DC9-30 CREW HAD ADI FAILURE IN STL CLASS B AIRSPACE.

Narrative

AS WE DEPARTED OUT OF STL; THE FO'S ADI FAILED. NO FLAGS -- IT JUST DIDN'T SHOW ANY BANK. I TOOK CTL OF THE ACFT AND WE CONTINUED ON TO OUR DEST (MSP) ABOUT 1 HR 15 MINS AWAY. THE QUESTION IS; 'WHY DIDN'T WE RETURN TO STL?' THAT OPTION WAS NEVER MENTIONED. AN INSTRUCTOR IN THE L SEAT; EXPERIENCED FO IN THE R AND ANOTHER COMPANY PLT IN THE JUMP SEAT; COMMUTING HOME. CONTRIBUTING FACTORS: 1) 4TH LEG OF A LONG DAY. 2) RUNNING 2 HR LATE DUE TO WX AT STL. 3) FULL LOAD OF PAX HAD BEEN WAITING A LONG TIME FOR THE CREW TO ARRIVE. 4) BLOCKED OUT AND RETURNED TO GATE BECAUSE WE WERE OVERWT FOR DEP. 5) RWY CHANGE AS WE STARTED TAXIING THE SECOND TIME CAUSED MORE DELAY. 6) RAIN AND GUSTY XWINDS ON TKOF; TURB ON DEP. 7) DEST WX NOT MUCH BETTER. 8) MAINT AT DEST TO HANDLE PROB. 9) 14 HR DUTY BY THE TIME WE GOT TO DEST. ARRIVED FOR XA00 DEP THE NEXT MORNING; AFTER 11 HR 15 MIN CREW REST. ASSIGNED THE SAME ACFT WITH MALFUNCTION SIGNED OFF. CHANGED GYRO; AND OPS CHKED OK. SUSPECTED WE MIGHT ENCOUNTER THE SAME PROB. NO MENTION OF RETURNING IF PROB RECURRED. BRIEFED TO LOOK FOR PROB EARLY AND ANTICIPATE PROB. ADI FAILED AGAIN AND WE CONTINUED TO HUB. CONTRIBUTING FACTORS: 1) DARK; EARLY MORNING DEPT. HAD SNOWED NIGHT BEFORE. LIMITED ARPT OPS. 2) WX WAS BETTER AT HUB AND DAYLIGHT LNDG. 3) 2 MORE LEGS AHEAD OF US TO COMPLETE A 4 DAY TRIP AND GO HOME. 4) HUB MAINT AVAILABLE TO CORRECT PROB. 5) KNEW WE FLEW ACFT OUT AND WOULD BE ABLE TO FLY IT BACK. IT SHOULD HAVE BEEN OBVIOUS THAT THE PRUDENT OPTION WOULD HAVE BEEN TO RETURN TO BASE ON THE FIRST LEG AND THE SECOND OCCURRENCE WOULDN'T HAVE HAPPENED. ALSO; OBVIOUS THAT HAD THIS OCCURRED IN THE SIMULATOR; WE WOULD HAVE MORE THAN LIKELY RETURNED AND LANDED OVERWT. WHY DIDN'T THIS CARRY OVER TO FLT OPS AND WHY IS IT SO MUCH EASIER TO SEE THE OBVIOUS SITTING HERE DRINKING A COLD ROOT BEER.