SMT CLIMBING ON AUTOPILOT OVERSHOT ASSIGNED ALT.

Narrative

AFTER DEP WAS GIVEN A HDG OF 030 DEG TO INTERCEPT THE CRP 045 DEG R TO THE PSX 218 DEG R. A CLRNC WAS GIVEN TO CLB TO FL210 AS FILED. THE 030 DEG HDG WAS FLOWN UNTIL IT BECAME APPARENT THAT; DUE TO WINDS; THE 045 DEG CRP WOULD NOT BE INTERCEPTED PRIOR TO THE PSX 218 DEG. I BEGAN INTERCEPTING THE PSX 218 DEG AND TRANSFERRED THIS INFO FROM THE #2 HSI TO THE #1 HSI AND THE AUTOPLT CAPTURED THE NAV MODE TO TRACK INBND TO PSX. AT THAT TIME I BEGAN PROGRAMMING THE VLF/OMEGA FOR THE GLAND INTXN. THIS WOULD BE THE NEXT FIX AFTER PSX AND IS PART OF THE CLRNC TO FLY THE GLAND FIVE ARR RTE INTO THE HOUSTON TERMINAL AREA. I RECEIVED A MESSAGE FROM HOUSTON CENTER ADVISING ME I HAD EXCEEDED MY ASSIGNED ALT. I VERIFIED THAT I WAS APCHING 22500'; DISCONNECTED THE AUTOPLT AND MADE AN IMMEDIATE HIGH RATE DSCNT TO 21000' AND ESTABLISHED MY CRUISE AT 21000'. THE AUTOPLT/FLT DIRECTOR HAD BEEN ENGAGED APPROX 1500'AGL WHEN DEPARTING CRP AND HAD BEEN CONTINUALLY ENGAGED UNTIL THE CALL FROM CENTER. IT HAD TRACKED THE ASSIGNED HDG UNTIL CAPTURING THE PSX 218 DEG R. IT HAD BEEN TRIMMED TO THE PROPER CLB SPD/ATTITUDE. THE ALT PRESELECT HAD BEEN SET FOR 21000' AND THE ALT PRESELECT MODE HAD BEEN SELECTED ON THE A/P CTLR. THERE WAS NO INDICATION OF ANY MALFUNCTION IN IT ON THIS FLT OR THE PREVIOUS LEG OF THIS FLT WHICH WAS FROM FT WORTH; TX TO CORPUS CHRISTI. AN AUDIBLE SIGNAL NORMALLY PRECEDES A SELECTED ALT BY 1000'. I DO NOT RECALL HEARING THIS ALERT SIGNAL AT 20000'; AT WHICH TIME THE ACFT WAS CLBING APPROX 1500' PER MINUTE. IT APPEARS THAT WHEN THE AUTOPLT SHIFTED FROM HDG MODE TO NAV MODE IT DISCONNECTED THE ALT PRESELECT MODE. I DO NOT KNOW IF THIS PARTICULAR AUTOPLT DOES THIS REGULARLY SINCE THIS IS THE FIRST TIME I HAVE FLOWN THIS ACFT. I REGULARLY FLY THE SAME MAKE OF ACFT AND AUTOPLT AND I HAVE NOT EXPERIENCED THIS DIFFICULTY WITH THE OTHER ACFT I NORMALLY FLY. MAINT WILL BE ADVISED OF THE PROB AND MAINT FLT CHKS MADE TO VERIFY PROBS IN THE AUTOPLT/FLT DIRECTOR IF ABLE TO DUPLICATE THE SAME CONDITIONS. WX AT THE TIME/PLACE OF OCCURRENCE WAS GOOD VMC AT 21000' WITH TOPS OF LOWER CLOUDS AROUND 10000'. THERE WAS NO OTHER TFC IN THE AREA AND NO CONFLICTS WERE NOTED OR CALLED OUT BY THE CENTER CTLR.