ALTDEV ALT UNDERSHOT IN DSCNT. ALT XING RESTR NOT MET. FO FAILED TO PLACE XING ALT IN FMC PRIOR TO DSCNT.

Narrative

WE WERE AT FL280 EXPECTING A CLRNC TO CRUISE ALT OF FL330. WE WERE GIVEN A XING CLRNC INSTEAD OF FL240 55 MI N OF SHB. WHILE PROGRAMMING FMS; WE INADVERTENTLY DIDN'T SET THE XING ALT OF FL240 IN THE ALT ALERT WINDOW. WE NOTICED THE ERROR AFTER THE FMS FAILED TO PROMPT US. THE RESTR WAS NOT PLACED IN THE ALERT WINDOW AND THE CTLR ASKING US IF WE WOULD MAKE THE RESTR AS WE WERE IMMEDIATELY STARTING THE DSCNT. THE CAPT AND I AGREE THAT SETTING FL240 IN THE ALT ALERT WINDOW FIRST IS A BETTER TECHNIQUE THAN PROGRAMMING THE FMS FIRST. I ALSO THINK IT WOULD BE A GOOD IDEA FOR CREWS TO ALWAYS HAVE THE VOR THE RESTR IS BASED ON IN THE RECEIVER SO YOU CAN XREF AND BACK UP THE FMS AND IRU SYS. SEEING THE DME IN THE WINDOW MIGHT CUE YOU INTO STARTING THE DSCNT. HUMANS ARE NOT GOOD MONITORS AND AUTOMATION FORCES US TO BE MORE OF A MONITOR THEN A PLT. WE NEED TO UTILIZE WAYS TO KEEP US MORE INVOLVED (SITUATIONAL AWARENESS) AND NOT TRUST OR TAKE THE AUTOMATION FOR GRANTED. SUPPLEMENTAL INFO FROM ACN 355997: FO BEGAN SETTING UP THE XING RESTR IN THE FMS COMPUTER; BUT SEVERAL ADDITIONAL PROGRAMMING STEPS WERE REQUIRED IN ORDER TO DELETE THE PLANNED CRUISING ALT AND REPROGRAM FOR A DSCNT WITH A XING RESTR. WHEN THE DSCNT WAS PROGRAMMED TO BEGIN; IT DID NOT. TO COMPLICATE THE PROB; THE FMS; WHICH NORMALLY DISPLAYS A MESSAGE TO 'RESET CLRED ALT' (IF IT IS NOT SET FOR LOWER THAN CRUISE) DID NOT DISPLAY; WHICH MAY BE DUE TO OUR BEING PAST THE POINT WHERE THAT MESSAGE IS DISPLAYED. ADDITIONALLY; SITUATIONAL AWARENESS WAS DEGRADED BY SHB VOR NOT BEING MANUALLY TUNED. WHEN CTLR NEXT CHKED WITH US; WE REALIZED THE MIS-SET ALT ALERT WINDOW; BEGAN OUR DSCNT; AND REACHED 24000 FT ABOUT 5 MI PAST 55 MI FIX.