MLG DESCENDED THROUGH ALT OF MDT WITH LESS THAN STANDARD SEPARATION.

Narrative

DURING OUR DSCNT FROM A CRUISE ALT OF FL280; ATL CENTER ISSUED US A CLRNC TO 12000'. SHORTLY THEREAFTER; THEY AMENDED OUR CLRNC TO 13000'. WE WERE THEN HANDED OFF TO ZJX WHO ISSUED A DSCNT TO 11000' AND ASKED FOR OUT BEST RATE OF DSCNT. WE SET 11000' INTO THE ALT ALERTER. AS WE DSNDED THROUGH APPROX 16000'; WE WERE ISSUED TFC AT 2 O'CLOCK AND 7 MI AT 12000'. I DO NOT RECALL THE TYPE OF ACFT. I SPOTTED THE TFC AND POINTED IT OUT TO THE COPLT WHO ACKNOWLEDGED THAT HE HAD IT IN SIGHT. THE FLT PATH OF THE ACFT APPEARED TO PARALLEL OUR OWN AND THEREFORE DID NOT APPEAR TO BE A HAZARD. I INFORMED THE COPLT THAT I WAS LEAVING THE FREQ TO PERFORM THE COMPANY-REQUIRED DSCNT PAX ANNOUNCEMENT. I THEN RETURNED TO THE ATC FREQ AND AS WE DSNDED THROUGH APPROX 12300' I SPOTTED AN ACFT AT 12:30 CONVERGING. I TOLD THE COPLT AND WE INCREASED OUR RATE OF DSCNT TO PASS UNDER THE ACFT; WHICH PASSED RIGHT TO LEFT AT APPROX 300-500' ABOVE AND 3/4 MI AHEAD. THE ACFT APPEARED TO BE AN MDT Y. WE QUERIED ZJX AND WERE TOLD WE HAD BEEN ISSUED 13000'. AFTER REVIEWING THE TAPES THE ZJX SUPVR CONFIRMED THAT WE HAD BEEN ISSUED 11000'. BOTH ACFT WERE UNDER ZJX CTL. I BELIEVE; BUT CANNOT CONFIRM; THAT THE MDT Y ACFT WAS THE SAME ACFT I HAD ORIGINALLY SPOTTED AT 2 O'CLOCK AND 7 MI. PRIMARY RESPONSIBILITY FOR THE INCIDENT RESTS WITH ZJX; WHO ISSUED US THE CONFLICTING ALT. FIRST; HE WAS UNAWARE THAT HE HAD ISSUED US 11000'. SECOND; HE FAILED TO ISSUE A WARNING AS WE DSNDED THROUGH 13000'. ALTHOUGH BOTH ACFT WERE UNDER THE CTL OF ZJX CENTER; NO TFC C/A WAS ISSUED. THE INCREASING WORKLOAD IN PREPARATION FOR THE APCH INTO COLUMBIA; SC; WAS A CONTRIBUTING FACTOR. DURING DSCNT OUR CREWS MUST RUN CHKLISTS; TUNE NAV RADIOS; MAKE IN-RANGE CALLS AND MAKE PAX ANNOUNCEMENTS. NOTE ALSO THAT THE MDT Y ACFT NEVER SAW US. HIGH WORKLOAD AND HIGH RATES OF CONVERGENCE MAKE SEE AND AVOID AN UNWORKABLE CONCEPT OF EQUAL IMPORTANCE; BOTH ACFT WERE IFR; AT RELATIVELY HIGH ALT. INCREASING THE VOL OF AIRSPACE IN WHICH ACFT ARE UNDER POSITIVE CTL WOULD HAVE NO EFFECT WHATSOEVER ON THIS TYPE OF ACCIDENT.