Definition
A threaded coupling nut used on flared tube fittings in aircraft fluid lines. The B-nut slides over the tubing and, when tightened onto a matching fitting, presses the flared end of the tube against the fitting's cone to form a leak-tight seal for fuel, oil, hydraulic, or pneumatic lines.
Plain English
A nut that holds a metal tube tightly against a fitting so fluid can pass through without leaking. It squeezes the flared end of the tube against a matching cone-shaped surface to seal the joint.
Context Anchor
Seen in aircraft maintenance, leak checks, and inspections of fluid or air lines around the engine, landing gear, brakes, and other aircraft systems.
Derivation
The 'B' refers to the AN (Air Force-Navy) standard fitting designation, where the B-nut was the coupling nut paired with flared tube assemblies. The name stuck in maintenance shop language.
Why Pilots Care
A properly torqued B-nut prevents leaks and maintains system integrity under vibration and pressure changes.
Intuition Check
Do not read B-Nut as just any nut with a letter name. In aircraft use, it means the coupling nut that secures a tube or hose line to a fitting.
Example Sentence 1
The mechanic found a small fuel stain at the firewall and traced it to a loose B-nut on the fuel line.
Example Sentence 2
During preflight, the pilot noticed a fuel stain near a loose B-nut fitting.