POTENTIAL CONFLICT SENSED BY TCASII WITH ARR DEP TFC IN XING TFC SIT. PROX OF APCH RTE.

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DURING A NIGHT VISUAL APCH TO RWY 17L AT DEN ARPT; WE OBSERVED AN AIRPLANE TAKING THE RWY FOR TKOF ON RWY 35L. THIS WAS INDICATED TO US WHEN WE SAW HIS LNDG LIGHTS COME ON AT THE START OF THE TKOF ROLL; AND BY A CALL FROM DEN TWR STATING THAT THE AIRPLANE WAS TAKING OFF TO THE N AND THAT HE WOULD BE MAKING A TURN TO THE NW (AWAY FROM US). RWYS 17L AND 35L ARE STAGGERED A BIT AND SEPARATED BY ABOUT 1/2 MI. SINCE WE KNEW HE WAS TAKING OFF; AND TWR TOLD US HE WOULD TURN NW; WE HAD NO REASON TO FEEL THAT THERE WOULD BE A CONFLICT WITH THIS OPPOSITE DIRECTION TFC. AFTER THE TFC LIFTED OFF; WE HEARD DEN TWR ASK HIM IF HE HAD US IN SIGHT. HE SAID YES; AND TWR TOLD HIM TO MAINTAIN VISUAL SEPARATION AND CLRED HIM FOR A TURN TO THE NE (A TURN DIRECTLY AT US). WE SAW THE AIRPLANE BANK TOWARD US; AND ALL 3 OF US IN THE COCKPIT HAD THE IMMEDIATE IMPRESSION THAT A TFC CONFLICT WOULD RESULT. WE EACH BEGAN CALCULATING WHERE WE COULD GO TO TAKE EVASIVE ACTION. THIS WAS SOMEWHAT DIFFICULT BECAUSE THE AIRPLANE WAS CONFIGURED TO LAND AND HAD LITTLE EXCESS SPD TO MANEUVER. THE BEST CHOICE TO TAKE WAS A DSCNT; NOT A HAPPY THOUGHT AT 2000 FT AGL IN THE BLACKNESS N OF DEN. AS WE WATCHED THE GEOMETRY DEVELOP; WE REALIZED THAT HE WOULD PASS AHEAD (ABOUT 1 MI) AND ABOVE (ABOUT 400 FT) OF US. AT THE SAME TIME; OUR TCASII CALLED OUT AN RA TO 'MONITOR VERT SPD.' (WE WERE ALREADY BEGINNING TO DSND.) WE FEEL THAT IN THE VISUAL ENVIRONMENT; PARTICULARLY AT NIGHT; NO PLT CAN ACCURATELY PREDICT FLT PATH AND CLOSURE WHEN APCHING HEAD-ON. DEN TWR MADE A VERY POOR CHOICE TO TURN THIS TFC IN FRONT OF US; AND IT APPEARS THAT IT WAS DONE WITHOUT REGARD TO ESTABLISHED SEPARATION CRITERIA FOR 2 IFR ACFT. THE ONLY WAY FOR THIS MANEUVER TO BE DONE SAFELY IN THE VISUAL ENVIRONMENT WOULD BE TO WAIT UNTIL WE HAD PASSED ABEAM AND THE TURN ACCOMPLISHED BEHIND US. I CANNOT DESCRIBE IN THIS RPT THE VERY UNCOMFORTABLE FEELING WE HAD WHEN THE ACFT AHEAD BEGAN ITS CLBING TURN INTO US.