Narrative:

Cleared out of 16000 ft to descend on the ege arrival. Approximately 12- 15 NM south of kremmling VOR and at approximately 15000 ft, we got a RA on the TCASII. It said to reduce the rate of descent. I did this by retracting the speed brakes. The TCASII screen did show traffic at 12 O'clock and 1000 ft. As the target airplane passed beneath us, I saw what looked like a twin with it's lights on. The TCASII screen did show one more altitude of 800 ft as the airplane flew beneath and behind us. TCASII definitely saved a near midair collision on this occasion. After discussing this situation with the area manager of ATC and the department of quality assurance in denver center, I am confident that it was not an IFR airplane. It may, in fact, have been a military twin that was operating under VFR flight rules in the area at the same time as our flight. I do not know if the military pilot ever saw us. The visibility in the clouds was marginal VMC at best. A see and avoid maneuver would have been difficult or impossible. Den center radar altitude coverage in the mountain areas is limited. TCASII worked as advertised.

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Original NASA ASRS Text

Title: POTENTIAL CONFLICT AVOIDED AS TCASII RA PROVIDES EVASIVE ACTION LEVEL OFF INFO TO ACR LGT.

Narrative: CLRED OUT OF 16000 FT TO DSND ON THE EGE ARR. APPROX 12- 15 NM S OF KREMMLING VOR AND AT APPROX 15000 FT, WE GOT A RA ON THE TCASII. IT SAID TO REDUCE THE RATE OF DSCNT. I DID THIS BY RETRACTING THE SPD BRAKES. THE TCASII SCREEN DID SHOW TFC AT 12 O'CLOCK AND 1000 FT. AS THE TARGET AIRPLANE PASSED BENEATH US, I SAW WHAT LOOKED LIKE A TWIN WITH IT'S LIGHTS ON. THE TCASII SCREEN DID SHOW ONE MORE ALT OF 800 FT AS THE AIRPLANE FLEW BENEATH AND BEHIND US. TCASII DEFINITELY SAVED A NMAC ON THIS OCCASION. AFTER DISCUSSING THIS SITUATION WITH THE AREA MGR OF ATC AND THE DEPT OF QUALITY ASSURANCE IN DENVER CTR, I AM CONFIDENT THAT IT WAS NOT AN IFR AIRPLANE. IT MAY, IN FACT, HAVE BEEN A MIL TWIN THAT WAS OPERATING UNDER VFR FLT RULES IN THE AREA AT THE SAME TIME AS OUR FLT. I DO NOT KNOW IF THE MIL PLT EVER SAW US. THE VISIBILITY IN THE CLOUDS WAS MARGINAL VMC AT BEST. A SEE AND AVOID MANEUVER WOULD HAVE BEEN DIFFICULT OR IMPOSSIBLE. DEN CTR RADAR ALT COVERAGE IN THE MOUNTAIN AREAS IS LIMITED. TCASII WORKED AS ADVERTISED.

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.