LGT HAS TFC CALLED AS VFR IN IMC. TCASII INDICATES PROX AND EVASIVE ACTION.

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I WAS THE COPLT ON A REGULARLY SCHEDULED PAX FLT FROM EWR TO MSP USING LGT EQUIP. WE WERE DELAYED IN OUR DEP FROM EWR ON NOV/XX/91 DUE TO A MECHANICAL PROBLEM AND PUSHED BACK AND WE TOOK OFF. CONDITIONS AT THE TIME WERE M16 OVCST; 3RF; 63 DEGS/63 DEGS; WIND 100/3. WE FLEW THE DEP AND WERE IMC SHORTLY AFTER TKOF. WE REMAINED IMC CONTINUOUSLY TO FL350. DEP CTL CLRED US TO 10000 AND TURNED US NW. EVENTUALLY WE WERE CLRED DIRECT SPARTA VOR (SAX) AND TO 11000 AND THEN 13000 FT. I ELECTED TO ENGAGE THE AUTOPLT AT ABOUT 10000 FT AND TOOK OVER THE MODE CTL PANEL FROM THE CAPT (PNF) TO DECREASE BOTH OF OUR WORKLOADS IN THE TURBULENT IMC CONDITIONS. AFTER CLRING US TO 13000 THE CTLR CALLED TFC AT 11 O'CLOCK AND 5 MI (I BELIEVE IT WAS 5 MI) VFR; AN SMT AT 13500. AS WE APCHED 13000 A TARGET APPEARED ON THE TCASII AS A BLUE DIAMOND OUTSIDE THE TCASII INNER CIRCLE. THE TARGET ADVANCED AND CHANGED TO YELLOW. THE FIRST VERT SEPARATION I RECALL SHOWED A +300 FT. DEP WAS IN CONTACT WITH THIS ACFT AND CALLED US AS TFC TO HIM. DEP INFORMED US THAT THE SMT PLT HAD US IN SIGHT. IF SO; I'M HIGHLY IMPRESSED SINCE WE WERE STILL SOLID IMC; AS WE RPTED TO THE CTLR. THE CTLR TOLD US THAT THE SMT PLT CLAIMED HE WAS 'BTWN LAYERS.' THE TCASII SOON SHOWED THE SMT AS CLBING; STARTING WITH A CLB TO +300. AT THIS TIME WE WERE ABOUT TO LEVEL AT 13000. AS THE TARGET ENTERED THE TCASII INNER CIRCLE IT TURNED RED; CONTINUED TO SHOW A CLBING VECTOR AND CONTINUED TO CONVERGE. THE TCASII ANNOUNCED 'MONITOR VERT SPD' AND SHOWED RED OVER THE ENTIRE CLB RANGE OF THE VERT SPD INDICATOR. WE INFORMED THE CTLR WE WERE TURNING 20 DEGS R. I COMMANDED THE AUTOPLT TO TURN BUT IMMEDIATELY DISCONNECTED AND TOOK OVER MANUAL CTL RATHER THAN WAIT. I ALSO INITIATED A DSCNT AS I DID NOT BELIEVE OUR SEPARATION WAS ADEQUATE FOR COMFORT. WHEN THE TARGET MERGED WITH THE AIRPLANE SYMBOL ON THE TCASII; I RECALL IT SHOWING A CLB ARROW AND +600. THE CAPT RECALLS SEEING UP TO 700 FT SEPARATION. THE CTLR RPTED TO US LATER THAT HIS EQUIP ALSO SHOWED 600 FT SEPARATION. WE NEVER SAW THE SMT. IT SEEMS CLR TO ME THAT THE SMT WAS NOT VFR; WAS NOT BTWN LAYERS (AND EVEN IF HE WAS; MOST CERTAINLY DID NOT HAVE LEGAL VISIBILITY OR CLOUD CLRNC); AND MOST PROBABLY WAS NOT ACTUALLY AT 13500 BUT WAS CLOSER TO 13000 FT; JUDGING BY THE TCASII DATA AND THE SMT INITIATION OF A CLB. BY NOT FLYING LEGAL VFR OR FILING IFR AND BY MOST LIKELY LYING ABOUT HIS TRUE ALT AND FLT CONDITIONS; THE PLT OF SMT PUT HIMSELF AND THE 51 SOULS ON OUR ACFT AT UNNECESSARY RISK. USING THE LGT AUTOFLT AND FMC ALLOWED US EXTRA TIME TO ANALYZE THE SITUATION DURING A TIME OF HVY WORKLOAD. THE TCASII WORKED AS ADVERTISED; AND SINCE IT MADE THE SERIOUSNESS OF THE SITUATION CLR WHEN NO VISUAL CONTACT WAS POSSIBLE IT MAY HAVE PREVENTED A MIDAIR BY CALLING THE NEED FOR EVASIVE ACTION TO MY ATTN.