CONFUSION ON CURRENCY OF AIRWORTHINESS RELEASE. FLC THOUGHT THEIR FLT DATE WAS THE LAST VALID DATE FOR THE AIRWORTHINESS; BUT IT WAS DISCOVERED THAT THEY HAD ACTUALLY FLOWN THE SEQUENCE WITH IT OUTDATED.

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THE FLT WAS THE ORIGINATING FLT FROM TAMPA TO BIRMINGHAM. I HAD WATCHED THE WX CHANNEL BEFORE LEAVING THE HOTEL AND SAW THAT THERE WAS SEVERE WX IN ALABAMA AND GEORGIA ASSOCIATED WITH A LARGE FRONT STRETCHING ACROSS THE SOUTHEASTERN STATES. ARRIVING AT THE ACFT I WAS VERY CONCERNED ABOUT THE SAFETY OF FLT THROUGH THIS WX SYS INTO BIRMINGHAM. PREFLT WAS NORMAL. I CHKED THE LOGBOOK FOR ANY WRITE-UPS AND FOR THE AIRWORTHINESS (A/W) RELEASE. AFTER GOING BACK THROUGH THE FLT LOG I CAME TO AN AIRWORTHINESS RELEASE DATED MAR/XX/96 AND NOTED THAT TODAY WAS THE LAST DAY THE RELEASE WOULD BE VALID. WHAT I FAILED TO NOTICE WAS THAT THIS AIRWORTHINESS HAD A T- CHK CIRCLED AND THAT THE CTLING AIRWORTHINESS RELEASE WAS A COUPLE OF PAGES FURTHER BACK DATED FEB/XX/96. BOTH AIRWORTHINESS RELEASES ARE IDENTICAL WITH THE EXCEPTION THAT THE CTLING AIRWORTHINESS RELEASE WILL HAVE AN A; B; OR C CHK CIRCLED. WE FLEW THE SEQUENCE AS SCHEDULED UP TO ST LOUIS; THROUGH BIRMINGHAM; AND BACK WITH AN INVALID AIRWORTHINESS RELEASE. WE TURNED THE ACFT OVER TO THE PM CREW IN TAMPA AND THE CAPT MENTIONED TO ME THAT THEY HAD TERMINATED THE SAME ACFT THE NIGHT BEFORE AND THAT THIS WAS THE LAST DAY THAT THE AIRWORTHINESS RELEASE WOULD BE VALID. EITHER THE CAPT ON FURTHER CHKING THE FLT LOG OR MAINT CTL DISCOVERED THAT THE AIRWORTHINESS RELEASE WAS OUT OF DATE AND THE ACFT WAS TAKEN OTS. BECAUSE THE TWO AIRWORTHINESS RELEASES ARE IDENTICAL WITH THE EXCEPTION OF WHICH LETTER IS CIRCLED; I FEEL THIS MISTAKE COULD EASILY HAPPEN AGAIN; ESPECIALLY WHEN THERE ARE NUMEROUS OTHER PROBS FACING THE FLT; IE; WX; RTE CHANGES; ETC. I BELIEVE THERE SHOULD BE ONLY ONE TYPE OF AIRWORTHINESS RELEASE AND THAT ANY OTHER TYPE OF MAINT WORK OR T-CHKS BE SIGNED OFF IN ANOTHER MANNER OR PLACE.