AN MHTG MANUAL CTLR CLRED AN ACR B757 THROUGH THE ALT OF AN ONCOMING ACR MD11. THE RPTR'S TCASII SHOWED THE INCIPIENT PROB AND ALLOWED THE RPTR TO PREVENT A LOSS OF SEPARATION. OPERROR.

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AT FL350; RIDE WAS CONTINUOUS LIGHT TO MODERATE CHOP. WE WERE VMC. 40 DME S PZA; I REQUESTED FL390 FROM CENAMER. I WAS TOLD TO STAND BY BRIEFLY; THEN WAS CLRED TO FL390. AT FL355; FO NOTED TARGET ON TCASII 40 MI SCALE WHICH POPPED UP SHOWING TO CALL ATC TO INFORM THEM OF THE CONFLICT AND TO THAT WE WERE RETURNING TO FL350. AT 33 DME S OF PZA; ATC RETURNED OUR CALL AND ASKED FOR OUR DME. AT THIS TIME WE GOT VISUAL ON SBOUND TFC; AN MD11 AT FL370. CENAMER ASKED THEM FOR THEIR DME (THIS IS THE FIRST TIME WE HEARD ANOTHER ACFT ON THE FREQ). HE GAVE HIS POS AS 35 DME S (HE HAD JUST PASSED US) AND CENAMER RECLRED US TO FL390. THE FACT THAT FO NOTED THE TFC IMMEDIATELY AS IT POPPED UP ALLOWED US TO KEEP SEPARATION BEFORE A TA OR RA WAS TRIGGERED. TCASII SAVED THE DAY ON THIS OCCASION AS I BELIEVE THAT THIS MOST LIKELY WOULD HAVE RESULTED IN A COLLISION. CLOSURE RATE AND LIGHTING PREVENTED US FROM CONTACTING TFC VISUALLY UNTIL LATERAL SEPARATION HAD CLOSED TO APPROX 10 MI EVEN THOUGH WE KNEW APPROX WHERE TO LOOK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR NOTES THAT THE SPD BRAKE HANDLE ON THE B757 CAN OBSCURE THE TOP HALF OF THE TCASII DISPLAY FROM A NORMAL SITTING POS IN THE CAPT'S SEAT. THE RPTR WAS THE CAPT AND WAS FLYING THE ACFT. THIS IS AT LEAST THE THIRD TIME THAT SOMETHING LIKE THIS HAS HAPPENED TO HIM IN CENTRAL AMERICA; AND PANAMA HAS DONE THIS TO HIM; ALSO. THERE IS NO RADAR IN THIS PART OF CENAMER AIRSPACE. 'THE CTLR'S ENGLISH WAS GOOD; THERE WAS NO PROB THERE.' THE RPTR'S ACR IS WORKING ON THIS PROB AND THE RPTR WILL CALL THE FAA SAFETY HOTLINE.