ALTDEV ALT OVERSHOT IN DSCNT. ALT XING RESTR NOT MET.

Narrative

EXPERIENCED FLYING BELOW ASSIGNED XING RESTR ALT BY APPROX 1000 FT DURING RELATIVE RAPID DSCNT TO CROSS COZMO INTXN AT 12000 FT AND 250 KTS. DURING THE DSCNT JUST PRIOR TO REACHING COZMO ZJX GAVE US A VECTOR TO EXPECT RWY 17 OR 18R AT MCO. DURING RETURNING RADIOS AND RESPONDING TO CTR'S INSTRUCTION; BOTH MYSELF AND MY FO MISSED THE FACT THAT THE AUTOPLT HAD FAILED TO CAPTURE AT 12000 FT. I NOTED THE DEV AND QUICKLY CLBED BACK UP TO 12000 FT. WE CONTINUED INBOUND AND LANDED WITHOUT ANY PROBS ON RWY 18R. PROB CAUSED BY DIVERTED ATTN DURING CAPTURE PHASE OF A NEWLY ASSIGNED ALT. TO ALLEVIATE THIS IN THE NEAR FUTURE; BOTH PLTS MUST PAY CLOSER ATTN DURING THE DSCNT PHASE OF FLT DUE TO RAPIDLY CHANGING SITS AND ALTS; FREQ CHANGES; HDGS; CONFIGNS; ETC. SUPPLEMENTAL INFO FROM ACN 307469: WE PROGRAMMED THE ALT IN OUR ALERTER SO THE AUTOPLT WOULD LEVEL US AUTOMATICALLY AT 12000 FT. WE STARTED OUR DSCNT; 1000 FT PRIOR THE AUDIBLE TONE WENT OFF AND WE BOTH ACKNOWLEDGED 1000 FT PRIOR. LESSONS LEARNED. 1) WHEN I SELECTED 12000 FT IN OUR ALT ALERTER I MIGHT NOT HAVE ARMED THE ALT. CORRECTIVE ACTION EVERY TIME I SELECT AND ARM THE ALT I WILL CHK TO MAKE SURE ALT ARM IS SELECTED ON MODE ANNUNCIATOR. MAKE SURE THE PNF SETS THE ILS FREQ IN.