Narrative:

We received a TCASII traffic alert followed by an RA. At the traffic alert I looked out and saw an air carrier Y aircraft, I believe in the captain's front side window. We were approximately 60 NM northeast of ormond beach (omn 040 degree radial at 60 DME). At the traffic alert, the TCASII showed the aircraft 800 ft above us. TCASII quickly delivered an RA to 'descend.' just prior and at the start of the RA, TCASII showed air carrier Y at 700 ft above us. After the RA, I concentrated all of my scan on the air carrier Y aircraft. Captain initiated a descent after disconnecting the autoplt and autothrottles. I immediately alerted ZJX that 'air carrier X is descending in response to an RA!' I believe the center responded with 'who's descending?' I responded again with 'air carrier X is descending in response to an RA.' captain then follows TCASII's call to 'adjust vertical speed' followed by 'clear of conflict' and climbs back to FL310. ZJX asks air carrier Y (somewhere in the middle of all this) 'air carrier Y say your altitude!' and air carrier Y responds 'air carrier Y is climbing from our descent in response to an RA.' air carrier Y is later given clearance to continue their descent to FL280. I would guess that our nearest point of approach is 500-700 ft vertically and inside 1 NM horizontally.

Related reports:

Google
 

Original NASA ASRS Text

Title: DURING CRUISE AT FL310, AN MD80 RECEIVES A TCASII ALERT AND SPOT A DSNDING ACR XING L TO R 1 MI AHEAD.

Narrative: WE RECEIVED A TCASII TFC ALERT FOLLOWED BY AN RA. AT THE TFC ALERT I LOOKED OUT AND SAW AN ACR Y ACFT, I BELIEVE IN THE CAPT'S FRONT SIDE WINDOW. WE WERE APPROX 60 NM NE OF ORMOND BEACH (OMN 040 DEG RADIAL AT 60 DME). AT THE TFC ALERT, THE TCASII SHOWED THE ACFT 800 FT ABOVE US. TCASII QUICKLY DELIVERED AN RA TO 'DSND.' JUST PRIOR AND AT THE START OF THE RA, TCASII SHOWED ACR Y AT 700 FT ABOVE US. AFTER THE RA, I CONCENTRATED ALL OF MY SCAN ON THE ACR Y ACFT. CAPT INITIATED A DSCNT AFTER DISCONNECTING THE AUTOPLT AND AUTOTHROTTLES. I IMMEDIATELY ALERTED ZJX THAT 'ACR X IS DSNDING IN RESPONSE TO AN RA!' I BELIEVE THE CTR RESPONDED WITH 'WHO'S DSNDING?' I RESPONDED AGAIN WITH 'ACR X IS DSNDING IN RESPONSE TO AN RA.' CAPT THEN FOLLOWS TCASII'S CALL TO 'ADJUST VERT SPD' FOLLOWED BY 'CLR OF CONFLICT' AND CLBS BACK TO FL310. ZJX ASKS ACR Y (SOMEWHERE IN THE MIDDLE OF ALL THIS) 'ACR Y SAY YOUR ALT!' AND ACR Y RESPONDS 'ACR Y IS CLBING FROM OUR DSCNT IN RESPONSE TO AN RA.' ACR Y IS LATER GIVEN CLRNC TO CONTINUE THEIR DSCNT TO FL280. I WOULD GUESS THAT OUR NEAREST POINT OF APCH IS 500-700 FT VERTLY AND INSIDE 1 NM HORIZLY.

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.