Airworthiness Directives; Boeing Model 747 Airplanes
We are adopting a new airworthiness directive (AD) for certain Boeing Model 747 airplanes. This AD requires modification of the refuel valve control unit for the reserve fuel tanks. This AD also requires a revision to the FAA-approved maintenance program to incorporate airworthiness limitation (AWL) No. 28-AWL-20 or AWL No. 28-AWL-25, as applicable. This AD results from fuel system reviews conducted by the manufacturer. We are issuing this AD to prevent lightning-induced electrical energy from entering a reserve fuel tank through the refuel valve, which could result in a fuel tank explosion and consequent loss of the airplane.