FO OF A B727-200 OVERSHOT ASSIGNED ALT DURING DSCNT RESULTING IN TCASII RA AND ARTCC TFC ALERT WARNING. THE CAPT NOTICED THE ERROR AND IMMEDIATELY RETURNED THE ACFT TO ASSIGNED ALT.

Narrative

WHEN WE WERE STARTING OUR DSCNT; WHICH WAS CONDUCTED BY THE FO; WE WERE INSTRUCTED BY ZMA (125.7 KHZ) TO DSND AND MAINTAIN 180 DEGS; TURN TO HDG 220 DEGS AND EXPECT RADAR VECTORS FOR THE DSCNT.' HE TOLD US AFTERWARDS THAT DUE TO TFC WE WERE TO FLY DIRECT TO THE ZQA VOR (NASSAU); MAINTAIN IN HOLDING PATTERN ESTABLISHED 090 DEG RADIAL; L TURNS; 20 MI LEGS; DSND AND MAINTAIN 16000 FT; EXPECTING APCH CLRNC AT XA20Z. AS I WAS INTRODUCING THE HOLDING PATTERN INTO THE CDU (GPS UNIVERSAL); THE TCASII SOUNDED 'TFC; TFC;' AND BECAUSE OF THAT I OBSERVED THE IVSI AND SAW AN AMBER CIRCLE AT 500 FT BELOW AND AT APPROX 8 NM AT THE R SIDE OF OUR POS. WHEN I CHKED THE ALTIMETER; I FOUND OUT THAT OUR ACTUAL ALT WAS 14600 FT; DSNDING. THIS WAS BECAUSE THE FO HAD ADJUSTED 6000 FT INSTEAD OF 16000 FT IN THE ALT ALERT. WHEN I REALIZED THAT; I PULLED THE ACFT AND STARTED TO CLB TO 16000 FT. DURING THE MANEUVER; WE WERE CALLED BY THE AIR TFC CTLR ASKING FOR OUR ALT. IN THAT MOMENT WE WERE XING 15200 FT; CLBING TO 16000 FT. HE INSTRUCTED US TO DSND IMMEDIATELY DUE TO TFC. BUT WHEN WE LOOKED OUTSIDE AS WELL AS THE IVSI; I INFORMED THE CTLR THAT WE HAD A TFC IN SIGHT BELOW US. HE ASKED US OUR ALT AND I INFORMED HIM THAT WE WERE REACHING 16000 FT; FLYING TO THE ZQA VOR. HE TOLD US THAT THE CONFLICT ALERT HAD BEEN ACTIVATED AND THAT THE TFC WAS AN MD83 OF ACR Y BELOW OUR POS. HE GAVE US INSTRUCTIONS TO MAINTAIN 16000 FT OVER THE ZQA VOR AFTERWARDS; AND TO CALL NASSAU APCH CTL. THIS INCIDENT HAPPENED AT APPROX XX55Z; 33 DME ZQA VOR RADIAL 355 DEGS; ZMA CTL FREQ 125.7; AT AN APPROX ALT OF 14600 FT.