This article originally appeared on the P&WC Airtime Blog.
In an aircraft engine, oil is much more than just a lubricant. It plays a number of other important roles, including cooling, cleaning and noise reduction. It’s therefore vital to monitor and analyze oil to ensure it’s doing its job properly.
WHEN AND WHAT TO ANALYZE
Engine oil sampling and analysis are recommended if a visual inspection reveals that the oil is very dark, has an unusual odor or exhibits other abnormal properties. You don’t necessarily have to change the oil, but at the very least, it should be analyzed to determine its total acid number (TAN) and water content.
Typically, the water concentration in brand-new oil varies between 0.02% and 0.04%, or 200 and 400 parts per million (ppm). However, water can enter the engine’s oil system due either to accidental contamination during compressor wash or normal condensation. Since aircraft engine oils easily absorb water and moisture from the air, their water content will rise over time. If it exceeds 1000 ppm, the TAN may rise as well, eventually leading to engine component corrosion.
HOW TO SAMPLE AIRCRAFT ENGINE OIL
When you take an oil sample, identify it with the brand name, engine serial number, total run time (oil life) and engine time since new (TSN) or time since overhaul (TSO). Have the sample analyzed for its TAN and water content by an approved laboratory. If necessary, ask the lab to analyze the oil viscosity and additives as well.
If a parameter exceeds the established limit, it’s recommended that you: 1) drain and discard the oil from the tank; 2) check the condition of the oil filter and, if needed, replace it with a new one; 3) refill the tank with fresh oil.
If you have access to the right kits, you could also perform the analysis yourself. Use a Titra-Lube TAN Test Kit to analyze the oil’s TAN and a HydroScout Analyzer kit for the water content.
RACK YOUR OIL CONSUMPTION
Every time you add oil into your engine, write down the amount so that you can calculate the average consumption. Check this figure against the limits indicated in the maintenance manual. If the engine is using more oil than it should, there may be a part that needs maintenance. For example, a damaged O-ring could be causing a leak, or something may be happening in the engine that’s burning up extra oil.
Oil is key to your engine’s health and performance, so give it the attention it deserves by following the advice above.
Read all technical tips and talk from our experts about aircraft engine oil on Airtime.
A BRIEF HISTORY OF AVIATION GAS TURBINE ENGINE LUBRICANTS
The earliest aviation piston engines were lubricated with natural oils such as castor oil and refined mineral oils. However, they lacked the thermal-oxidative stability needed for high-temperature mechanical systems and would form deposits like gum and lacquer on metal surfaces.
In the 1950s, following research efforts aimed at improving thermal-oxidative stability, synthetic polyester-based lubricants became the base stock of choice for aviation gas turbine engine oils. Thanks to their chemical properties, these lubricants are effective over a wide temperature range, from -65oF to 425oF. They possess good thermal-oxidative stability, high lubricating film strength, good surface wetting, and low friction and wear rates, making them ideal for aircraft engines.