B737 AT FL260 WITH ZJX EXPERIENCED TCAS RA WITH TFC NOT OBSERVED ON ATC RADAR.

Narrative

CRUISING AT FL260 NOTED TCAS TARGET AT FL265 AT 12:30 O'CLOCK POS CLOSING. THE TCAS SOON CALLED OUT 'TFC' SHORTLY FOLLOWED BY AN RA 'DSND.' ADVISED ZJX; WHO SAID THAT THEY HAD NO ONE NEAR US ON THEIR SCOPE AND THAT OUR DSCNT PATH WAS ALSO CLR. AS WE DSNDED OUR SEPARATION WITH THE TCAS TARGET INCREASED PROPORTIONALLY TO OUR DSCNT. THE RA RESOLVED TO 'CLR OF CONFLICT' AS THE TCAS TARGET PASSED NEARLY OVERHEAD. WE BOTTOMED AT ABOUT FL250 AND RETURNED TO FL260. TCAS TARGET EXITED OUR SCOPE AT 6 O'CLOCK POS STILL INDICATING SEPARATION APPROPRIATE FOR AN ACFT AT FL265. WE WERE IN THICK CLOUDS AND NEVER SAW THE OTHER ACFT. WE ADVISED ZJX OF ALL THE ABOVE AND THEY AGAIN SAID THEY NEVER SAW A RETURN IN OUR VICINITY. SUPPLEMENTAL INFO FROM ACN 634128: THE 'UNKNOWN CONTACT' (WHICH WE NEVER SAW) PASSED APPROX 900 FT ABOVE US AND ABOUT 500-700 FT PAST OUR STARBOARD SIDE (ACCORDING TO THE TCAS DISPLAY). AFTER DSNDING ABOUT 1500 FT; THE ADVISORY WENT AWAY AND WE INFORMED ATC WE WERE CLBING BACK TO FL260. ATC AGAIN SAID HE DIDN'T HAVE ANYONE AROUND US ON RADAR AND CLRED US BACK UP TO FL260.