Definition
A rigid or semi-rigid ring installed alongside an O-ring or other elastomeric seal to prevent the seal from being squeezed (extruded) into the gap between mating parts under high pressure. It supports the seal so it stays in place and continues to seal effectively.
Plain English
A stiff ring fitted next to a rubber seal to stop the seal from being pushed out of its groove when pressure pushes against it.
Context Anchor
Seen in aircraft maintenance when assembling or inspecting hydraulic parts, brake parts, landing gear struts, and other pressurized fittings that use seals.
Derivation
The name describes its job: it 'backs up' the primary seal, supporting it from behind so pressure cannot deform it out of position.
Why Pilots Care
Prevents hydraulic leaks and seal blowout that could cause loss of flight controls or landing gear function.
Analogy
It is like a firm backing piece behind a soft gasket: the soft part seals, and the firm part keeps it from being forced out of shape.
Intuition Check
Do not read “backup ring” as a spare ring kept in reserve. In this context, it is a support ring installed with the seal.
Example Sentence 1
When reassembling the hydraulic actuator, the technician installed a new O-ring with its backup ring positioned on the low-pressure side.
Example Sentence 2
During the hydraulic cylinder inspection, worn backup rings were replaced to maintain system pressure.