Summary
The FAA is revising an earlier proposed airworthiness directive (AD) for certain Boeing Model 737-300, -400, -500, -600, - 700, -700C, -800, and -900 series airplanes. The original NPRM would have required modifying the wiring for the master dim and test system. For certain airplanes, the original NPRM also proposed to require related concurrent actions as necessary. The original NPRM resulted from a report that the master dim and test system circuit does not have wiring separation of the test ground signal for redundant equipment in the flight compartment. This action revises the original NPRM by adding a new concurrent action for certain airplanes, extending the compliance time, and removing certain airplanes from concurrent requirements. We are proposing this supplemental NPRM to prevent a single fault failure in flight from simulating a test condition and showing test patterns instead of the selected radio frequencies on the communications panels, which could inhibit communication between the flightcrew and the control tower, affecting the continued safe flight of the airplane.