Definition
The ratio of a fluid's absolute (dynamic) viscosity to its density at a given temperature. It expresses how easily a fluid flows under the influence of gravity, and is commonly measured in centistokes (cSt). Oil specifications often list kinematic viscosity at 40°C and 100°C.
Plain English
A measure of how easily a fluid like oil flows when only its own weight is making it move. It takes the fluid's thickness and divides it by how heavy the fluid is for its size.
Context Anchor
Seen in powerplant maintenance when checking lubricating oil properties, oil specifications, and how oil will flow at different temperatures.
Derivation
From Greek 'kinema,' meaning motion, and Latin 'viscosus,' meaning sticky. The term describes stickiness as it relates to motion under gravity, rather than to an external pushing force.
Why Pilots Care
Correct kinematic viscosity ensures engine oil and fuel flow and lubricate properly across temperature changes, directly affecting engine reliability.
Analogy
Think of pouring two liquids down a tilted surface. Kinematic viscosity is about how fast each one moves under its own weight, not just whether it looks thick in the container.
Grounding Statement
Cold oil has higher kinematic viscosity, so it moves more slowly through the engine than warm oil.
Intuition Check
Do not read kinematic viscosity as simply “thickness.” It includes both the fluid’s resistance to flowing and its density, so it is about how the fluid actually moves under gravity.
Example Sentence 1
The maintenance manual specified an oil with a kinematic viscosity of 12 cSt at 100°C for summer operations.
Example Sentence 2
Winter operations require fuel with a lower kinematic viscosity so it continues to flow through lines at low temperatures.