NMAC IN PROX OF APCH RTE.

Narrative

IN THE AREA OF HELON INTXN WE RECEIVED CLRNC TO DSND FROM 18000-13000 FT. ON PASSING THROUGH ABOUT 17000 FT CAPT AND FO NOTICED TFC ON TCASII/RADAR SCREEN AT 12 O'CLOCK ABOUT 600 FT BELOW. CAPT SUGGESTED AND FO SHALLOWED DSCNT. JUST WHEN TFC APPEARED TO BE PAST OR PASSING UNDER OUR ACFT (ON TCASII SCREEN) RA COMMANDED A CLB. FO EXECUTED A CLB OF APPROX 2000-2500 FPM (WHATEVER IVSI CALLED FOR) CTLR WARNED OF TFC AT 16500 FT AND I BELIEVE COMMANDED CLB AS WE LEVELED AT 17500 FT (AFTER RA STOPPED). PROB PROBABLY BROUGHT ON BY PEAK TFC AND POSSIBLE LOW EXPERIENCE ON PART OF CTLR. THERE ALSO SEEMS TO BE A LACK OF CONSISTENT CORRIDORS AND DSCNT ALTS; IE; WE WERE GIVEN SYR AT 27000 FT (CRUISE ALT 41000 FT) THEN GIVEN SAX 25 DME AT 8000 FT FROM A DISTANCE AND ALT THAT WE COULD NOT MEET THE RESTR FROM. TOO LOW AT SYR IS BAD FOR OUR FUEL BURN. TOO LATE FOR THE XING RESTR IS BAD PLANNING AND MESSES UP BOTH PLTS AND CTLRS. IN THIS CASE THE XING RESTR WAS MADE IMPOSSIBLE BY THE RA BUT IT IS USUALLY TIGHT DUE TO LATE DSCNT CLRNC.