The lubricant in aircraft piston engines has several complex functions within the engine. One of the main characteristics is lubricating the moving parts to reduce friction and wear. Friction occurs between two pieces that move relative to each other, even though they have polished metal surfaces. Since there are microscopic peaks and valleys called asperities, these surfaces come into contact, and the rough edges adhere to each other through micro-welds. If these surfaces are in relative motion, the micro-welds constantly break and reshape, resulting in friction and wear. Friction is resistance to relative motion, and wear is loss of material.
There are different types of lubrication events within an engine; the first is hydrodynamic lubrication, which occurs when a fluid (especially a lubricant) is interposed between two moving parts. The relative movement of these parts creates sufficient pressure in the lubricant, forming a fluid wedge to separate the parts to prevent each from coming into contact. This is the same effect that occurs during hydroplaning when a car travels on a road covered in a layer of water, and the wheels become separated from the pavement by the pressure of the water.
Hydrodynamic lubrication works well when the relative speed of the parts is high enough to overcome the load, pushing them against each other. If this speed is insufficient, the lubricant pressure will not be adequate, and the parts will not stay separated. The second form of lubrication is film lubrication. This lubrication method relies on a thin film deposited between moving parts that adhere to surfaces, reducing friction through a chemical action produced by extreme pressure additives.
Another function of a lubricant is keeping engine parts clean. Compared to automotive engines, aircraft piston engines can become extremely dirty. Burning leaded fuel produces large amounts of lead salts, carbon, sulfur, water and unburned fuel, among other by-products of combustion. These deposits accumulate in the lower part of the engine due to the passage or “blow-by” of these products through piston rings. The oil functions as a method to keep these contaminants dispersed and in suspension to be drained at the following oil change. Not allowing for accumulation inside the engine in the form of sludge. Likewise, the dispersant additives contained in the oil help neutralize the acids generated by combustion.
Lubricant also contributes to the engine’s cooling in an aircraft, especially in piston-powered aircraft that cannot be cooled by air. The only way to keep the pistons from melting is by keeping oil circulating on the underside of the piston (crown) to remove the heat generated by combustion. The lubricant typically gains about 30°C (90°F) as it circulates through the engine, which is then dissipated through the oil cooler.
The lubricant also acts as a seal between the piston rings and the cylinder walls, preventing the loss of gases and liquids due to “blow-by.” This also keeps the o-rings, seals, and gaskets lubricated, so there is no loss of fluid or gases through them.
If the aircraft is equipped with a constant speed propeller, the oil controls the variable pitch. Likewise, in turbo-charged engines, the “wastegate” (turbo compressed air relief valve) is hydraulically actuated by the engine oil.
Used lubricant extracted from the engine after an oil change is an essential tool in knowing the ongoing condition of the engine in an airplane. Samples are often sent to a specialized laboratory that, through spectrographic analysis, informs us about the different elements contained. If we do this process regularly, operators will learn what the principal elements and pollutants are, such as iron, copper, aluminum, silicon, bronze, lead, tin, etc. Other benefits of testing include knowing the state of the used oil by measuring its viscosity and oxidation and the state of additives such as phosphorus, zinc, and calcium.
As part of regular servicing and knowing your aircraft, it is also vital for mechanics to cut and open the oil filter to see what particles may become trapped in the filter. They can be ferrous (those attracted by a magnet), iron type, or non-ferrous aluminum, bronze, etc.
A published Lycoming Service Bulletin (No. 480F) detailed a guide on how to proceed in finding particles in the oil filter. This SB can also be applied to Continental engines as a guide, as Continental does not have a specific bulletin.
Last and perhaps most importantly, lubricants protect metal components such as the crankshaft, camshaft, piston rods, and cylinder walls from the effects of corrosion during periods that the aircraft is not in use. Because we typically use our airplanes less often and more unevenly than our automobiles, the preservative requirement of aeronautical lubricant is much more demanding than in automotive lubricants.
Therefore, remember to change the oil every 50 hours for filtered engines, 25 hours for screened engines or every four months to keep lubricants in excellent condition to perform all the functions described above.
Conference given by Field Engineer Steven Strollo from Phillips 66 at the Mercosur and Latin American Congress of Agricultural Aviation, Salto 2019.
Mike Bush’s “Your engine’s lifeblood” article published in AOPA Pilot, October 2020.
Lycoming Service Bulletin N ° 480F: https://www.lycoming.com/content/service-bulletin-no-480-f