Another fifty hours, another oil change. You know you have to do it. You know it's a normal expense of operating your aircraft, but do you truly know why you do it? The answer has a little bit of science involved in it but can be beneficial to take the time to learn.


Now, if you're anything like me, then you probably like to geek out on the why of things. Here is a cool fact for you: The gas you put into your aircraft is made of the molecules Carbon and Hydrogen (C8H18). You then take this fuel and mix it in the right mixture with the oxygen in the air (O2).

Hydrogen and oxygen are two molecules that are so highly attracted to each other they can create an explosion when they are introduced to each other. That's right: The two elements that you combine to make water will actually become volatile in the mad rush to meet up with each other. Take a look at the following video for a demonstration. It's pretty cool to watch:

Now, if you do the math with me you have these two elements that you light on fire. The fire, or combustion, taking place is not destroying these elements but simply rearranging them. So what you get leftover is carbon, and dihydrogen oxide or H2O. In fact, for every gallon of fuel you burn you are sending about a gallon of water out your exhaust.


If you don't believe me, then just look at the muffler of the car sitting in front of you at a red light sometime, and you're bound to see liquid pour out.


There are six main functions for the oil you put in your engine.

Reduces Friction


This one is a no brainer for any one that has touched oil. It's slick, slimy, and hard to get off your hands, tools and clothes. In fact it's this difficult chore of getting removed from anything it touches that makes it so efficient for your engine.

It coats the outside and inside of the bearings, push rods, rockers, and just about every other surface inside the engine. And be honest, it's probably lubricating a fair portion of the belly of you bird too.

It provides a slick surface that not only helps things move easier but actually keeps the things that move from making direct contact with each other and instead sliding around on a thin film of oil.

Seals and Cushions

Turn your windshield wipers on your car one day when there is absolutely no rain or water on the windshield: They bounce, squeak, and bump because they have no lubrication to help them seal to the window. Once water is introduced into the equation they work quietly and efficiently to displace the water from the windshield. It takes water on the windshield to let the wipers work properly.

Now imagine if your windshield wipers were made out of steel and you turned them on without water: Ouch. This image is very similar to what could be happening to the inside of your cylinder.


Your piston ring will act very similarly to a windshield wiper, creating a seal between the piston and the inside cylinder wall. Oil is the substance that helps this seal to form and allows the piston ring to slide back and forth without the bouncing, scrapping and gouging.


Oil on the cylinder wall creates a nice smooth surface that allows the piston ring to not only create that air tight seal we are all looking for during our compression test, but also provides the lubrication for the piston ring to slide back and forth with minimal wear from metal on metal contact.


It also helps to cushion metal-on-metal contact, such as when the push rod hits the rocker or the cam shaft contacts the lifter. Oil makes the difference in this metal-on-metal contact much like punching your bare fist into a bowl of pudding instead of rock hard concrete.

Removes Heat

If you have ever gotten a little too anxious with a baked potato and stuck into your mouth scalding hot, then you probably followed this brilliant action up with pouring a drink down your throat immediately after. What you did here was not cool off the food in your mouth but displace the heat into the cool liquid.

The engine you depend on gets hot, very hot. And let's face it: Those funny looking fins on your cylinder are not going to cut it. Therefore, oil gets sprayed on the back of the piston to pour off taking the heat with it and back down into the sump. It then gets sucked up and shot through the oil cooler to have a nice blast of cold air cool it off before being slung up onto the piston again.

Cleans the Inside of the Engine

Not only is the oil cooling off the piston, but it is cleaning the cylinder as well. The seal that the piston ring is making is far from perfect. It allows a lot of the byproducts created from combustion to slide on by. The oil catches these impurities and contains them with the help of certain additives that are processed into the oil during manufacturing. It will also collect all the little particles of metal that are very normal to form and leave them deposited on the screen in the oil filter or sitting in the bottom of the oil pan.

Protects against Corrosion

Again, that piston ring does not create a perfect seal, and not only is it letting acidic by products from the combustion by, but it is also allowing that water that is created in.

That water mixes in the oil and then gets sprayed all over the inside of the engine along with the oil.

The boiling point of water is 212 degrees Fahrenheit. So, to those of you going around bragging about how cool your engine runs and that your oil temp never reads over two hundred, you can now realize the mistake you may be making. You want that oil to heat up hot enough every once in a while to boil the water in it. The boiled water turns to vapor that is sucked out the breather and shot down the belly of your airplane.

Performs Hydraulic Action

The majority of horizontally opposed engine have hydraulic valve lifter. The constant speed propeller is hydraulically actuated. Another function of the hydraulic action is your oil pressure gauge. The oil being directed into the line connecting to an oil transducer, or the panel mounted oil pressure gauge, is a hydraulic action.


There are several types of oil out there for aircraft engines. There are even more different types of weights and viscosities.

The multi-viscosity types advertise that they are thinner when the engine is cold so it can lubricate faster but thicken up as it gets warmer. This does as advertised and is great for an engine that gets to run more than five hours a week.

Part of oil's job is to coat the inside of the engine to prevent that water and acid by-products from being able to bond on the engine's metal surface and begin to corrode it. If your airplane sits idle for long periods then you do not want a thin engine oil because it will just run off the interior walls of the engine and expose bare metal to all the harsh chemical deposits it was meant to keep away from it in the first place.

A thicker oil will give this inside protection longer when sitting idle but will still eventually creep back down into the sump.

This is why adding extra additive to your oil can be a very smart investment. Additives can help to contain all the nasty by products leftover from combustion along with creating an impregnable surface on the inside of the engine that oil cannot do alone.

There is one brand in particular that I recommend, Cam Guard

You will add 1.6 oz of an additive like this to every quart of oil. This will raise the cost of your oil change slightly, but the added benefit of an additive that can help do all the little extras that your oil is trying so hard to do on its own can be well worth the investment.


Your oil has to provide a tremendous service to your engine. After being boiled, frozen, and polluted your oil will eventually lose the battle of holding itself together. At this point it will rebel against the abuse and start working against its own environment by thinning, not coating from corrosion, and having no more room for all the leftovers from the combustion process.


Through research, manufacturers have provided us with the ideal times to perform our engine oil changes. These ideal times are set to give your engine the optimum use out of the life of your oil change and to provide the best times to drain and dispose of oil that has performed its duty. So remember, it's not just another chore to perform an oil change on your engine, but a great opportunity to give you a chance to see what is going on inside the engine, what particles are being left in the filter and a great potential time to provide your engine with the little extras to help it stay cool and efficient for the many hours it has left.


Article by Anthony Arispe - Arispe Aviation, McKinney, TX