DC10 FLT CREW OVERSHOOTS ASSIGNED ALT WHEN AUTOPLT DROPS FROM COMMAND TO CTL WHEEL STEERING AND DSCNT IS CONTINUED.

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ON APCH ON A VECTOR HDG WE WERE CLRED FROM 8000 FT TO 4000 FT IN VMC WITH LIGHT TFC. AUTOPLT #2 WAS ENGAGED AND ALT WAS SET TO 4000 FT AND ARMED. PASSING 5000 FT; I CALLED 5000 FT FOR 4000 FT ALT ARMED. PASSING 4500 FT I LOOKED DOWN AT THE APCH PLATE IN THE COMMERCIAL CHART MANUAL ATOP MY FLT BAG TO DOUBLECHK THE ILS FREQ. DURING MY DISTR I HEARD A TONE THAT I ATTRIBUTED TO THE STABILIZER IN MOTION ALERT; WHICH IS NORMAL. A SECOND LATER; I LOOKED AT THE INSTS AND REALIZED THAT THE AUTOPLT DID NOT LEVEL OFF AT 4000 FT AND WE WERE DSNDING THROUGH 3500 FT! I IMMEDIATELY RECOVERED TO 4000 FT BY MANUALLY PULLING BACK ON THE CTL WHEEL; AND THAT'S WHEN I REALIZED THAT THE AUTOPLT DID NOT LEVEL OFF DUE TO THE FACT THAT IT DROPPED FROM COMMAND TO CTL WHEEL STEERING AND HENCE THE RED AUTOPLT DISCONNECTED LIGHTS THAT WERE FLASHING. NO AURAL WARNING WAS SOUNDED AS THE AUTOPLT DISCONNECTED SINCE THE DC10 IS NOT EQUIPPED WITH SUCH A SYS! WHICH IN MY OPINION IS A GREAT SHORTCOMING. WE DID NOT HEAR ABOUT THE DEV FROM ATC; WHICH SOON AFTER CLRED US TO 3000 FT AND A VISUAL APCH TO RWY 27. ON FINAL WITH AUTOPLT #2 RE-ENGAGED. TRACKING THE LOC AND GS; THE AUTOPLT DISCONNECTED AGAIN TO CTL WHEEL STEERING. WITH THE RED LIGHTS (AUTOPLT DISCONNECTED) FLASHING AND CLRLY VISIBLE IN MY SCAN; I CLICKED THE AUTOPLT FROM 'CTL WHEEL STEERING' TO 'OFF;' FLEW THE APCH MANUALLY TO A SAFE; UNEVENTFUL LNDG. WHAT CAUSED THIS PROB WAS MY TOTAL RELIANCE ON THE AUTOPLT TO LEVEL OFF AT THE PRE-SELECTED ALT WHICH LED TO MY COMPLACENCY TO MONITOR IT LEVELING OFF; AS I DISTR MY ATTN TO LOOK UP THE ILS FREQ IN THE APCH PLATE TO DOUBLECHK THAT IT WAS THE ONE TUNED IN THE ILS RECEIVER. ALSO; LACK OF AN AUTOPLT DISCONNECT AURAL WARNING; AS IN ANY MODERN JET CAN LEAD TO TROUBLE; ESPECIALLY AFTER A 16 HR DUTY DAY WHEN ONE IS NOT AS SHARP AND FRESH IN CATCHING THE AUTOPLT DROPPING OFF FROM COMMAND MODE WITH ONLY 2 LITTLE RED LIGHTS FLASHING AS A WARNING. ON SIMILAR AIRPLANES TO THE DC10; SUCH AS THE LOCKHEED L1011; IF THE AUTOPLT DROPS FROM COMMAND TO CTL WHEEL STEERING OR TO OFF OR IF IT IS MANUALLY DISCONNECTED; A LOUD DISTINCTIVE SIREN/WAILER GOES OFF ALERTING THE CREW OF THE SITUATION. I BELIEVE SUCH A SYS SHOULD BE INCORPORATED IN THE DC10. MEANWHILE; I WILL PAY VERY CLOSE ATTN TO THE AUTOPLT AS IT LEVELS OFF DURING ALL PHASES OF FLT AND GIVE EVERY APCH MY TOTAL UNDIVIDED ATTN AND AVOID ANY DISTRS DURING THE CRITICAL PHASES OF FLT.