the recommended level.
To determine what is right for your engine, you need to do some homework.
You can start with the manufacturer, but sometimes they follow the company/legal line and just tell you to always run it full.
I have found that the best source is to talk to a reputable re-builder. Their business depends on satisfied customers, and so they want operators to be happy with their engines, which includes a good oil consumption level.
So ask around and come up with a good level for your engine.
Say you determine that 10 quarts is the best level for your engine. At your next oil change, drain the oil and change the filter, or have your mechanic do it. Then add 10 quarts of oil and start the engine to fill the oil filter. After a few minutes shut the engine off and let it sit for a few minutes. Now check your oil level the way you plan to normally check it (screw the dipstick down or let it sit on top).
You can make a mental note about where the level is or some people will make a very small notch in the dipstick to ensure that whenever they check their oil level using the same oil check procedure, they can be sure that they are at the right level.
This whole process depends on people using some common sense.
For example, if your engine’s oil consumption goes up with age and you are planning a long cross-country trip, you may wish to put in an extra quart or two to ensure that you are safe for the entire trip.