AN A320 FLT CREW GETS INTO A LOW SPD REGIME WHEN THE FO FAILS TO FOLLOW PROCS FOR THE ACFT CONFIGN SELECTED DURING A DSCNT INTO IAD; VA.

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DSNDING THROUGH APPROX 8000 FT; CLRED TO 3000 FT. WE HAD ORIGINALLY BEEN TOLD TO EXPECT AN ILS RWY 19R; BUT AS WE APCHED IAD; APCH CTL ISSUED A CHANGE TO ILS RWY 1L. WHILE ON VECTORS AROUND WX BORDERING THE APCH COURSE TO RWY 1L; WE WERE ISSUED A CHANGE TO THE ILS RWY 1R. I ELECTED TO HAVE THE FO CONTINUE FLYING THE AIRPLANE WHILE I ENTERED THE CHANGES IN THE FMS. HE WAS EXPEDITING THE DSCNT WITH THE USE OF SPD BRAKES WHILE I WAS HEADS-DOWN WITH THE APCH SETUP. I POINTED OUT TO HIM THAT HE HAD SELECTED FULL SPD BRAKES WITH THE AUTOPLT ENGAGED. IN THE A320; ONLY HALF SPD BRAKES ARE AVAILABLE WITH THE AUTOPLT ENGAGED. THE FO THEN DISENGAGED THE AUTOPLT AND TURNED OVER FCU CTL TO ME. WITH THE SPD BRAKE SELECTOR AT FULL; WHEN THE AUTOPLT WAS DISENGAGED THE SPD BRAKES FULLY DEPLOYED. OUR SPD WAS DECREASING SO I CALLED OUT 'CHK SPD.' AS OUR SPD WAS DECAYING BELOW VLS (LOWEST SELECTABLE SPD) I SUGGESTED CONFIGURING TO FLAPS 1 DEG TO LOWER THE VLS. HE CALLED FOR FLAPS 1 DEG AND I CHKED SPD; THEN EXTENDED THE FLAPS. AS EXPECTED; THE VLS DECREASED; BUT OUR SPD CONTINUED TO DECAY. AS OUR SPD STARTED DECAYING INTO THE ALPHA PROT SPD RANGE; AND WITH THE ACFT PITCH APPROX 10 DEGS UP; I COMMANDED THE FO TO PITCH DOWN; REPEATED THE COMMAND; THE COMMANDED HIM TO PITCH DOWN TO THE FLT DIRECTOR NEEDLES. THE ACFT PITCHED UP ANOTHER 1 TO 1 1/2 DEGS; SO I TOOK CTL OF THE ACFT; PITCHED DOWN TO THE FLT DIRECTOR COMMANDED PITCH; AND RETRACTED THE SPD BRAKES. THE SPD RAPIDLY RECOVERED TO NORMAL. I ENGAGED THE AUTOPLT; COMMANDED THE FO TO TAKE CTL; AND I COMPLETED THE APCH SETUP AND BRIEFING. THE REST OF THE FLT WAS UNEVENTFUL. DURING DEBRIEFING; THE FO DEMONSTRATED THAT HE UNDERSTOOD THE SPD BRAKE SYS AND THE NEED TO UTILIZE THE FLT DIRECTOR SYS TO ALLEVIATE WORKLOAD. HE COMMENTED THAT; HE THOUGHT HE INADVERTENTLY SELECTED FULL SPD BRAKES; AND HAD NOT CONSIDERED THAT ADDITIONAL SPD BRAKE DEPLOYMENT WAS TO BE EXPECTED WHEN THE AUTOPLT WAS DISENGAGED. THE RAPID CHANGE IN ASSIGNED APCHS; ALONG WITH DEVIATING AROUND THE TSTMS IN THE AREA CAUSED HIM TO MOMENTARILY LOSE SITUATIONAL AWARENESS AND FAILURE TO FOLLOW THE FLT DIRECTOR COMMANDS. THE DESIGN OF THE A320'S SPD BRAKE SYS CONTRIBUTED TO THIS EVENT. WITH THE EXCEPTION OF THE ACTUAL SPD BRAKE LEVER; THERE IS NO INDICATION AVAILABLE TO THE PLTS OF JUST HOW MUCH THE SPD BRAKES ARE EXTENDED. PARTIAL OR FULL SPD BRAKE SELECTION IS ONLY ANNUNCIATED WITH A 'SPD BRAKE' MESSAGE ON THE EWD (ENG WARNING DISPLAY) IN GREEN TEXT. WHEN THE SPD BRAKES ARE INADVERTENTLY SELECTED TO FULL WITH THE AUTOPLT ENGAGED; THERE IS NO INDICATION THAT THE LEVER IS COMMANDING MORE THAN IS ALLOWABLE. WHEN THE AUTOPLT IS DISENGAGED; THE SPD BRAKES WILL DEPLOY TO FULL WITH NO ADDITIONAL ANNUNCIATION. I SUGGEST THAT THE SYS SHOULD FLASH 'SPD BRAKE' IN ORANGE (AS IT DOES WITH SPD BRAKE DEPLOYED WITH ENGS GREATER THAN IDLE SPD) WHENEVER THE SPD BRAKE IS SELECTED BEYOND HALF WITH THE AUTOPLT ENGAGED. ADDITIONALLY; IF SPD BRAKES ARE SELECTED WITH THE AUTOPLT ENGAGED; THE SYS SHOULD BE INHIBITED TO HALF SPD BRAKES EVEN IF THE AUTOPLT IS DISENGAGED. THERE IS ALREADY LOGIC BUILT INTO THE SYS FOR AUTOMATIC RETRACTION WITH TOGA PWR AND A REQUIREMENT TO RETRACT THE SPD BRAKES TO RESET THE SYS. A SIMILAR SOFTWARE CHANGE COULD REQUIRE SELECTING FULL RETRACTION BEFORE FULL EXTENSION IS ALLOWED AFTER INHIBITION DUE TO FULL EXTENSION SELECTED WITH AUTOPLT ENGAGED. AN APPROPRIATE MESSAGE ON THE EWD WOULD ALSO NEED TO BE ADDED TO THE SYS.