Definition
A circular seal with a round cross-section, typically made of synthetic rubber or similar elastomer, fitted into a groove between two surfaces to prevent the leakage of fluid, air, or pressurized gas. When compressed slightly between the mating parts, the O-ring deforms to fill the groove and block any path for leakage past it.
Plain English
A small rubber ring shaped like a doughnut that sits in a groove and squashes between two parts to stop fluid or air from leaking out.
Context Anchor
Seen during aircraft maintenance, especially around fuel caps, brake parts, hydraulic parts, and other places where a tight seal is needed.
Derivation
Named for its shape: the letter 'O' describes the circular form, with 'ring' meaning a circular band. The name simply describes what it looks like in cross-section.
Why Pilots Care
A failed O-ring can allow fuel, oil, or hydraulic fluid to leak, leading to loss of system pressure or engine malfunction.
Analogy
Works like a rubber washer squeezed tight between two pipe fittings to keep water from escaping.
Intuition Check
Do not think of an O-ring as just any rubber circle. In aircraft maintenance, its size, material, condition, and correct installation all matter because it is a sealing part.
Example Sentence 1
The technician replaced the O-ring on the hydraulic fitting after noticing a small fluid leak during the inspection.
Example Sentence 2
A leaking O-ring in the hydraulic cylinder caused the flaps to move slowly during approach.