COMMUTER DSNDS BELOW ASSIGNED ALT ON APCH.

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BEING VECTORED BY SFO APCH CTL TO INTERCEPT THE 19L LOC FOR THE ILS APCH SFO. CLRNC WAS TO DSND TO AND MAINTAIN 2900 FT UNTIL SHAKE; TURN TO A CERTAIN HDG; CLRED FOR THE APCH. CLRNC WAS ACKNOWLEDGED; ALT HOLD FEATURE WAS CORRECTLY SET AT 2900 FT. I WAS BUSY WITH SOMETHING SUCH AS A CHKLIST OR TALKING TO COMPANY OR LOOKING FOR TFC -- I DON'T RECALL WHAT; WHEN I NOTICED THE ACFT DSNDING THROUGH ABOUT 2500 FT. I SAID TO THE FO WHO WAS FLYING 'WE'D BETTER GET BACK UP TO 29' AND HE IMMEDIATELY PUSHED THE ALT ARM FEATURE OF THE FLT DIRECTOR. IMMEDIATELY AFTER THAT HE PUSHED THE ALT FEATURE. HE DISCONNECTED THE AUTOPLT AND ADDED PWR AND ADJUSTED OUR ALT. AN ACFT ALERT WAS ISSUED PROBABLY FOR AN ACFT ON APCH TO 11 AT OAK. SEVERAL POSSIBLE PLT INDUCED ERRORS ARE BUILT INTO THIS SITUATION. WE USUALLY CALL 1000 TO GO AND 500 TO GO TO LEVEL OFF. I DON'T RECALL THAT I DID SO ON THIS OCCASION; I MAY HAVE. THE FO ALSO GOOFED BY NOT MAKING THE SAME CALLS AND MONITORING HIS ALT. THE ALT FEATURE DOES NOT ARM IF THE TARGET ALT IS RESET. WHEN THE WARN (ALERT) FEATURES COMES ON FOR THE PREVIOUSLY SET ALT (WITHIN 200 FT ABOVE OR BELOW) IE; IF 3000 FT HAD BEEN SET PREVIOUSLY AND IF WHEN DSNDING THROUGH 3200 FT THE ARM LIGHT GOES ON. IF THE ALT HOLD FEATURES IS THEN CHANGED TO 2900 FT (BEFORE THE 3000 FT FEATURE CAPTURES) THE ALT HOLD FEATURE IS LOST. IT IS NOT LOST WHEN THE ALT PRESELECT (HOLD) IS CHANGED AT MORE THAN 200 FT FROM THE PREVIOUSLY DESIGNATED ALT. IN OTHER WORDS; IT WORKS DIFFERENTLY AT DIFFERENT TIMES. ALL OF THAT NOTWITHSTANDING; THE RESPONSIBILITY RESTS WITH THE CREW AND ULTIMATELY WITH ME AS CAPT. WE DID NOT MONITOR OURSELVES CLOSELY ENOUGH. ALSO; WHEN THE ERROR WAS NOTICED I SHOULD HAVE BEEN MORE AGGRESSIVE IN REGAINING OUR ALT. THE FO DEMONSTRATED THAT HE DID NOT HAVE A FULL UNDERSTANDING OF THE FLT DIRECTOR; AUTOPLT SYS.