ALT XING RESTRICTION NOT MET. NIGHT OP.

Narrative

WE WERE DSNDING ON THE MILTON 8 ARR INTO LGA WITH A CLRNC TO CROSS VIBES AT 13000 FT. THE FMC WAS PROPERLY PROGRAMMED WITH THE ARR AND ALT OVER VIBES. LNAV AND VNAV WERE ENGAGED AND THE ACFT WAS DSNDING PROPERLY. (THE ALT XING AT VIBES WAS PROJECTED TO BE 13000 FT BY THE COMPUTER.) AS A LINE CHK AIRMAN DOING INITIAL OPERATING EXPERIENCE (IOE) WITH A NEW CAPT; WE BEGAN TO DISCUSS THE LDA 'A' APCH INTO LGA (WE WERE AT FL240 AT THE TIME) BECAUSE IT IS VERY IMPORTANT FOR NEW CAPTS TO KNOW THE FMC THOROUGHLY. DURING THIS DISCUSSION; BOTH OF US WERE ENGAGED IN LOOKING AT WAYS TO SELECT THE APCH; TUNE THE RADIOS AND BUILD WAYPOINTS ASSOCIATED WITH THE LDA AND/OR ILS BACK-UPS. DURING THIS DISCUSSION NEITHER OF US WAS WATCHING THE ACFT VERY WELL BECAUSE OF OUR INTEREST IN THE APCH AND BECAUSE THE ACFT WAS ENGAGED IN VNAV. JUST PAST VIBES; OUR DISCUSSION ENDED AND OUR ATTNS WENT BACK TO THE ACFT SIT AS WE ANTICIPATED FLT BELOW 10000 FT AND THE CHKLISTS. MUCH TO OUR AMAZEMENT; WE WERE DSNDING TO 13000 FT FROM 17000 FT. WE HAD MISSED THE XING BY OVER 4000 FT! THE COMPUTER WAS STILL IN VNAV AND LNAV WITH APPROPRIATE ANNUNCIATIONS ON THE FMA. I IMMEDIATELY KNEW WHAT HAD HAPPENED. THE AIRBUS FMC DELETES XING ALTS ON STARS WHENEVER A RWY IS CHANGED OR DIFFERENT APCH IS SELECTED AT DEST. WE HAD GIVEN THE COMPUTER A HARD XING ALT AT FIBES; BUT DURING OUR DISCUSSION WE HAD SELECTED THE RWY 22 ILS; RESELECTED THE RWY 22 AND RESELECTED THE ILS AT LGA AND THE COMPUTER AUTOMATICALLY DESELECTED AND DISREGARDED OUR HARD ALT XING. THIS IS EXACTLY WHAT IT IS PROGRAMMED TO DO; AND IN MY OPINION IT IS A VERY DANGEROUS PROGRAM. I CONSTANTLY WARN NEW PLTS ABOUT THIS TRAP IN THE AIRBUS FMC. IT HAD NOW CAUGHT ME. WE DSNDED TO 13000 FT AS RAPIDLY AS POSSIBLE AND NOTHING WAS SAID BY US OR ATC. WE LANDED AT LGA UNEVENTFULLY. SOLUTION: THE AIRBUS PROGRAMMING NEEDS TO BE MODIFIED SO IT WILL NOT DELETE ALTS THAT ARE PUT IN BY THE PLT; AND OF COURSE; SOMEONE SHOULD BE MONITORING THE ACFT AT ALL TIMES. THIS LAST IS TOUGH TO DO ON AN IOE FLT WITH ADVANCED COCKPITS. SUPPLEMENTAL INFORMATION FROM ACN 266453: I MANUALLY PUSHED OVER; DISENGAGED THE AUTO THROTTLES AND DEPLOYED FULL SPD BRAKE. WE CROSSED VIBES A COUPLE OF THOUSAND FT HIGH. IT WAS LATE AT NIGHT WITH MINIMUM TFC AND NO CONFLICT WAS GENERATED. THIS INCIDENT REINFORCES THE REQUIREMENT THAT SOMEONE MUST BE FLYING THE PLANE!