Definition
The ability of a lubricating oil to maintain a continuous, unbroken film between two moving metal surfaces under load, preventing direct metal-to-metal contact.
Plain English
How well an oil can stay between two moving parts and keep them from grinding against each other, even when they're being squeezed hard.
Context Anchor
Seen in aircraft maintenance and engine discussions about oil, grease, bearings, gears, cylinders, and other moving parts.
Derivation
‘Film’ here means a thin layer (the oil coating), and ‘strength’ means how well that layer resists being pushed out from between the surfaces. The term describes the oil's ability to hold its protective layer together under pressure.
Why Pilots Care
Adequate film strength prevents rapid wear and scoring of bearings and cylinder walls under load.
Intuition Check
Film does not mean a movie or a visible sheet here. It means a very thin layer of oil or grease. Strength does not mean the metal part is strong. It means the oil layer can hold up under pressure.
Example Sentence 1
Aviation engine oils are formulated with high film strength to protect bearings during high-power takeoffs.
Example Sentence 2
Before approving the engine for return to service, the mechanic verified the oil met the required film strength rating.