Airworthiness Directives; Boeing Model 757-200 Series Airplanes

The FAA is adopting a new airworthiness directive (AD) for certain Boeing Model 757-200 series airplanes. This AD requires modifying the wiring of the test ground signal for the master dim and test system circuit in the flight compartment. This 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 issuing this AD to prevent a single fault failure during flight, which could result in test patterns instead of the selected radio frequencies showing on the communications panel. These conditions could adversely affect voice and transponder communication capability between the flightcrew and air traffic control, which could result in increased pilot workload.