Narrative:

During landing in IMC conditions with the first officer flying and the aircraft on a coupled 3 channel ILS approach. Approximately 1000 ft AGL the autopilot disconnected on its own. The autopilot disconnect lights and warning horns sounded. The warning horns could not be silenced by any normal means in the cockpit. The volume was at least 200 DB so loud we were unable to hear tower communications or to talk/yell to each other. It was so loud it was impossible to hear anything in the cockpit or any communications on the radio. If we would have had to go around we would have been in severe distress because of the volume of the horns. We landed the aircraft pulled off the runway and communicated with the tower by transmitting in the blind. We eventually found the 2 circuit breakers controlling the horns. After taxing to the gate and notifying maintenance we shut the aircraft down normally. The horn still went off when the mechanics reset the circuit breakers. The mechanics determined the right flight control computer (FCC) was malfunctioning and replaced it. We flew the aircraft without incident. The warning horn volume on this particular aircraft needs to be turned down. Not being able to silence the warning horns created a stressful situation.

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Original NASA ASRS Text

Title: B757 Captain reports autopilot malfunction during approach causing extremely loud disconnect warning to sound that cannot be silenced. After landing crew is able to disable warning horn with circuit breakers.

Narrative: During Landing in IMC conditions with the First Officer flying and the aircraft on a coupled 3 channel ILS approach. Approximately 1000 FT AGL the autopilot disconnected on its own. The autopilot disconnect lights and warning horns sounded. The warning horns could not be silenced by any normal means in the cockpit. The volume was at least 200 DB so loud we were unable to hear tower communications or to talk/yell to each other. It was so loud it was impossible to hear anything in the cockpit or any communications on the radio. If we would have had to go around we would have been in severe distress because of the volume of the horns. We landed the aircraft pulled off the runway and communicated with the tower by transmitting in the blind. We eventually found the 2 circuit breakers controlling the horns. After taxing to the gate and notifying maintenance we shut the aircraft down normally. The horn still went off when the mechanics reset the circuit breakers. The mechanics determined the right flight control computer (FCC) was malfunctioning and replaced it. We flew the aircraft without incident. The warning horn volume on this particular aircraft needs to be turned down. Not being able to silence the warning horns created a stressful situation.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.