Narrative:

While being vectored by approach control and descending sbound into dca (downwind leg) and leveling at 5000 ft MSL with the autoplt engaged and slowing, the autoplt out of trim light came on. It stayed on longer than normal so autoplt was disconnected, accompanied by an abrupt pitch over. The pitch over was more than expected and before I recovered we had descended to 4700 ft MSL. An immediate climb was initiated back to the assigned altitude, however prior to reaching 5000 ft MSL approach control reclred us to 1600 ft MSL. The remainder of the flight was uneventful. 1 additional distraction was an 'alpha speed' annunciation which kept flashing in the FMA panel. The speed shown was plus 20 KTS in error on the high side for our current gross weight which may have been a contributing factor to the autoplt out of trim light coming on, the black box thinking we were heavier than we actually were. I would say if the unusual is taking place be prepared for the unusual results.

Google
 

Original NASA ASRS Text

Title: ALT OVERSHOOT. AUTOPLT OUT OF TRIM WARNING SO PLT DISCONNECTED WITH SUBSEQUENT PITCH-OVER AND ALTDEV.

Narrative: WHILE BEING VECTORED BY APCH CTL AND DSNDING SBOUND INTO DCA (DOWNWIND LEG) AND LEVELING AT 5000 FT MSL WITH THE AUTOPLT ENGAGED AND SLOWING, THE AUTOPLT OUT OF TRIM LIGHT CAME ON. IT STAYED ON LONGER THAN NORMAL SO AUTOPLT WAS DISCONNECTED, ACCOMPANIED BY AN ABRUPT PITCH OVER. THE PITCH OVER WAS MORE THAN EXPECTED AND BEFORE I RECOVERED WE HAD DSNDED TO 4700 FT MSL. AN IMMEDIATE CLB WAS INITIATED BACK TO THE ASSIGNED ALT, HOWEVER PRIOR TO REACHING 5000 FT MSL APCH CTL RECLRED US TO 1600 FT MSL. THE REMAINDER OF THE FLT WAS UNEVENTFUL. 1 ADDITIONAL DISTR WAS AN 'ALPHA SPD' ANNUNCIATION WHICH KEPT FLASHING IN THE FMA PANEL. THE SPD SHOWN WAS PLUS 20 KTS IN ERROR ON THE HIGH SIDE FOR OUR CURRENT GROSS WT WHICH MAY HAVE BEEN A CONTRIBUTING FACTOR TO THE AUTOPLT OUT OF TRIM LIGHT COMING ON, THE BLACK BOX THINKING WE WERE HEAVIER THAN WE ACTUALLY WERE. I WOULD SAY IF THE UNUSUAL IS TAKING PLACE BE PREPARED FOR THE UNUSUAL RESULTS.

Data retrieved from NASA's ASRS site as of July 2007 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.