FAASTeam: The “B” Nut Can Be Deadly

No matter how simple this component is, it requires a specific "torque" to keep it reliable in maintaining plumbing system integrity.


All aircraft maintainers have heard or used the term “B” nut. It is the common description of the nut that is used to connect fluid lines and hoses. It is a very simple component; but does a critical job. It provides a reliable seal in plumbing systems and usually results in ease of maintenance for AMTs. It is designed to provide years of reliable service. However, “B” nut reliability is only as good as how you treat it.

We disconnect and connect them many times during our careers - right? However, do you “tighten” or “torque” this simple critical component? No matter how simple this component is, it requires a specific “torque” to keep it reliable in maintaining plumbing system integrity. You may have seen a maintenance record entry where the sign-off stated, “Tightened B nut.” Does this mean it was tightened enough to stop a leak, or was it tightened to a specific torque? If you are just tightening the “B” nut, you could be setting the stage for future failure or fracture.

Just get the tools you need to get it done right!

Another “gotcha” is finger tightening then “forgetting” to torque the “B” nut. A high risk of this occurring can be distractions (like telephone calls) and interruptions (like multi-tasking) while on the job.

A good technique to follow is if you connect a “B” nut, finish the job with the proper torque. Otherwise, tag it or write it up so it is not overlooked, especially if you leave the job site or another AMT finishes the job!

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