Engine oil performs a number of functions in an aircraft engine: lubrication, cooling, cleaning, corrosion protection, noise reductions and propeller operation; the most important of course being lubrication. Needless to say, without oil all moving parts would wear out quickly. Oil forms a layer between the engine parts and reduces friction. Oil can be visualized as millions of tiny, molecular size, ball bearings rolling between the moving parts. The size of these balls is determined by engine clearances and dictates which viscosity the oil must have for a long engine service life. Engine oil must be able to withstand high temperatures, pressure and loads. It has certain properties as viscosity and contains additives to clean the engine. With the correct oil friction (viscosity) losses in an engine are reduced to a minimum. To do this engine usage, temperature, climate, location and engine design are taken into consideration. The engine manufacturer usually recommends a certain oil type to use regarding the different circumstances.
Oil viscosity or its readiness to flow under different temperatures is probably the most important for pilots to know. During a cold winter start oil will be thicker than during a warm start in summertime. Both cases demand that the oil pressure is attained within 30 seconds after start to prevent damage. Oils have certain viscosities or commonly known as grades. Multi-grade oils are capable of keeping their specific viscosity under a wide temperature range. Engine oils with higher grades are used at higher startup ambient temperatures and are not recommended for use in freezing arctic conditions. Most single grade oils have been replaced with multi-grade oils like 15W50 or 20W50 being more common today. Different oil viscosities may be mixed, however the result will end up being somewhere between the two grades. Mixing multi-grade 10W4 and 15W50 will result in 12W45, etc.. However do not mix mineral or synthetic oils.
Engine cooling is another purpose served by oil. As oil is pumped around in the engine lubricating gears, bearings, piston and valves its temperature rises, particularly near the pistons and cylinders. To ensure that the oil stays within the operating limits it will need to be cooled by running it through an oil cooler. Generally the oil temperature indicator shows the oil temperature when it leaves the cooler. Oil temperature too low isn’t good either to prevent moisture. Remember the engine is only operating on its designed specifications when at its proper operating temperature.
Oils also clean an engine and provides corrosion protection. This is why it is important to follow the maintenance oil and filter change schedules. When the engine is shutdown the oil will drain into the sump, leaving a thin film on all internal parts preventing corrosion. If the engine oil has not been changed accordingly, there is a chance that contaminants in the oil could corrode the metal. The oil films also provides the necessary gas tight seals between piston ring and cylinder walls preventing what is often called gas blowby. Oil on the valve train serves to cushion the valves reducing valve noise.
Engine oil capacities are determined by the manufacturer. The minimum capacity is set according to the sump capacity. According to Advisory Circular 23.1011-1, for engine oil systems without transfer capabilities, only the usable oil tank or sump capacity should be considered in the determination of the usable oil supply. The oil quantity in the oil lines, cooler and/or propeller should not be included. If the aircraft manufacturer determines to operate the engine with less that total oil capacity, the lower oil level should be used for oil analysis.
AC 23.1011-1 provides an acceptable means to comply with the minimum allowable oil capacity. “The minimum usable oil capacity can be determined from the endurance and the maximum allowable oil consumption. ” It goes on to say, ” the maximum allowable usable oil supply ratio is equal to the minimum obtainable oil consumption ratio.
In summary, aircraft operators must keep in mind that the key to operating an engine with low engine oil is adequate lubrication, cleaning, and cooling during high engine loading: engine startup, TO, climb and other critical engine speeds and angles. The oil capacity is directly related to the sump capacity. Operating below the engine manufacturer and/or as defined by AC 23.1011-1 could cause additional wear and tear that reduces engine life, not to mention if too low, cause an engine failure during a critical engine operating phase. [email protected]