Summary
We are revising an earlier proposed airworthiness directive (AD) for certain Model 747-100, 747-100B, 747-100B SUD, 747-200B, 747- 200C, 747-200F, 747-300, 747SR, and 747SP series airplanes. The original NPRM would have required adding two new indicator lights on the P10 panel to inform the captain and first officer of a low pressure condition in the left and right override/jettison pumps of the center wing tanks. The original NPRM would also have required replacing the left and right override/jettison switches on the M154 fuel control module on the P4 panel with improved switches and doing the associated wiring changes. The original NPRM would have also required a revision to the maintenance program to incorporate airworthiness limitation No. 28-AWL-22. The original NPRM resulted from fuel system reviews conducted by the manufacturer. This action revises the original NPRM by adding a revision to the airplane flight manual to advise the flightcrew what to do in the event thatthe pump low pressure light on the flight engineer's panel does not illuminate when the pump is selected off; and requiring, for certain airplanes, installation of a mounting bracket for the new indicator lights. We are proposing this supplemental NPRM to prevent uncommanded operation of the override/ jettison pumps of the center wing tanks, and failure to manually shut off the override/jettison pumps at the correct time, either of which could lead to an ignition source inside the center wing tank. This condition, in combination with flammable fuel vapors, could result in a center fuel tank explosion and consequent loss of the airplane.