Narrative:

Center left us high on the arrival; so I was using speed brakes for speed control during descent. Leveled at 13000 and closed the speed brakes. Aircraft began an uncommanded roll to the left so I disconnected autopilot to stop the roll but it continued. We may have rolled 30-40 degrees before I realized the left spoiler had probably failed to stow and was not responding to the speed brake lever input. Finally; it closed after several attempts. In the commotion; we descended about 800 feet below the assigned altitude. Prior to the event; center had switched us to TRACON; and approach was not aware of our assigned altitude; so ATC seemed unaware of our deviation. With the aircraft back under control; we continued in to land and contacted maintenance upon landing.the aircraft experienced a failure that put us into an undesired state. Fortunately; the spoiler finally stowed and full control was restored. Had it continued; we would have had to notify ATC of an emergency. I was caught off guard mainly because I didn't think that a spoiler asymmetry could actually happen like that. We tend to expect a trim runaway so figuring out what was actually happening distracted me from maintaining altitude. Perhaps being more aggressive in my response would have resulted in little or no altitude loss.

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Original NASA ASRS Text

Title: EMB-145 Captain reported an uncommanded roll due to the failure of a spoiler retraction; after use of the speed brake/spoiler system in flight.

Narrative: Center left us high on the arrival; so I was using speed brakes for speed control during descent. Leveled at 13000 and closed the speed brakes. Aircraft began an uncommanded roll to the left so I disconnected autopilot to stop the roll but it continued. We may have rolled 30-40 degrees before I realized the left spoiler had probably failed to stow and was not responding to the speed brake lever input. Finally; it closed after several attempts. In the commotion; we descended about 800 feet below the assigned altitude. Prior to the event; Center had switched us to TRACON; and Approach was not aware of our assigned altitude; so ATC seemed unaware of our deviation. With the aircraft back under control; we continued in to land and contacted maintenance upon landing.The aircraft experienced a failure that put us into an undesired state. Fortunately; the spoiler finally stowed and full control was restored. Had it continued; we would have had to notify ATC of an emergency. I was caught off guard mainly because I didn't think that a spoiler asymmetry could actually happen like that. We tend to expect a trim runaway so figuring out what was actually happening distracted me from maintaining altitude. Perhaps being more aggressive in my response would have resulted in little or no altitude loss.

Data retrieved from NASA's ASRS site 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.