ALTDEV ALT OVERSHOT IN DSCNT PROC.

Narrative

ON APCH TO BUFFALO; NY WE WERE GIVEN A DSCNT TO 11000 FT. I WAS FLYING. AT OUR USUAL DSCNT RATE OF 3 MI PER 1000 FT; THE DSCNT CLRNC WAS GIVEN ABOUT 5 MI LATE. TO INCREASE OUR DSCNT RATE; I INCREASED OUR SPD WITH THE AUTOPLT CTLS. THE AUTOPLT DOVE FOR THE AIRSPD TOO AGGRESSIVELY; AND I RESTRAINED THE CTL COLUMN. THIS ACTION MUST HAVE DISCONNECTED THE PERFORMANCE MGMNT PART OF THE AUTOPLT BUT LEFT THE BASIC FUNCTIONS STILL ENGAGED. AS STATED; WE WERE CLRED TO 11000 FT. AT 10800 FT BOTH PLTS NOTICED THE AUTOPLT NOT LEVELING OFF; I DISENGAGED THE AUTOPLT AND AT 10600; RECOVERED; AND CLBED BACK UP TO 11000 FT. CONTRIBUTING FACTORS WERE LATE DSCNT CLRNC AND HIGH DSCNT RATE. WE WERE ALERTED BY THE AURAL WARNING. CORRECTIVE ACTION WOULD BE TO MONITOR THE ALT MORE CLOSELY AND WHEN USING THIS PARTICULAR AUTOPLT TO MAKE MORE FREQUENT CHKS TO MAKE SURE THE AUTOPLT HAS NOT INADVERTENTLY PARTIALLY DISENGAGED. SUPPLEMENTAL INFO FROM ACN 301920: I WAS OFF GETTING THE ATIS. WE HAD BOTH AGREED ON THE DSCNT TO 11000 FT IN THE ALT SELECT WINDOW. WE WERE DSNDING AT 6000 FPM. I CAN'T REMEMBER IF WE DID BUT I'M FAIRLY SURE WE BOTH CALLED '12 FOR 11.' AS THE ACFT PASSED THROUGH 11000 FT I CALLED OUT TO PULL UP TO 11000 FT; SINCE THE ALT SELECT WAS KNOCKED OFF.