DSCNT THROUGH ASSIGNED ALT.

Narrative

WHILE IN AUTOPLT VNAV CTLED DSCNT ON CIVET APCH FOR RWY 25L AT LAX; ATC REQUESTED WE MAINTAIN 320 KTS AND THEN CHANGED TO 250 KTS PRIOR TO LEVELOFF PLANNED AT 12000 FT. FO AND CAPT NOTED ACFT SWITCHED TO ALT CAPTURE MODE ABOVE 12000 FT AS EXPECTED AND SWITCHED ATTN TO MAKING 250 KT SPD RESTR BY BREMR. ACFT IN HIGH RATE OF DSCNT DUE TO FULL SPD BRAKE AND IDLE PWR. ALT CAPTURE MODE WENT RIGHT THROUGH EXPECTED 12000 FT LEVELOFF AND WAS MANUALLY RETURNED TO 12000 FT WITH A CALL TO ATC. NO PROB NOTED BY ATC; BUT THIS SHOULDN'T HAPPEN! POSSIBLY DUE TO MODE VNAV NOT DIFFERENTIATING OR READOUT BTWN 'DES NOW' AND 'DES DIR' UNCLR WHICH MODE ACFT WAS IN. LEARNED NOT TO RELY ON SOFTWARE. GENERAL COMMENT: TOO MUCH AUTOMATION LEADS TO TOO MANY OPTIONS LEADS TO UNCERTAINTY! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE B-767 AND B-757 FOR AN AIRLINE. THERE WAS AN ACI ON THE JUMPSEAT ON THIS TRIP AND WITNESSED THE MISSED LEVELOFF. THE ACI WAS NOT AT ALL CONCERNED ABOUT THIS AND THOUGHT THAT THE CREW DID A GOOD JOB. BOTH PLTS SAW ALT CAPTURE INITIATED; THEN DIVERTED THEIR ATTN. AFTER A DSCNT IS STARTED; IT IS VERY DIFFICULT FOR THE CREW TO DETERMINE IF THE ACFT IS IN 'DES NOW' OR 'DES DIR.' 'DES NOW' COMMANDS THE ACFT TO DSND SO AS TO MAKE ANY PREPROGRAMMED RESTRS. 'DES DIR' WILL WIPE OUT ANY RESTRS. THE RPTR DOES NOT LIKE THE 'DON'T WORRY ABOUT IT; YOU'LL GET THE REST ON THE LINE' APCH TO TRAINING. NEITHER PLT KNOWS IN WHICH DSCNT MODE THE ACFT WAS OPERATING; NOR WHY THE ACFT FAILED TO CAPTURE ITS ALT.