Summary
We are revising an earlier proposed airworthiness directive (AD) for certain Model 747 airplanes. The original NPRM would have required measuring the electrical bond resistance between the motor operated valve (MOV) actuators and airplane structure for the main, center, auxiliary, and horizontal stabilizer fuel tanks, as applicable, and corrective action if necessary. The original NPRM also would have required a revision to the maintenance program to incorporate airworthiness limitation (AWL) No. 28-AWL-21 or AWL No. 28-AWL-27, as applicable. The original NPRM resulted from fuel system reviews conducted by the manufacturer. This supplemental NPRM would revise the original NPRM by adding airplanes to the applicability, and would require replacing production-installed laminate phenolic spacers with metallic spacers between the fuel jettison MOV and the airplane structure, as applicable. We are proposing this supplemental NPRM to prevent electrical current from flowing through anMOV actuator into a fuel tank, which could create a potential ignition source inside the fuel tank. This condition, in combination with flammable fuel vapors, could result in a fuel tank explosion and consequent loss of the airplane.