Narrative:

Captain flying in descent to FL200 using autopilot in the vertical speed mode; 1;000 fpm down. Heard an audible 'pop' with vibration in airframe. Assumed we were about to see either one or both engines roll back. Engine indications normal no EICAS messages. Constant vibration of airframe; seen and felt in both yokes. A very fast (fore and aft) movement; almost a clicking sound but not a 'stick shaker'. Checked spoiler handle up; since it felt like a floating spoiler panel but more severe. Slowed to 250 KTS; asked for 10;000 ft and continued descent. Still on autopilot; vertical speed. Checked flight control synoptic page; nothing out of the ordinary. The first officer had decision to disconnect the autopilot. We checked the trim thinking the autopilot trim might be the cause. Trim indicators looked normal. What the heck; I disconnected the autopilot. Immediately; noise and vibration went away. Feeling better; but still not sure what had been going on. It had felt like a door or panel fluttering. Did some 'south' turns. Felt normal. Continued to our destination with extra wide downwind easy turns to final. All configuration changes done in straight and level flight. Used first officer's autopilot and it seemed to work just fine. Talking to maintenance after landing; they advised that there was some sort of service letter out about an autopilot caused vibration. Symptoms matched; i.e.; that we were on autopilot and in vertical speed. As soon as the autopilot was disengaged it went away. The service letter mentions multiple EICAS messages; we did not receive any messages during the flight. With concurrence we deferred the number one autopilot and dispatched to a maintenance base where we were assured it would receive further attention. General 'heads up' to line pilots about this. It is caused by a fight between two channels of the autopilot. Disconnect the autopilot or change modes to get out of vertical speed. Remind pilots these are computers and fly by wire. Not what you might have been flying all of your life. Strange things can and do happen.

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Original NASA ASRS Text

Title: ERJ-190 encountered a vibration apparently caused by the autopilot operating in the vertical speed mode while in descent.

Narrative: Captain flying in descent to FL200 using Autopilot in the vertical speed mode; 1;000 fpm down. Heard an audible 'pop' with vibration in airframe. Assumed we were about to see either one or both engines roll back. Engine indications normal no EICAS messages. Constant vibration of airframe; seen and felt in both yokes. A very fast (fore and aft) movement; almost a clicking sound but not a 'stick shaker'. Checked Spoiler handle up; since it felt like a floating spoiler panel but more severe. Slowed to 250 KTS; asked for 10;000 FT and continued descent. Still on autopilot; vertical speed. Checked flight control synoptic page; nothing out of the ordinary. The First Officer had decision to disconnect the autopilot. We checked the trim thinking the autopilot trim might be the cause. Trim indicators looked normal. What the heck; I disconnected the autopilot. Immediately; noise and vibration went away. Feeling better; but still not sure what had been going on. It had felt like a door or panel fluttering. Did some 'S' turns. Felt normal. Continued to our destination with extra wide downwind easy turns to final. All configuration changes done in straight and level flight. Used First Officer's autopilot and it seemed to work just fine. Talking to maintenance after landing; they advised that there was some sort of service letter out about an autopilot caused vibration. Symptoms matched; i.e.; that we were on autopilot and in vertical speed. As soon as the autopilot was disengaged it went away. The service letter mentions multiple EICAS messages; we did not receive any messages during the flight. With concurrence we deferred the number one autopilot and dispatched to a maintenance base where we were assured it would receive further attention. General 'Heads up' to line pilots about this. It is caused by a fight between two channels of the autopilot. Disconnect the autopilot or change modes to get out of vertical speed. Remind pilots these are computers and fly by wire. Not what you might have been flying all of your life. Strange things can and do happen.

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.