Airworthiness Directives; Boeing Model 737-300, -400, -500, -600, -700, -700C, -800, and -900 Series Airplanes

The FAA is adopting a new airworthiness directive (AD) for certain Boeing Model 737-300, -400, -500, -600, -700, -700C, -800, and -900 series airplanes. This AD requires modifying the wiring for the master dim and test system. For certain airplanes, this AD also requires related concurrent actions as necessary. This AD results 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. We are issuing this AD 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.