Do Aircraft Require Oil Changes?

One of the most basic maintenance tasks that are performed on a road-moving vehicle is changing the oil.

One of the most basic maintenance tasks that are performed on a road-moving vehicle is changing the oil. This ensures that the moving parts of the engines, such as the pistons, crankshaft, and cylinders, are well lubricated. Proper lubrication makes sure that the friction is reduced between the moving parts, which reduces temperature and damage to the components.

So, do airplanes need oil changes like cars or other road vehicles?

The engines of piston engine-powered aircraft are very similar to the engines found in vehicles. You have a piston that moves up and down in a cylinder which then moves a crankshaft. This crankshaft is connected to the propeller and makes the propeller turn generating propeller thrust.

In a piston engine, the compression, combustion, and exhaust all occur in one single location. The combustion process usually produces carbon deposits, and as oil is sprayed onto the piston and the cylinder, and after they are coated and lubricated, the used-up oil carries the deposits with it to the oil tank, which gets reused again. The oil also gets contaminated with unburnt fuel.

One of the main by-products of combustion is water. This water can sometimes combine with the gases of combustion, producing acids that can cause corrosion. Such contaminants cause the oil to get "dirty," which can reduce the ability of the oil to cool and lubricate the engine components. And the acids carried in the oil can cause corrosion and component failure.

Thus, regular oil changes are necessary for piston-engine aircraft.

The way a turbine engine is lubricated is very different from its piston engine counterpart. Firstly, the combustion in a jet engine occurs in a hollow chamber with no moving parts. It requires cooling, but this cooling is provided by clever airflow mechanisms. Over 80% of the airflow that reaches the combustion chamber flows around its casing for cooling purposes. Hence, the oil in a jet engine never makes contact with the combustion products.

The oil in jet engines is mainly used to lubricate the bearings of turbine and compressor assemblies. Typically, the oil in a turbine engine is subject to more heat than the oil in a piston engine. This is because the compressor and turbine assemblies rotate at very high speeds, which generates a lot of heat. During the early days of aviation, piston engine oils, which are mineral-based, were also used in jet engines. It was soon found out that these oils, when subject to high temperatures, had a tendency to burn and oxidize, forming carbon deposits and sludge in the oil. Due to this reason, synthetic oils had to be developed for turbine engines. Today, almost all jet engines use synthetic oil. Some properties of these oils include:

Because the oil used in a turbine engine remains fairly clean, draining the old oil and filling it up with new oil is not something that is done regularly. As a matter of fact, an engine can go through its entire life on an aircraft without a single oil change. There are times when oil gets drained from the engines during some hard maintenance checks. But this is rare.

However, there are some exceptions. While there is no actual draining of oil from the tanks, jet engines have a tendency to use up the oil at a very high rate once the engines start running. For example, in an Airbus A320 fitted with CFM 56 engines, the normal oil consumption rate is equal to about 0.5 quarts per hour. To be legal to conduct a flight, the A320 oil tank should have a minimum of 9.5 quarts of oil.

One of the first checks pilots perform once inside the cockpit is to check the oil quantity. This oil quantity should be the minimum amount of oil (in this case 9.5 quarts) plus the estimated oil burn during the flight. So, if the flight is, say, four hours, and if we have 10 quarts of oil shown in the oil quantity display, at some point in the flight it will go below the required 9.5 quarts. If this were the case, the engineering must be contacted to get an oil top-up.

Every time the engine gets a top-up, new oil is introduced. Because of this, the engine components always have a supply of old and new oil. Due to the very high rate of oil usage in a turbine engine, most of the oil remains new. However, there is a small risk of having really old oil in the tank. And at times, this can cause deterioration of the oil to the point where it loses some of its properties. This can lead to oil oxidation and burning of oil. The burning can cause carbon deposits to form, reducing the oil's ability to lubricate the engine components.

To solve this problem, the type of oil used must match the type of operation. For instance, an aircraft engine that is operated quite frequently (higher number of cycles), with low oil consumption may have issues as oil is not used that much even though the engine is operated at a higher rate. This will result in less frequent top-ups and more and more old oil retained in the system. If this oil deteriorates, the engine operation may be severely affected.

Also, if the engine is operated in hot environments, its components are subject to more heat which is transferred to the oil, again leading to engine oil losing its properties. Most turbine engine oil manufacturers provide their customers with high-performance capacity oils that can withstand harsher operating conditions.

Interestingly, newer engines have low oil consumption with very high performance and these engines should be paired with high-performance oils for optimum engine performance.

Want answers to more key questions in aviation? Check out the rest of our guides here.

Journalist - An Airbus A320 pilot, Anas has over 4,000 hours of flying experience. He is excited to bring his operational and safety experience to Simple Flying as a member of the writing team. Based in The Maldives.