Definition
A fitting used to join two pieces of pipe or tubing together so that the connection can be taken apart for service or inspection. A typical union has three parts: two threaded end pieces that attach to the lines being joined, and a central nut that draws them together to form a sealed joint.
Plain English
A connector that joins two sections of pipe or tubing in a way that lets you unscrew them later without cutting the lines.
Context Anchor
Seen in aircraft fuel, oil, hydraulic, pneumatic, and instrument line installations.
Derivation
From the Latin 'unio,' meaning 'oneness' or 'joining into one.' The fitting earns the name because it brings two separate lines together as one continuous passage.
Why Pilots Care
A properly installed union prevents leaks in critical fluid systems while allowing components to be serviced without replacing entire lines.
Intuition Check
Do not read “union” here as a labor organization or a general agreement between people. In aircraft systems, a union is a physical connector used to join line sections.
Example Sentence 1
The mechanic replaced the union in the fuel line after finding a small leak at the threaded joint.
Example Sentence 2
Before flight, the pilot checked all visible union fittings for signs of fuel seepage.