SMA PLT DEPARTS SUS WITHOUT CLRNC; BELIEVING TWR WAS CLOSED.

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ON THE EVENING OF AUG/XA/00; MY SAFETY PLT AND I BEGAN OUR RETURN TRIP TO SMD FROM SUS. AFTER THE PREFLT; WE TAXIED TO THE END OF RWY 8R AND COMPLETED THE RUN-UP. WHILE COMPLETING THE RUN-UP; PLT B SET THE RADIOS TO 120.9. WE CALLED DEPARTING RWY 8R; THEN WE TAXIED ONTO THE RWY AND BEGAN OUR DEP. ABOUT 2 MINS AFTER CALLING DEP THE PATTERN TO THE E; WE HEARD ANOTHER ACFT ON THE SAME FREQ TALKING TO DOWNTOWN-PARKS TFC. BY THAT TIME WE WERE NEARING THE AIRSPACE OVER DOWNTOWN-PARKS; AND SINCE WE WERE UNDER THE FALSE ASSUMPTION THAT THE SUS TWR WAS ALREADY CLOSED; WE FAILED TO MAKE A CALL BACK TO SUS TFC. WE CONTINUED OUT OF THE ST LOUIS AREA AND ON TOWARD INDIANAPOLIS TO REFUEL. WHEN WE CONTACTED IND APCH THEY INFORMED US WE WERE TO CALL THE ARPT SUPVR IN ST LOUIS. UPON RETURNING HIS CALL; HE INFORMED US THAT WE HAD DEPARTED AN ARPT WITH AN OPERATING TWR WITHOUT CONTACTING THEM. IT WAS THEN THAT WE REALIZED THAT WE HAD INCORRECTLY ASCERTAINED THAT THE TWR WAS CLOSED BECAUSE WE USED INFO FROM THE DOWNTOWN-PARKS ARPT WHEN READING THE AFD. THEREFORE; WE NOT ONLY THOUGHT THE TWR WAS CLOSED; BUT WE WERE USING THE WRONG CTAF. THERE WERE SEVERAL FACTORS THAT CONTRIBUTED TO OUR FAILURE TO COMMUNICATE WITH THE PROPER CTLING AGENCY. THE FIRST OF WHICH WAS THE LACK OF ATTN THAT WS PAID TO; OR FAILURE IN DOUBLECHKING ALL FREQS THAT WERE TO BE USED INFLT. ANOTHER CONTRIBUTING FACTOR WAS THE LATE HR WHICH MAY HAVE CAUSED US TO BE LESS THAN THOROUGH. THE AREA IN WHICH WE WERE FLYING WAS ALSO UNFAMILIAR TO BOTH OF US. IN RETROSPECT; WE SHOULD HAVE DOUBLECHKED USING MORE THAN 1 SOURCE; IE; AFD; SECTIONAL CHARTS; WHILE ENRTE -- ESPECIALLY NOT BEING FAMILIAR WITH THE AREA.