ALTDEV ARRIVING SEATAC IN A320.

Narrative

WE WERE GIVEN A DSCNT CLRNC FROM FL200 TO 12000 FT. JUST AFTER THE FLT GUIDANCE SYS WENT INTO THE ALT CAPTURE MODE AT 12400 FT; WE WERE GIVEN ANOTHER CLRNC TO CROSS JAKSN AT 12000 FT; DSND AND MAINTAIN 7000 FT. PROB: WHEN YOU SET A NEW ALT (7000 FT) IN THE ALT SELECT WINDOW ON THE FLT CTL UNIT -- WHILE THE FLT GUIDANCE IS IN THE ALT CAPTURE MODE (IT WAS CAPTURING 12000 FT) -- THE VERT FLT MODE REVERTS TO VERT SPD. THE ACFT WILL NO LONGER CAPTURE THE ALT -- IN OUR CASE 12000 FT -- EVEN THOUGH IT WAS ALREADY IN THE PROCESS OF CAPTURING AND INITIATING A LEVELOFF AT 12000 FT. IT REVERTS TO VERT SPD AS SOON AS YOU SELECT 7000 FT AND WILL FLY THROUGH 12000 FT. WE CROSSED JAKSN SLIGHTLY BELOW 12000 FT. NO CONFLICTS WITH TFC AND ATC DID NOT QUESTION OUR ALT. THIS IS A REAL 'GOTCHA' IN THE GLASS/AUTOMATED COCKPIT. IN YOUR MIND YOU KNOW THE ACFT IS LEVELING OFF AT AN ALT (12000 FT) BUT AS SOON AS YOU CHANGE THE ALT SELECT WINDOW; YOU DISCONTINUE YOUR LEVELOFF. I HAVE ALSO SEEN THIS OCCUR ON NON PRECISION APCHS. I WOULD SUGGEST THAT THE AUTOMATED FLT GUIDANCE SYS BE CONFIGURED SO THAT THE ALT CAPTURE MODE WILL NOT DISCONTINUE A LEVELOFF ONCE THE ALT CAPTURE MODE IS ENGAGED WITHOUT DIRECT PLT INVOLVEMENT. IT IS VERY EASY TO NOT REALIZE THAT YOU ARE NOT GOING TO LEVEL OFF ANYMORE AT AN ALT THAT THE ACFT WAS IN THE PROCESS OF CAPTURING AND LEVELING OFF.