Abstract: The lubrication system of an aero-engine serves as a versatile safeguard for engine safety. It not only handles lubrication and cooling, but also carries away the tiny metal debris generated by wear, preventing them from accumulating on critical components and causing secondary damage. What’s even more important is that these dust-like debris actually serve as early “warning signals” of the engine’s health. By examining their size and quantity, engineers can detect potential faults before they develop.To identify risks earlier and more accurately, a variety of online oil-debris monitoring technologies have been developed to continuously “keep an eye on” debris while the engine is running. Monitoring methods based on electromagnetic induction, ultrasonic reflection, and optical imaging each have their own strengths: some excel at identifying magnetic properties, some can determine debris size, and others can capture debris morphology. Working together, they form a multidimensional health-check system that makes the engine’s operating condition clear at a glance.As technology continues to advance, modern online monitoring approaches increasingly complement traditional offline inspection methods, and are becoming more intelligent with the help of machine learning. Today, these systems can not only detect anomalies, but also predict fault trends—issuing alerts at the earliest stages of risk and providing stronger protection for the safe and reliable operation of aircraft engines.