Narrative:

Our aircraft was an large transport. Our aircraft was on the bitho 5 arrival to orl and had been level at 12000' MSL at 250 KTS for about 10-12 NM. At that time we received a 'traffic, traffic' warning on our TCAS system, followed immediately by a resolution advisory and descend command from our TCAS. I was flying at the time and began a descent of about 1000 FPM, following the directions on the TCAS vsi. The captain advised approach control that we were in a VFR descent due to a TCAS RA. Passing through about 11500' the RA went away, so I leveled off at about 11450' and began climbing back to 12000'. As we leveled off, another air carrier flight passed directly overhead, going the same direction as us, but significantly faster. Approach later told us that the other aircraft was on an arrival to mlb who was descending to 13000' over us. It appeared to me that other aircraft must have been descending at an extremely high rate over us, and our TCAS issued an RA based on its descent. The lowest # I recall seeing on TCAS was a +8 with a red circle. I was very favorably impressed with the entire TCAS operation, and feel sure that had the other aircraft missed its level off altitude, our TCAS would have prevented a midair collision.

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Original NASA ASRS Text

Title: ACR FO DIVES LGT ABOUT 500' UNDER ASSIGNED ALT IN RESPONSE TO A TCAS II ALERT TO AVOID AN OVERTAKING ACFT DESCENDING FROM THE REAR THAT MISSED HIS LEVEL OFF BY 200'.

Narrative: OUR ACFT WAS AN LGT. OUR ACFT WAS ON THE BITHO 5 ARR TO ORL AND HAD BEEN LEVEL AT 12000' MSL AT 250 KTS FOR ABOUT 10-12 NM. AT THAT TIME WE RECEIVED A 'TFC, TFC' WARNING ON OUR TCAS SYS, FOLLOWED IMMEDIATELY BY A RESOLUTION ADVISORY AND DSND COMMAND FROM OUR TCAS. I WAS FLYING AT THE TIME AND BEGAN A DSNT OF ABOUT 1000 FPM, FOLLOWING THE DIRECTIONS ON THE TCAS VSI. THE CAPT ADVISED APCH CTL THAT WE WERE IN A VFR DSNT DUE TO A TCAS RA. PASSING THROUGH ABOUT 11500' THE RA WENT AWAY, SO I LEVELED OFF AT ABOUT 11450' AND BEGAN CLBING BACK TO 12000'. AS WE LEVELED OFF, ANOTHER ACR FLT PASSED DIRECTLY OVERHEAD, GOING THE SAME DIRECTION AS US, BUT SIGNIFICANTLY FASTER. APCH LATER TOLD US THAT THE OTHER ACFT WAS ON AN ARR TO MLB WHO WAS DSNDING TO 13000' OVER US. IT APPEARED TO ME THAT OTHER ACFT MUST HAVE BEEN DSNDING AT AN EXTREMELY HIGH RATE OVER US, AND OUR TCAS ISSUED AN RA BASED ON ITS DSNT. THE LOWEST # I RECALL SEEING ON TCAS WAS A +8 WITH A RED CIRCLE. I WAS VERY FAVORABLY IMPRESSED WITH THE ENTIRE TCAS OPERATION, AND FEEL SURE THAT HAD THE OTHER ACFT MISSED ITS LEVEL OFF ALT, OUR TCAS WOULD HAVE PREVENTED A MIDAIR COLLISION.

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.