CORPORATE JET HAS NMAC WITH A POSSIBLE INTRUDER IN AN UNAUTH PENETRATION OF CLASS B AIRSPACE. TCASII RA GIVES A DSCNT COMMAND BUT ACFT Y; A C152; IS BELOW ACFT X. TFC SIGHTED; VISUAL CONTACT MAINTAINED AND EVASIVE ACTION NORMAL CLB TO 2000 FT ATTEMPTED. SLIGHT OVERSHOOT.

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TKOF -- CLRED RWY HDG AND MAINTAIN 2000 FT. (THIS WAS A REVISED CLRNC FROM THE SID). TCASII DISPLAY ON MFD WITH ACFT ALT READOUTS SELECTED. CLBING AT 1200 FPM; PASSING 1500 FT MSL; TCASII SOUNDED 'TFC; TFC.' (SCREEN WAS CLR AT TKOF.) TARGET DISPLAYED AT 11 O'CLOCK INDICATING 1900 FT MSL. BOTH PLTS SAW THE C150 NOW SLIGHTLY BELOW US HDG IN OPPOSITE DIRECTION. THE TCASII THEN SAID 'DSND; DSND NOW.' I WAS THE CAPT BUT PNF. THE PF SAID 'I SEE IT' AND INSTINCTIVELY INCREASED CLB RATE. THE IVSI WAS COMMANDING A 1500 FPM DSCNT. AS OUR ACFT PASSED 2000 FT MSL THE TCASII THEN VOICED THE TA 'CLB; CLB.' WE CLBED TO 2200 FT MSL. THIS ALL OCCURRED AS MMU TWR ADVISED US TO CONTACT EWR TRACON ON 119.2. I ADVISED TRACON WE MADE A TCASII AVOIDANCE TO 2200 FT AS WE RETURNED TO 2000 FT MSL. TRACON SAID THEY HAD ATTEMPTED TO CONTACT US TO ISSUE A 30 DEG R TURN. THE NORMAL SID REQUIRES A R TURN AT 1500 FT TO A 160 DEG HDG. THE NONSTANDARD CLRNC TO 'MAINTAIN RWY HDG' MAY HAVE CONTRIBUTED TO THIS. I HAVE MADE SEVERAL TCASII AVOIDANCE MANEUVERS IN THE NEW YORK AND DCA AREAS OVER THE LAST FEW YRS. THIS IS THE FIRST 'INCORRECT RA' FOLLOWED BY A 'REVERSAL.' I SPECULATE THE FOLLOWING: THE CESSNA WAS AT 1500 FT BELOW THE CLASS B. THAT PLT THOUGHT HE WAS OUTSIDE THE CLASS D. HE (SHE) WAS NOT! THE SUDDEN APPEARANCE ON TCASII SUGGESTS THE XPONDER WAS OFF AND SUDDENLY TURNED ON. OUR TCASII IS TA ONLY UNTIL 1100 FT AGL (1300 FT MSL AT MMU). THE AUTO SWITCH TO RA WITH THE HIGH CLOSURE RATE INITIALLY CONFUSED THE TCASII. IF WE HAD DSNDED AS COMMANDED...? RECOMMEND INCREASED AWARENESS NEAR MMU. I FREQUENTLY SEE ACFT THAT TCASII DOES NOT. THESE ACFT ARE ALL BELOW CLASS B; USUALLY WITHIN THE 30 MI RING FROM EWR. IT IS MY OPINION THAT MORE PLTS THAN WE REALIZE FAIL TO TURN ON THEIR XPONDER OR FAIL TO SELECT ALT RPTING. I ALSO FLY LIGHT ACFT AND SAILPLANES IN THE NE. I HAVE HEARD 'HANGAR FLYING' STORIES ABOUT FORGETTING TO TURN ON THE XPONDER. GOOD CRM AND 2 HIGHLY EXPERIENCED PLTS (AND WELL TRAINED) IN CONJUNCTION WITH TCASII MADE A NON EVENT. SUPPLEMENTAL INFO FROM ACN 352803: WITH THE TFC IN SIGHT; BELOW AND L; I ELECTED TO LEVEL THE ACFT AT 2000 FT AND DISREGARD THE RA. WHEN RADIO CONTACT WAS ESTABLISHED WITH NEW YORK DEP; WE WERE ADVISED THEY WERE PREPARED TO ISSUE US AN AVOIDANCE VECTOR; BUT DUE TO FREQ CONGESTION WE HAD NOT YET CHKED IN ON FREQ. TRACON ADVISED US THEY WOULD TRACK THE TARGET TO ASCERTAIN IF IT WAS A CLASS B AIRSPACE VIOLATOR. IT IS APPARENT THAT TCASII IS AN INVALUABLE TOOL FOR COLLISION AVOIDANCE; BUT IT IS ONLY ONE TOOL. WE MUST ALSO REMAIN VIGILANT AND BE PREPARED TO TAKE ACTION AS NECESSARY.