Narrative:

Filed over maxim FL370 proceeding to key west for duval arrival. Captain's leg, first officer on the radio. ATC frequency 132.2 very busy -- controller being stepped on and telling everyone trying to check in to stand by. Previously assigned transponder code XXXX by havana for mia. About 30-40 mi, 3-4 mins past maxim (fir) break in radio traffic allowed us to check in at FL370. Controller said we were assigned FL390. We asked if he wanted us to climb to FL390, and he then gave us an immediate descent for traffic conflict less than 1 mi, 12 O'clock position, FL370. All of these xmissions included our correct call sign from both we and the controller. This was not a readback/hearback type error. We acknowledged and initiated an immediate descent (approximating a TCASII type manual escape), then queried the controller for what altitude I was to descend to. I did not receive an immediate reply and asked again what altitude he needed us at. This whole time I am not convinced this maneuver was necessary because there are no targets on my TCASII at any altitude within about 20 mi (TCASII was set to below for arrival), but the controller was emphatic and sounded very concerned in his initial transmission. His next transmission was asking again our current altitude, so I immediately leveled off at approximately FL364, suspecting that he was not looking at me and now concerned that he had given me a descent while looking at another aircraft. Another pause and controller asked us to identify, followed by controller comment that he did not previously 'have' us, also that havana had given us a transponder code identical to another aircraft he was controling he said, and that the ATC computer had not picked up the problem. He then gave us a continued descent for a normal arrival into mia. I do not believe, and there was no report, that any traffic conflict or loss of separation ever actually occurred with my aircraft or the aircraft with the same transponder code. I suspect that the controller was looking at the other aircraft that had the same transponder code, and was actually level at FL390, confusing the controller. I confirmed this with a call to the ZMA supervisor on duty. There are no entries in the ARTCC incident logbook concerning this event. The ARTCC manager said he would check into it.

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Original NASA ASRS Text

Title: B737 FLC TAKE EVASIVE MANEUVER DUE TO ZMA CTLR CHALLENGE AS TO WHERE THEY ACTUALLY ARE IN REF TO ANOTHER ACFT WITH THE SAME XPONDER CODE BELIEVED TO BE IN THE SAME PROX.

Narrative: FILED OVER MAXIM FL370 PROCEEDING TO KEY WEST FOR DUVAL ARR. CAPT'S LEG, FO ON THE RADIO. ATC FREQ 132.2 VERY BUSY -- CTLR BEING STEPPED ON AND TELLING EVERYONE TRYING TO CHK IN TO STAND BY. PREVIOUSLY ASSIGNED XPONDER CODE XXXX BY HAVANA FOR MIA. ABOUT 30-40 MI, 3-4 MINS PAST MAXIM (FIR) BREAK IN RADIO TFC ALLOWED US TO CHK IN AT FL370. CTLR SAID WE WERE ASSIGNED FL390. WE ASKED IF HE WANTED US TO CLB TO FL390, AND HE THEN GAVE US AN IMMEDIATE DSCNT FOR TFC CONFLICT LESS THAN 1 MI, 12 O'CLOCK POS, FL370. ALL OF THESE XMISSIONS INCLUDED OUR CORRECT CALL SIGN FROM BOTH WE AND THE CTLR. THIS WAS NOT A READBACK/HEARBACK TYPE ERROR. WE ACKNOWLEDGED AND INITIATED AN IMMEDIATE DSCNT (APPROXIMATING A TCASII TYPE MANUAL ESCAPE), THEN QUERIED THE CTLR FOR WHAT ALT I WAS TO DSND TO. I DID NOT RECEIVE AN IMMEDIATE REPLY AND ASKED AGAIN WHAT ALT HE NEEDED US AT. THIS WHOLE TIME I AM NOT CONVINCED THIS MANEUVER WAS NECESSARY BECAUSE THERE ARE NO TARGETS ON MY TCASII AT ANY ALT WITHIN ABOUT 20 MI (TCASII WAS SET TO BELOW FOR ARR), BUT THE CTLR WAS EMPHATIC AND SOUNDED VERY CONCERNED IN HIS INITIAL XMISSION. HIS NEXT XMISSION WAS ASKING AGAIN OUR CURRENT ALT, SO I IMMEDIATELY LEVELED OFF AT APPROX FL364, SUSPECTING THAT HE WAS NOT LOOKING AT ME AND NOW CONCERNED THAT HE HAD GIVEN ME A DSCNT WHILE LOOKING AT ANOTHER ACFT. ANOTHER PAUSE AND CTLR ASKED US TO IDENT, FOLLOWED BY CTLR COMMENT THAT HE DID NOT PREVIOUSLY 'HAVE' US, ALSO THAT HAVANA HAD GIVEN US A XPONDER CODE IDENTICAL TO ANOTHER ACFT HE WAS CTLING HE SAID, AND THAT THE ATC COMPUTER HAD NOT PICKED UP THE PROB. HE THEN GAVE US A CONTINUED DSCNT FOR A NORMAL ARR INTO MIA. I DO NOT BELIEVE, AND THERE WAS NO RPT, THAT ANY TFC CONFLICT OR LOSS OF SEPARATION EVER ACTUALLY OCCURRED WITH MY ACFT OR THE ACFT WITH THE SAME XPONDER CODE. I SUSPECT THAT THE CTLR WAS LOOKING AT THE OTHER ACFT THAT HAD THE SAME XPONDER CODE, AND WAS ACTUALLY LEVEL AT FL390, CONFUSING THE CTLR. I CONFIRMED THIS WITH A CALL TO THE ZMA SUPVR ON DUTY. THERE ARE NO ENTRIES IN THE ARTCC INCIDENT LOGBOOK CONCERNING THIS EVENT. THE ARTCC MGR SAID HE WOULD CHK INTO IT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.