AN ACR FLC FLYING A B757 NEAR LGA RPTS A TFC CONFLICT AND TCASII RA STATING THAT TCASII WAS NOT FUNCTIONING PROPERLY.

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DURING THIS INCIDENT THE COCKPIT INDICATIONS HAPPENED VERY RAPIDLY; BUT TO THE BEST OF MY KNOWLEDGE THIS RPT REPRESENTS AN ACCURATE RECONSTRUCTION OF THE EVENTS. DEPARTED ORD ON TIME AND PROCEEDED NORMALLY FROM ORD TOWARD LGA IN VFR WX CONDITIONS. THIS INCIDENT BEGAN AT A POINT APPROX 50 MI W OF LGA IN COM WITH NEW YORK TRACON AND LEVEL AT 10000 FT. THE COCKPIT HAD BEEN PREPARED FOR THE APCH; BRIEFING COMPLETED AND THE APCH DSCNT CHKLIST ACCOMPLISHED. I CHOSE TO FLY THE FIRST LEG OF THE TRIP AND WAS IN CTL OF THE ACFT. THE INCIDENT BEGAN WITH ATC ISSUING A WARNING OF VFR TFC TO OUR L CONVERGING AT 10500 FT. AS THIS IS A MINIMUM AMOUNT OF SEPARATION (ALMOST WITHIN THE 'NEAR MISS' CATEGORY) WE IMMEDIATELY BEGAN TRYING TO FIND THE TFC VISUALLY. THE TCASII IDENTED THE TFC AND INDICATED THE INTRUDING ACFT WAS IN A DSCNT (THE FO NOTED THE DSCNT; I WAS SEARCHING FOR THE TFC VISUALLY). WHILE WE CONTINUED THE EFFORT TO FIND THE TFC VISUALLY; TCASII ISSUED A YELLOW TFC ALERT. THE ALERT SHOWED THE TFC VERY CLOSE AT ABOUT 11 O'CLOCK POS ON THE 10 MI SCALE. ALMOST IMMEDIATELY TCASII ISSUED A PREVENTIVE RA OF 'MONITOR VERT SPD.' THE PITCH GUIDANCE BAR INDICATED THAT WE SHOULD BE IN LEVEL FLT AND I VERBALIZED TO THE FO THAT 'WE WOULD MAINTAIN LEVEL FLT.' AT THIS POINT; WE STILL HAVE NOT IDENTED THE TFC VISUALLY. (THE SKY WAS HAZY AND LOOKING ABOVE US IT WAS NEARLY WHITE). I CONTINUED TRYING TO FIND THE TFC VISUALLY. AT THIS POINT; FO VERBALIZED EMPHATICALLY THAT WE NEEDED TO CLB. SINCE I HAD SAID ONLY A FEW SECONDS EARLIER THAT WE WERE GOING TO REMAIN IN LEVEL FLT; I FELT THAT SHE HAD SEEN THE TFC AND PERHAPS A COLLISION WAS IMMINENT. AS I LOOKED BACK IN THE COCKPIT; I DISCONNECTED THE AUTOPLT AND STARTED A SLOW ASCENT WHILE I CHKED THE TCASII INTRUDER BEARING AND ALT. (AT THIS POINT THE FO ADVISED ATC THAT WE WERE FOLLOWING A TCASII ALERT AND WERE CLBING.) I DID NOTE THAT THE TCASII PITCH BAR SHOWED THAT WE SHOULD CLB. IN THE ABSENCE OF A CLB RA; I CHOSE TO IGNORE THE VISUAL COMMAND. I IMMEDIATELY REALIZED THAT THE TFC WAS STILL ABOVE US; STOPPED THE CLB AND AGGRESSIVELY DSNDED BACK TO 10000 FT. (WE REACHED ABOUT 10200 FT.) THE FO VERBALIZED THAT THE PITCH BAR WAS SHOWING A CLB AND WE NEEDED TO FOLLOW ITS GUIDANCE. (REMEMBER; WE DID NOT HAVE THE INTRUDING TFC VISUALLY AT THIS TIME.) NEEDLESS TO SAY; ATC ADVISED US NOT TO CLB; THAT THE TFC WAS ABOUT TO PASS ABOVE US. A FEW SECONDS LATER; A WHITE PVT ACFT PASSED DIRECTLY ABOVE US AT 10600 FT. THE REST OF THE FLT PROCEEDED NORMALLY EXCEPT DURING TAXI IN; GND PROVIDED US WITH A PHONE NUMBER TO CALL TRACON. THE INCIDENT WAS DISCUSSED WITH THEM TO THEIR SATISFACTION AND A TCASII RPT WAS THE ONLY RPT FILED BY TRACON. WE IMMEDIATELY DISCUSSED THE INCIDENT WITH THE FLEET; THE DUTY FLT MGR; AND MAINT. THE TCASII UNIT WAS DISABLED AND DEFERRED. BOTH PLTS WILL FILE NASA RPTS. THE FO AND I DISCUSSED THE INCIDENT THOROUGHLY ON THE NEXT LEG. ALL OF THE ABOVE HAPPENED IN A MATTER OF A FEW SECONDS; ESPECIALLY PERTAINING TO THE CLB COMMANDS. I BELIEVE THAT THE FO BASED HER CLB VERBALIZATIONS ON THE FACT THAT THE TCASII PITCH BAR SHOWED A CLB EVEN THOUGH WE WERE TO REMAIN IN LEVEL FLT. I DO NOT RECALL HEARING A CLB RA AND NEITHER DOES THE FO. THE FOM DOES HAVE A REF TO REMAINING LEVEL DURING A PREVENTIVE RA AND THIS IS WHAT I BASED MY ACTIONS ON. HOWEVER; IN REREADING THE SECTION; I FIND THAT IT IS RATHER VAGUELY WRITTEN WITH NO DIRECT REF TO IGNORING THE PITCH BARS. I HAVE NO EXPLANATION WHY THE PITCH BARS WOULD INDICATE LEVEL FLT AT THE BEGINNING OF THE INCIDENT AND THEN IN VERY CLOSE PROX TO THE TFC INDICATE A CLB. I FELT IT BEST TO WRITE UP THE TCASII IN THAT THE CLB INDICATION IN THIS SIT DID NOT SEEM NORMAL TO ME. IF THE SYS IS DESIGNED LIKE THIS; IT'S A VERY POOR DESIGN AND IN THE HEAT OF THE BATTLE COULD BE CONFUSED WITH A CLB COMMAND. IT WAS ALSO NOTED THAT PRIOR TO THE INTRUDER PASSING ABOVE US; ALL CLB INDICATIONS DISAPPEARED FROM THE SCREEN; AND ONLY THE TA INDICATION REMAINED WITH THE INTRUDER'SALT. I THINK IT'S CERTAINLY FAIR TO SAY THAT IN THE CONFUSION HAD WE FOLLOWED THE PITCH GUIDANCE; A NEAR MISS OR WORSE COULD HAVE OCCURRED.