AUTOFLT FAILS TO CAPTURE ALT AND CREW DOES NOT NOTICE WHEN DISTR OUTSIDE THE COCKPIT.

Narrative

DURING A DSCNT INTO GRR WE WERE CLRED TO DSND TO 9000 FT. 9000 FT WAS SET IN THE ALT SELECTOR AND ARMED ON THE AUTOPLT CTL PANEL. SHORTLY AFTER THIS WE WERE HANDED OFF TO GRR APCH. AT THIS TIME MY FO RPTED TO ME THAT HE HAD THE FIELD IN SIGHT. I THEN BEGAN LOOKING FOR THE ARPT. WHEN I LOOKED BACK IN THE COCKPIT; WE WERE DSNDING THROUGH 8500 FT. THE ALT ALERT LIGHT ON THE ALTIMETER WAS LIT; HOWEVER; THE ALT ARM LIGHT WAS STILL ON ON THE AUTOPLT CTL PANEL AND THE AUTOPLT DID NOT CAPTURE THE ALT. AT THIS POINT; I DISENGAGED THE AUTOPLT AND IMMEDIATELY CLBED BACK TO 9000 FT. TOTAL TIME OFF ALT WAS ABOUT 40 SECONDS. ONCE BACK AT 9000 FT; THE FO CALLED GRR APCH. GRR THEN GAVE US A DSCNT TO THE ARPT AND DID NOT QUESTION ANY DEV. I BELIEVE THE CONTRIBUTING FACTORS WERE THE AUTOPLT'S FAILURE TO CAPTURE THE ALT ALONG WITH MY FAILURE TO MONITOR THE AUTOMATION AS WE WERE APCHING THE ASSIGNED ALT. IN THE FUTURE; I WILL MONITOR THE AUTOMATION MORE CLOSELY -- EVEN IN VMC. PARTICULARLY WHEN APCHING ASSIGNED ALTS AND HDGS. IF I WISHED TO LOOK FOR THE ARPT; I SHOULD HAVE THE FO MONITOR THE INSTS. THIS PROC IS USED ON INST APCHS; BUT SHOULD ALSO BE USED DURING VISUAL CONDITIONS.