Airworthiness Directives; Boeing Model 737-200, -300, -400, and -500 Series Airplanes Equipped With an Auxiliary Fuel System Installed in Accordance With Supplemental Type Certificate (STC) SA83NE, SA1078NE, SA725NE, ST00040NY, or ST01337NY

The FAA is adopting a new airworthiness directive (AD) for certain Boeing Model 737-200, -300, -400, and -500 series airplanes equipped with an auxiliary fuel system installed in accordance with STC SA83NE, SA1078NE, SA725NE, ST00040NY, or ST01337NY. This AD requires a one-time deactivation of the auxiliary fuel system, repetitive venting of the auxiliary fuel tanks, and revising the Limitations section of the airplane flight manual to limit the maximum cargo weight. This AD results from a re-evaluation of the floor structure and cargo barriers conducted by the STC holder. We are issuing this AD to prevent structural overload of the auxiliary fuel tank support structure, which could cause the floor beams to fail and resultant damage to the primary flight controls and the auxiliary power unit fuel lines that pass through the floor beams, and consequent loss of control of the airplane. We are also issuing this AD to prevent structural overload of the cargo barriers, which could cause the barriers to fail, allowing the cargo to shift, resulting in possible damage to the auxiliary fuel tanks, residual fuel leakage, and consequent increased risk of a fire.