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§ 121.231
Fuel system lines and fittings.
(a) Fuel lines must be installed and supported so as to prevent excessive vibration and so as to be adequate to withstand loads due to fuel pressure and accelerated flight conditions.
(b) Lines connected to components of the airplanes between which there may be relative motion must incorporate provisions for flexibility.
(c) Flexible connections in lines that may be under pressure and subject to axial loading must use flexible hose assemblies rather than hose clamp connections.
(d) Flexible hose must be of an acceptable type or proven suitable for the particular application.