Narrative:

On vectors for approach, we were assigned 6000 ft MSL. Approaching level off, approach cleared us to continue to 5000 ft. After acknowledging the 5000 ft clearance, approach instructed us to descend to 4000 ft within 4 NM. The speed brakes were pulled, and the descent rate increased by 1000-1200 FPM. We received a TCASII TA then RA for traffic that we were being descended under. As we were already in the 'expedited' descent, the RA was academic except for the fact that it illustrated a potential conflict. 2 factors come to mind as the first officer and I later reviewed what had transpired. First, although the clearance to 4000 ft was given with an unrealistic restriction (within 4 NM) we did not argue or complain to approach, we simply tried to comply not knowing what affect non compliance might have. Secondly, we really had no reliable means to precisely mark off, 4 NM as we were not heading directly to the VOR,but 'arcing' while on the vector for approach. Approach had not asked if we could make the restriction prior to us accepting it, he probably didn't have time. I ultimately have the final say in accepting or rejecting a clearance, but without knowing if the delay in trying to clarify the clearance would have aggravated the situation, I felt (before and after) that all I could do is try to comply as closely as possible. I'm not sure of the controller's separation criteria, but with the RA issued by TCASII, I'm sure we infringed on it. As best as I could determine we were 'well clear' at all times, but it wasn't pretty.

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Original NASA ASRS Text

Title: MLG ON DSCNT HAS TCASII RA, RESPONDS.

Narrative: ON VECTORS FOR APCH, WE WERE ASSIGNED 6000 FT MSL. APCHING LEVEL OFF, APCH CLRED US TO CONTINUE TO 5000 FT. AFTER ACKNOWLEDGING THE 5000 FT CLRNC, APCH INSTRUCTED US TO DSND TO 4000 FT WITHIN 4 NM. THE SPD BRAKES WERE PULLED, AND THE DSCNT RATE INCREASED BY 1000-1200 FPM. WE RECEIVED A TCASII TA THEN RA FOR TFC THAT WE WERE BEING DSNDED UNDER. AS WE WERE ALREADY IN THE 'EXPEDITED' DSCNT, THE RA WAS ACADEMIC EXCEPT FOR THE FACT THAT IT ILLUSTRATED A POTENTIAL CONFLICT. 2 FACTORS COME TO MIND AS THE FO AND I LATER REVIEWED WHAT HAD TRANSPIRED. FIRST, ALTHOUGH THE CLRNC TO 4000 FT WAS GIVEN WITH AN UNREALISTIC RESTRICTION (WITHIN 4 NM) WE DID NOT ARGUE OR COMPLAIN TO APCH, WE SIMPLY TRIED TO COMPLY NOT KNOWING WHAT AFFECT NON COMPLIANCE MIGHT HAVE. SECONDLY, WE REALLY HAD NO RELIABLE MEANS TO PRECISELY MARK OFF, 4 NM AS WE WERE NOT HDG DIRECTLY TO THE VOR,BUT 'ARCING' WHILE ON THE VECTOR FOR APCH. APCH HAD NOT ASKED IF WE COULD MAKE THE RESTRICTION PRIOR TO US ACCEPTING IT, HE PROBABLY DIDN'T HAVE TIME. I ULTIMATELY HAVE THE FINAL SAY IN ACCEPTING OR REJECTING A CLRNC, BUT WITHOUT KNOWING IF THE DELAY IN TRYING TO CLARIFY THE CLRNC WOULD HAVE AGGRAVATED THE SIT, I FELT (BEFORE AND AFTER) THAT ALL I COULD DO IS TRY TO COMPLY AS CLOSELY AS POSSIBLE. I'M NOT SURE OF THE CTLR'S SEPARATION CRITERIA, BUT WITH THE RA ISSUED BY TCASII, I'M SURE WE INFRINGED ON IT. AS BEST AS I COULD DETERMINE WE WERE 'WELL CLR' AT ALL TIMES, BUT IT WASN'T PRETTY.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.