Airworthiness Directives; The Boeing Company Model 747 Airplanes

We are adopting a new airworthiness directive (AD) for the products listed above. This AD requires 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; revising the maintenance program to incorporate airworthiness limitation (AWL) No. 28-AWL-21 or AWL No. 28-AWL-27, as applicable; and replacing production-installed laminate phenolic spacers with metallic spacers between the fuel jettison MOV and the airplane structure, as applicable. This AD was prompted by fuel system reviews conducted by the manufacturer. We are issuing this AD to prevent electrical current from flowing through an MOV 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.