Narrative:

20 mi east of torch on el nido arrival 12000 ft. Received VFR TA 1 O'clock position at 11500 ft. Shortly thereafter received a TA with target at 200 ft above us, 1 O'clock position and closing. ZOA then cleared us to descend to 11000 ft. As the descent was started the TA changed to an RA with a descend command. Autoplt was disengaged and descend command followed at 1300 FPM rate. Passing 11700 ft we finally spotted the traffic at 11500 ft and closing rapidly at which time I increased descent rate beyond commanded rate to avoid the traffic. Traffic passed behind us with between 1/4 to 1/2 mi horizontal separation. ZOA was advised of the RA. They then advised the traffic to check altitude, mode C showed 12200 ft. This whole chain of events took less than 45 seconds. We were in visual conditions. If we were IMC or did not visually spot the traffic, I think that this event would have ended much less favorably. I suspect this event was caused by the erroneous mode C of the VFR traffic that told our TCASII it was 200 ft above us when it was really 500 ft below us. This led to a descend command when we needed to maintain altitude. This event further reinforces the see and avoid concept in visual conditions and the need to aggressively maintain situational awareness.

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Original NASA ASRS Text

Title: CAPT OF A B757 DISCONNECTED THE AUTOPLT AND INCREASED THE RATE OF DSCNT IN RESPONSE TO A TCASII RA CAUSING A POTENTIAL COLLISION WITH THE OTHER ACFT BY DECREASING SEPARATION INSTEAD OF INCREASING IT. THE OTHER ACFT'S MODE C XPONDER WAS NOT RPTING THE SAME ALT AS THE PLT HAD GIVEN TO ATC.

Narrative: 20 MI E OF TORCH ON EL NIDO ARR 12000 FT. RECEIVED VFR TA 1 O'CLOCK POS AT 11500 FT. SHORTLY THEREAFTER RECEIVED A TA WITH TARGET AT 200 FT ABOVE US, 1 O'CLOCK POS AND CLOSING. ZOA THEN CLRED US TO DSND TO 11000 FT. AS THE DSCNT WAS STARTED THE TA CHANGED TO AN RA WITH A DSND COMMAND. AUTOPLT WAS DISENGAGED AND DSND COMMAND FOLLOWED AT 1300 FPM RATE. PASSING 11700 FT WE FINALLY SPOTTED THE TFC AT 11500 FT AND CLOSING RAPIDLY AT WHICH TIME I INCREASED DSCNT RATE BEYOND COMMANDED RATE TO AVOID THE TFC. TFC PASSED BEHIND US WITH BTWN 1/4 TO 1/2 MI HORIZ SEPARATION. ZOA WAS ADVISED OF THE RA. THEY THEN ADVISED THE TFC TO CHK ALT, MODE C SHOWED 12200 FT. THIS WHOLE CHAIN OF EVENTS TOOK LESS THAN 45 SECONDS. WE WERE IN VISUAL CONDITIONS. IF WE WERE IMC OR DID NOT VISUALLY SPOT THE TFC, I THINK THAT THIS EVENT WOULD HAVE ENDED MUCH LESS FAVORABLY. I SUSPECT THIS EVENT WAS CAUSED BY THE ERRONEOUS MODE C OF THE VFR TFC THAT TOLD OUR TCASII IT WAS 200 FT ABOVE US WHEN IT WAS REALLY 500 FT BELOW US. THIS LED TO A DSND COMMAND WHEN WE NEEDED TO MAINTAIN ALT. THIS EVENT FURTHER REINFORCES THE SEE AND AVOID CONCEPT IN VISUAL CONDITIONS AND THE NEED TO AGGRESSIVELY MAINTAIN SITUATIONAL AWARENESS.

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.