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

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The FAA proposes to adopt a new airworthiness directive (AD) for certain Boeing Model 737-300, -400, -500, -600, -700, -700C, -800 and -900 series airplanes. This proposed AD would require modifying the wiring for the master dim and test system. For certain airplanes, this proposed AD would also require related concurrent actions as necessary. This proposed AD is prompted by 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 proposing this AD to prevent a single fault failure inflight 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.