MD88 FLT CREW WITH A80 EXPERIENCED TCAS RA WHEN XING ALT CLRNC WAS MISUNDERSTOOD AND AN EARLY DSCNT WAS INITIATED.

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THE CAPT WAS THE PF. THE FLT WAS DSNDING ON THE ERLIN 1 RNAV ARR TO ATL. THE CLRNC WAS; 'CROSS DALAS AT 14000 FT; DSND TO 12000 FT.' AFTER XING DALAS AT 14000 FT AND WHILE DSNDING TO 12000 FT IN THE VICINITY OF STUTZ; THE FLT RECEIVED THE CLRNC; 'CROSS NOFIV AT 210 KTS THEN DSND TO 7000 FT.' THE COPLT READ BACK THE CLRNC BUT THE CAPT DID NOT HEAR THE WORD 'THEN.' THE CAPT UNDERSTOOD THE CLRNC AS; 'CROSS NOFIV AT 210 KTS; DSND TO 7000 FT.' THE COPLT APPARENTLY DID NOT UNDERSTAND THE STIPULATION 'THEN' AND DID NOT REMEMBER READING IT BACK. APPARENTLY ATC EXPECTED THE FLT TO CROSS NOFIV AT 12000 FT; AT 210 KIAS AND 'THEN' DSND TO 7000 FT. THE CAPT UNDERSTOOD THE CLRNC AS AN UNRESTR DSCNT TO 7000 FT WITH A SPD RESTR AT NOFIV. WHILE DSNDING THROUGH APPROX 11000 FT; TCAS ACTIVATED WITH 'TFC; TFC.' THE CAPT ACKNOWLEDGED THE TCAS ALERT; SIGHTED THE ALERT ACFT AND ANNOUNCED 'TFC IN SIGHT.' AT APPROX 10600 FT; TCAS ACTIVATED WITH AN RA AND CALLED FOR A CLB. AT ABOUT THE SAME TIME; ATC INSTRUCTED THE FLT TO CLB TO 11000 FT. THE CAPT DISCONNECTED THE AUTOPLT AND RETURNED THE ACFT TO 11000 FT. SOME DISCUSSION THEN ENSUED BTWN THE CREW AND ATC REGARDING THE CLRNC BEFORE FURTHER CLRNC WAS RECEIVED AND NORMAL OPS CONTINUED. CONTRIBUTING FACTORS: AT NO TIME DID ATC STIPULATE AN ALT XING RESTR AT NOFIV IN ANY OTHER MANNER THAN WITH THE WORD 'THEN.' I BELIEVE THIS IS CRITICAL SINCE THIS IS NOT A PROFILE DSCNT AND ALL ALTS ARE PUBLISHED AS 'EXPECT.' CONTRAST THE CLRNC RECEIVED TO WHAT I CONSIDER A TYPICAL CLRNC FOR THIS SITUATION -- 'CROSS NOFIV AT 12000 FT AND 210 KS; THEN DSND TO 7000 FT.' ATC WAS VERY BUSY AT THE TIME AND THE FREQ WAS NEAR SATURATION. FATIGUE: THE CAPT WAS ON THE LAST LEG OF A 4 DAY TRIP. PICK-UP THAT DAY WAS IN NORFOLK AT XA00 FOR AN XB00 DEP. THIS WAS THE THIRD LEG FLOWN THAT DAY AND IT WAS NOT YET XH00.