Just An Oil Change

Whether you do your own oil changes or entrust it to professional maintenance
providers, the objective is the same – remove as much of the old oil as possible. The
traditional and industry accepted sequence for oil change is to pull the “cold” airplane
out of the hangar and perform a ground run to heat up the oil so it drains faster. I believe
we’ve allowed ourselves to buy into a system of convenience instead of one that
accomplishes the stated goal. The problem, as I see it, with this tradition is that much of
the old oil remains in the engine. How can that be? Well, let’s consider some
possibilities.

Old School

Running the engine to heat the oil should reduce the time it takes to vacate the sump
during the oil drain. It’s considered better to take the plane for a short flight to really get
everything warmed up so all that nasty old oil will drain out even quicker. This works
perfectly if you don’t think about it too much.

The accepted assumption is that the heated oil has lower viscosity which means it’ll
drain out of the sump faster. I agree that part is true. As the oil temperature increases, it
becomes thinner, and whatever oil is in the sump will drain out quickly. In the business
world this means the maintenance provider spends less time waiting for the sump to
drain. In our shop we open the drain valve first then proceed to removing the oil filter.
We cut open the filter and perform a detailed inspection of the oil filter element. We then
install and safety the new filter and clean around the filter area where there might have
been some spillage. After everything else is completed, we close the drain valve and
add the new oil. Total time for valve open might be an hour if the customer is waiting. If
the plane is in for annual or the customer leaves the plane for the work, we’ll leave the
drain valve open as long as practical.

All that is great and does a good job of draining the sump. What isn’t considered is all
the oil that’s still in the rest of the engine. Oil is covering the entire inside of the engine
just sitting there taking its own sweet time moseying down to the sump.

Thought Experiment and Homework

During a short fuel stop on a long trip, we accept that our oil level check is going to be
lower than when we started the journey with a “cold” engine. We understand that there
is a significant amount of oil still held up in the upper parts of the engine that hasn’t had
time to drain down to the sump. If the oil level checked at 6 ½ quarts when the journey
began, a short fuel stop oil level check of 5 quarts might be perfectly acceptable. The
same concept applies after that short flight or ground run made in preparation for the oil
change.

How long does it take for all that oil to make its way to the sump? If we answer that
question, we’ll know how long it takes to vacate all the old oil from the engine during an
oil change. An easy and interesting experiment will give us the information we’re looking
for.

After your next flight try checking the oil after all the post flight housekeeping is done
(unload the baggage, put the plane back in the hangar, clean the bugs off the wings,
etc.). Take note of the time from engine shut down until oil level check. Make your way
back to the plane the next day and check the oil level a second time and note the
results. About a week later, with no flights between, check the oil level again. You may
be surprised to find a full additional quart difference in the oil sump between the day
after the last flight and a week after the last flight. That full quart difference is the oil that
wouldn’t have been removed from the engine during the old school process of running
the engine just prior to an oil change.

 

Allow plenty of time for all the oil to make its way to the sump

Primary ObjectiveThe primary objective of an oil change is to remove as much debris from the engine as
possible. The debris comes in the form of the byproducts of combustion, moisture, and
wear elements from the internal components. The combustion byproducts include acids
that attack the polished steel parts of the engine such as the camshaft lobes and the
camshaft followers. Wear elements include metallic debris which can damage bearings
and gear contact surfaces. If we perform an oil change but leave a quart of the old,
contaminated oil in the engine, we haven’t accomplished our objective.

The first engine start after a traditional oil change immediately circulates that quart of
old oil back through the engine and contaminates all the new oil just added. Imagine
what happens with the next oil analysis when it starts with the contaminations from the
previous oil cycle. How accurate are the reports going to be, especially if you’re tracking
elevated readings?

Let’s consider some options that will help us get to that Primary Objective.

New School

Pardon my play on words with my last name, but I couldn’t resist. After your experiment
checking oil levels over time, you have an idea how long it takes for that last bit of oil to
drain down from the crank case walls, crankshaft outer surface, oil cooler, etc.  My
educated guess is that it’ll take about a week for the oil level to stabilize at its
maximum.  This seems like the perfect time to do an oil change.

It is true that the oil temperature will be near ambient, and the viscosity will be higher
than it will be just after a flight. The tradeoff is that the undrainable film is just what
would be on the wall of the oil sump. That’s way less surface area than the rest of the
entire engine.

It would be great if we had a way to increase the temperature of the oil in the sump so it
would drain faster after sitting there for a week during a cold January in the T hangar.
What about that engine preheater that almost everyone has installed on the Cirrus
engines these days? There is no rule that says these heaters can only be used in the
winter or only for pre-flying operations. Let’s plug that heater in on the day before an oil
change to get the oil temp as warm as possible and its viscosity as low as possible.

Now the engine is ready for draining. If the work is being done in your hangar, then just
pull the plug. If you outsource the oil change to a shop on the field, then have the plane
towed over instead of starting the engine and taxing over.

Oil filter access is easy on the non-turbo and non-air conditioned planes

Unnatural

I know this suggested process goes against years of tradition, but it works. Your
mechanics may object to the idea at first, but they will come around if you present it to
them logically. In the end, you’ll get more of the bad oil out of the engine, which is the
actual goal and best for the long-term health of your engine.