ALTDEV ALT UNDERSHOT IN DSCNT DUE TO RESPONSE TO TCASII RA. THIS RA CREATED BY HDG TRACK DEV DURING RADAR VECTORING DURING APCH PROC.

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ON DSCNT INTO ORD WE LEFT FL240 FOR 10000 FT; AIRSPD 300 KTS; HDG 060. CTR TOLD US TO EXPEDITE DSCNT; THEN LATER TOLD US TO SLOW TO 250 KTS. WE DISCOVERED AT THAT POINT A MINOR FLT CTL PROBLEM WITH THE FLT SPOILERS. THEY WOULD DEPLOY; BUT WE HAD TO HOLD THE HANDLE IN THE DEPLOYED POS. NORMALLY; IT STAYS IN THE DEPLOYED POS AUTOMATICALLY UNTIL WE STOW THE SPOILERS. WE WERE HANDED OVER TO APCH AND THEY ACKNOWLEDGED OUR DSCNT TO 1000 FT. NORMALLY; WE ARE ADVISED TO EXPECT A CERTAIN RWY FOR LNDG; BUT NOT TODAY. I WAS FLYING THE ACFT WHEN THE FLT ATTENDANTS INFORMED THE CAPT OF A MAINT ITEM IN THE CABIN AT THE SAME TIME APCH TOLD US TO FLY HDG 100 AND INTERCEPT THE 14 R LOC. IT SEEMS LIKE EVERYTHING HAPPENED AT ONCE NOW. I WAS FLYING THE ACFT HOLDING THE SPOILER LEVER CONSTANTLY WITH 1 HAND; DIALING IN THE LOC FREQ (WE HAD NO PRIOR NOTICE TO EXPECT THIS RWY); THE CAPT WAS COORD THE MAINT ITEM WITH THE FLT ATTENDANT; ACKNOWLEDGING OUR NEW CLRNC; AND; STILL SLOWING TO 250 KTS WITH A SIGNIFICANT TAILWIND; WE HAD A FAST GND SPD. THE AUTOPLT CAPTURED THE LOC; BUT SHOT RIGHT THROUGH IT. APCH TOLD US TO TURN TO 150 TO RE-INTERCEPT THE LOC; ABOUT THAT TIME; WE NOTICED TFC ON THE TCASII APPROX 4-5 MI AT 10 O'CLOCK AND 500 FT BELOW. WE LEVELED OFF AT APPROX 11000 FT AND THE TCASII ISSUED A TA. THE TFC PASSED OFF OUR 9 O'CLOCK AT ABOUT 3 1/2 TO 4 MI; 300 FT BELOW. WHEN THE ALERT WAS OVER; WE CONTINUED OUR DSCNT TO 1000 FT. APCH THEN TOLD US TO FLY 160 TO INTERCEPT THE LOC; THEN FINALLY 170 TO INTERCEPT THE LOC. THE REMAINDER OF THE FLT WAS WITHOUT INCIDENT. I LEARNED SEVERAL THINGS FROM THIS INCIDENT. AFTER 2 1/2 HRS OF NON-EVENTFUL CRUISE; A NORMAL DSCNT BECAME AN EXPEDITED DSCNT AND THE WORKLOAD ON THE PLTS BECAME EXTREMELY HIGH DURING THE SECONDS PRIOR TO JOINING THE LOC. NORMALLY; YOU EXPECT CRUNCH POINTS DURING A FLT AND ARE READY FOR THEM. BUT THROW IN 2 OR 3 EXTRA; UNEXPECTED THINGS DURING THOSE CRUNCH POINTS AND THE CHANCE FOR DISTR BECOME GEOMETRICALLY GREATER. IT'S AT THESE TIMES; WHEN YOUR ATTN IS DRAWN IN SEVERAL DIRECTIONS AT ONCE THAT YOU MUST FIRST OF ALL; FLY THE AIRPLANE. DO ONLY 1 THING AT A TIME BEFORE YOU MOVE ON TO THE NEXT. I NEVER REALIZED HOW IMPORTANT THAT ADVISORY CALL FROM CTR TO EXPECT A CERTAIN RWY IS. THE NEXT TIME; IF IT IS NOT OFFERED; I WILL ASK. SUPPLEMENTAL INFO FROM ACN 210471: DURING THE DSCNT; THE FLT ATTENDANTS INFORMED ME THAT THE FORWARD GALLEY CARPET WAS WET. AS THE PNF; I WAS HANDLING COMS; IDENTING NAVAIDS; READING CHKLISTS; GIVING PAX PA'S AND ATTEMPTING TO UTILIZE OUR AIRLINE'S NEW AUTOMATIC MALFUNCTION RPTING SYS THROUGH ACARS. I BELIEVE THAT THIS INCIDENT WAS CAUSED BY THE EXTREMELY HIGH WORKLOAD ON THE PLTS AT THE TIME. I INTEND TO INFORM THE COMPANY OF THIS INCIDENT AND RECOMMEND THAT THEY REMIND OUR OTHER PLTS OF THE NEED TO REMAIN EXTREMELY VIGILANT OF THE ACFT FLT PATH DURING THESE CRITICAL PHASES OF FLT AND HIGH WORK LOADS.