APCH CTLR DSNDED A B727 TO AN ALT THAT HAD NOT BEEN VACATED BY A PRECEDING E110 AND HAD LTSS OCCUR. VECTORS TO KEEP LEGAL SEPARATION WERE INSUFFICIENT TO PREVENT THE SYS ERROR.

Narrative

2 ACFT (AN E110 PAX COMMUTER AND B727 ACR) INBOUND TO BUF FROM OVER DKK BOTH VECTORED TO R DOWNWIND FOR RWY 23. THE E110 WAS GIVEN DSCNT CLRNC OUT OF 11000 FT TO 5000 FT. THE B727 ON CONVERGING COURSE WAS SWITCHED OVER FROM ENRTE CTR ONLY 20 MI FROM ARPT OUT OF 12000 FT. CTLR EVALUATED DSCNT RATES AND ISSUED DSCNT CLRNC FOR THE B727 TO 7000 FT. SINCE THE E110 WAS OVER 3000 FT BELOW AND DSNDING; CTLR ANTICIPATED THAT THE E110 WOULD VACATE 7000 FT BEFORE B727 GOT TO IT. CTLR BECAME DISTRACTED BY OTHER ACFT AND BY LOOKING AT WX FILTERS TO PROVIDE REQUESTED WX INFO. WHEN ATTN WAS REDIRECTED TO DOWNWIND THE CTLR NOTICED THAT THE B727 WAS GOING THROUGH 9000 FT AND THE E110 WAS NOT YET OUT OF 8000 FT. CTLR ISSUED DIVERGING VECTORS TO EACH ACFT AND EXCHANGED TFC. E110 PLT RPTED SIGHTING THE B727. TAILWIND AFFECTED TURN RATE. COMPUTER DATA DUMP REVEALED E110 DSCNT RATE OF 860 FPM; B727 INCREASED DSCNT RATE TO 2400 FPM AFTER RECEIVING DSCNT CLRNC. RPTR (THE CTLR) ACKNOWLEDGES RESPONSIBILITY FOR HUMAN ERROR IN NOT BEING COGNIZANT OF DSCNT RATE DISPARITY; BUT FEELS THAT OTHER FACTORS CONTRIBUTED. 1) ENRTE CTR TRAPPED THE B727 TOO HIGH FOR TOO LONG AND TOO CLOSE TO ARPT OF INTENDED LNDG (ALSO IN VIOLATION OF FACILITY LOA'S). PLT MAY HAVE FELT NEED TO 'CATCH-UP' AND GOT DOWN QUICK. 2) B727 PLT SHOULD HAVE ADVISED CTLR OF PLAN TO DSND AT HIGHER THAN NORMAL RATE.