Summary
We propose to adopt a new airworthiness directive (AD) for certain Boeing Model 747-400, 747-400D, and 747-400F series airplanes. For all airplanes, this proposed AD would require installing new pump control and time delay relays, doing related investigative and corrective actions if necessary, and changing the wiring for the center and main fuel tanks override/jettison fuel pumps; and, for certain airplanes, installing new relays and wiring for the horizontal stabilizer override/jettison fuel pumps. This proposed AD would also require a revision to the maintenance program to incorporate Airworthiness Limitation No. 28-AWL-24 and No. 28-AWL-26. For certain airplanes, this proposed AD would also require installing an automatic shutoff system for the horizontal stabilizer tank fuel pumps and installing a new integrated display system. This proposed AD results from fuel system reviews conducted by the manufacturer. We are proposing this AD to prevent uncommanded operation of certain override/ jettison pumps which could cause overheat, electrical arcs, or frictional sparks, and could lead to an ignition source inside a fuel tank. This condition, in combination with flammable fuel vapors, could result in a fuel tank explosion and consequent loss of the airplane.