A B727-200 CARGO FLT DSNDS OUT OF ITS ASSIGNED ALT TOWARDS OPPOSITE DIRECTION TFC WHEN THE PNF BELIEVES HE HAS HEARD A CLRNC TO LOWER ALT S OF JLI; CA.

Narrative

NEARING THE END OF A 3 HR CARGO FLT FROM GUADALAJARA; MEXICO (GDL) TO LOS ANGELES; CA (LAX); WE WERE HANDED OFF FROM MAZATLAN CTR TO ZLA. AT THIS TIME THE RADIO TFC VOLUME INCREASED DRAMATICALLY ALONG WITH A FAIR AMOUNT OF STATIC. AFTER OUR INITIAL CONTACT WITH CTR; WE WERE GIVEN A NEW STAR THAN THE ONE CONTAINED IN OUR ORIGINAL CLRNC. THE CAPT WAS HANDLING THE RADIOS AND HAD TO ASK FOR A READBACK OF THE NEW STAR AND CONFIRMATION OF A DSCNT FROM FL350 TO FL310. HE ACKNOWLEDGED THE CHANGE IN STAR FROM THE JULIAN ARR TO THE PDZ ARR AND ALSO ACKNOWLEDGED DSCNT TO FL310. WE BEGAN OUR DSCNT WHICH WE WERE ALREADY EXPECTING DUE TO THE UPCOMING XING RESTR OF THE JULIAN VOR. APCHING FL330; THE CTLR ASKED OUR ALT; AT WHICH TIME WE REPLIED FL330 FOR FL310. HE TOLD US WE WERE ASSIGNED FL350 AND TO CLB IMMEDIATELY BACK TO THAT ALT. DURING THE CTLR'S INSTRUCTIONS TO CLB BACK TO FL350; THE TCASII ALERTED US TO TFC JUST AHEAD OF US; AT FL310. THE AUTOPLT WAS DISCONNECTED AND AN IMMEDIATE CLB WAS INITIATED BACK TO FL350. WE WERE THEN GIVEN A VECTOR TO THE L AND A FEW MINS LATER; A DSCNT FOLLOWED BY A CLRNC DIRECT TO THE VOR AND RESUMPTION OF THE STAR. AT THIS TIME; I CANNOT REMEMBER HEARING THE CTLR ISSUE THE INITIAL DSCNT TO FL310. I ASKED THE CAPT TO CONFIRM THE DSCNT TO FL310; WHICH HE DID; AND I ALSO HEARD HIM READ BACK THE NEW STAR AND DSCNT TO FL310 TO THE CTLR. ALTHOUGH IT DOESN'T GET THE CREDIT FOR CATCHING THE MISTAKE FIRST IN THIS INSTANCE; I BELIEVE THE INSTALLATION OF TCASII IN THIS PARTICULAR AIRPLANE (EVEN THOUGH NOT REQUIRED IN CARGO ACFT) WOULD HAVE PREVENTED AN NMAC HAD THE CTLR NOT INTERCEDED WHEN HE DID.