Summary
We propose to adopt a new airworthiness directive (AD) for Model 747-400, 747-400D, and 747-400F series airplanes. This proposed AD would require installing a hot short protector (HSP) for the fuel quantity indicating system (FQIS) of the center fuel tank and, for certain airplanes, the horizontal stabilizer fuel tank. This proposed AD results from fuel system reviews conducted by the manufacturer. We are proposing this AD to prevent an electrical hot short from a source outside the FQIS to the densitometer wiring from causing failure of the FQIS densitometer resistors, which could result in an ignition source inside the center or horizontal stabilizer fuel tanks. An ignition source, in combination with flammable fuel vapors, could result in a fuel tank explosion and consequent loss of the airplane.