TCASII INCIDENT.

Narrative

ACFT #4 WAS NBOUND AT FL290. 3 OTHER ACFT SBOUND AT FL310 AND ALL 3 HAD TCASII EVENTS WITH ACFT #4 WHEN THEY CAME IN CLOSE PROX. ACFT #2 RESPONDED TO TCASII RA AND DSNDED BUT DID NOT LOSE SEPARATION. ACFT #3 RECEIVED AN RA BUT DID NOT CHANGE ALT. ACFT #1 RESPONDED TO THE RA AND DSNDED TO FL302; SEPARATION WAS LOST BTWN ACFT #1 AND ACFT #4. OUR EQUIP SHOWED NO ABNORMAL ALT INFO FROM ACFT #4. SUPPLEMENTAL INFO FROM ACN 292824: THE ATC CTLR SPECULATED THAT THE C-9 MAY HAVE BEEN THE INTRUDER ACFT ALTHOUGH HIS ASSIGNED ALT OF FL290 WAS CONFIRMED BY THE CREW. ERRONEOUS ENCODED ALT INFO FROM THAT ACFT MAY HAVE BEEN RESPONSIBLE FOR CAUSING THE RA'S ACCORDING TO THE CTLR. IN MY OPINION TCASII PERFORMED ITS FUNCTION AS IT WAS DESIGNED TO DO. HUMAN FACTORS MAY HAVE BEEN INVOLVED IF AN ALTIMETER WAS IMPROPERLY SET. BOTH OF OUR ALTIMETERS WERE VERIFIED SET AT 29.92 AS APPROPRIATE FOR FLT AT HIGH ALT. OFFICIAL TCASII EVENT FORM HAS BEEN FILED. SUPPLEMENTAL INFO FROM ACN 293125: FLYING AT FL310 AUTOPLT ENGAGED; VNAV AND LNAV SELECTED. TCASII SHOWED A TARGET AT FL290; WE WERE HDG S ON J121. AS ACFT GOT WITHIN 10 NM; ANOTHER TARGET APPEARED AT PLUS 100 FT; FOLLOWED BY A TA THEN AN RA. CAPT ASSUMED CTL OF ACFT AND BEGAN A SHALLOW L TURN AND DSNDED TO APPROX FL303 AT WHICH TIME THE RA DISAPPEARED. IN MY OPINION THERE WAS A MALFUNCTION IN ACFT #4'S XPONDER CAUSING FALSE TCASII RETURNS.