Summary
We propose to adopt a new airworthiness directive (AD) for certain Boeing Model 747 airplanes. This proposed AD would require modification of the refuel valve control unit for the reserve fuel tanks. This proposed AD would also require 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 proposed AD results from fuel system reviews conducted by the manufacturer. We are proposing 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.