Narrative:

Air carrier X dc-9 at 12000 ft. Air carrier Y BA31 at 11000 ft. Unknown VFR at 12400 ft. X was overtaking Y, both vectored on a 050 degree heading. An unknown VFR aircraft was southwest bound indicating 12400 ft. I turned Y 15 degree to the left to get spacing for the final controller. 14 seconds later, X advised he was descending in response to a RA, to the traffic at 12400 ft. The FAA's instruction to controllers is to acknowledge the deviation but not to contradict it. I rogered the announced descent and advised X that Y was also below him at 11000 ft. I assumed he would only descend 200 ft but according to the radar display he went down to 11000 ft then climbed back. The resulting initial readout indicated separation of 1.2 mi laterally and 0 ft vertically. Had I not turned Y earlier, the separation would have been even less. This is the situation that all controllers have wondered when it would happen. An uncontrolled VFR at a legal altitude give an IFR aircraft a descend (or climb) resolution into another IFR aircraft. TCASII works with one situation only at a time (kind of like tunnel vision), ignoring other aircraft. A majority of the controllers at my facility feel that TCASII should only be used in an advisory mode in the terminal environment.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X TCASII RA DSND WITH VFR ACFT HAD LTSS FROM ACR Y. PLTDEV. EVASIVE ACTION TAKEN.

Narrative: ACR X DC-9 AT 12000 FT. ACR Y BA31 AT 11000 FT. UNKNOWN VFR AT 12400 FT. X WAS OVERTAKING Y, BOTH VECTORED ON A 050 DEG HDG. AN UNKNOWN VFR ACFT WAS SW BOUND INDICATING 12400 FT. I TURNED Y 15 DEG TO THE L TO GET SPACING FOR THE FINAL CTLR. 14 SECONDS LATER, X ADVISED HE WAS DSNDING IN RESPONSE TO A RA, TO THE TFC AT 12400 FT. THE FAA'S INSTRUCTION TO CTLRS IS TO ACKNOWLEDGE THE DEV BUT NOT TO CONTRADICT IT. I ROGERED THE ANNOUNCED DSCNT AND ADVISED X THAT Y WAS ALSO BELOW HIM AT 11000 FT. I ASSUMED HE WOULD ONLY DSND 200 FT BUT ACCORDING TO THE RADAR DISPLAY HE WENT DOWN TO 11000 FT THEN CLBED BACK. THE RESULTING INITIAL READOUT INDICATED SEPARATION OF 1.2 MI LATERALLY AND 0 FT VERTLY. HAD I NOT TURNED Y EARLIER, THE SEPARATION WOULD HAVE BEEN EVEN LESS. THIS IS THE SIT THAT ALL CTLRS HAVE WONDERED WHEN IT WOULD HAPPEN. AN UNCTLED VFR AT A LEGAL ALT GIVE AN IFR ACFT A DSND (OR CLB) RESOLUTION INTO ANOTHER IFR ACFT. TCASII WORKS WITH ONE SIT ONLY AT A TIME (KIND OF LIKE TUNNEL VISION), IGNORING OTHER ACFT. A MAJORITY OF THE CTLRS AT MY FACILITY FEEL THAT TCASII SHOULD ONLY BE USED IN AN ADVISORY MODE IN THE TERMINAL ENVIRONMENT.

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.