"B-nut" is a common term for a nut that provides the clamping force to create a reliable seal in lines (fuel, oil, or air, lines on a reciprocating or turbine engine) installed on an aircraft. If a B-nut is improperly secured (either torqued too much or not enough), a loss of engine power or an engine fire could result.
Under- and over-torqued B-nuts could cause fuel, oil, or air leaks depending on where the B-nuts are installed; over-torqued B-nuts could also result in deformation and damage to a line. Fuel or oil leaked onto a hot engine could result in a fire.
B-nuts are exposed to vibration and thermal expansion and contraction during aircraft operations; therefore, maintenance personnel must ensure that the B-nuts are properly secured.
Here is a link to the NTSB Safety Alert SA-086: https://www.faasafety.gov/files/notices/2023/May/SA-086.pdf
For more information, visit www.ntsb.gov
