Narrative:

We were on the olympia 1 arrival, approximately 16 mi out on the olm 022 degree radial, descending to 10000 ft. At 10500 ft our TCASII gave a TA, with a yellow target, of a supposed target at our 10 O'clock, 5 mi, 200 ft above our altitude. I looked for the traffic (captain was flying) but saw nothing, and then looked back at the TCASII display. The target had disappeared from the display. Approximately 1 min later, 20 mi from olm, the captain was transitioning the aircraft for a level off at 10000 ft, when we got a TCASII RA 'descend, descend.' I looked at the TCASII, saw a red target right over us, and immediately looked for traffic. The captain looked up also, but neither of us saw any traffic. Since we had no real proof that this was a false alarm, the captain initiated a shallow descent and I told approach control that we had a TCASII alert and were descending. Controller said he was not showing any traffic. We descended to approximately 9700 ft when the target disappeared. I told the controller we were clear of the possible conflict and asked if he wanted us to return to 10000 ft. He gave us descent to 7000 ft and the remainder of the flight was uneventful, considering our elevated blood pressures. During discussion, it was discovered that the captain never saw any command arc segments on his vsi. My vsi had displayed a red arc from 0 to 1500 FPM down, and a green arc between 1500-2000 FPM down. Both of us felt 90 percent sure it was a false RA, which is why he only began a shallow descent and why I didn't insist on anything steeper. The controller seemed only mildly irritated, perhaps he heard the RA's in the backgnd of my transmission. We were VMC and I had been scanning for traffic prior to the TA's and RA's. The TCASII unit was written up.

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Original NASA ASRS Text

Title: AN ACR MLG DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative: WE WERE ON THE OLYMPIA 1 ARR, APPROX 16 MI OUT ON THE OLM 022 DEG RADIAL, DSNDING TO 10000 FT. AT 10500 FT OUR TCASII GAVE A TA, WITH A YELLOW TARGET, OF A SUPPOSED TARGET AT OUR 10 O'CLOCK, 5 MI, 200 FT ABOVE OUR ALT. I LOOKED FOR THE TFC (CAPT WAS FLYING) BUT SAW NOTHING, AND THEN LOOKED BACK AT THE TCASII DISPLAY. THE TARGET HAD DISAPPEARED FROM THE DISPLAY. APPROX 1 MIN LATER, 20 MI FROM OLM, THE CAPT WAS TRANSITIONING THE ACFT FOR A LEVEL OFF AT 10000 FT, WHEN WE GOT A TCASII RA 'DSND, DSND.' I LOOKED AT THE TCASII, SAW A RED TARGET RIGHT OVER US, AND IMMEDIATELY LOOKED FOR TFC. THE CAPT LOOKED UP ALSO, BUT NEITHER OF US SAW ANY TFC. SINCE WE HAD NO REAL PROOF THAT THIS WAS A FALSE ALARM, THE CAPT INITIATED A SHALLOW DSCNT AND I TOLD APCH CTL THAT WE HAD A TCASII ALERT AND WERE DSNDING. CTLR SAID HE WAS NOT SHOWING ANY TFC. WE DSNDED TO APPROX 9700 FT WHEN THE TARGET DISAPPEARED. I TOLD THE CTLR WE WERE CLR OF THE POSSIBLE CONFLICT AND ASKED IF HE WANTED US TO RETURN TO 10000 FT. HE GAVE US DSCNT TO 7000 FT AND THE REMAINDER OF THE FLT WAS UNEVENTFUL, CONSIDERING OUR ELEVATED BLOOD PRESSURES. DURING DISCUSSION, IT WAS DISCOVERED THAT THE CAPT NEVER SAW ANY COMMAND ARC SEGMENTS ON HIS VSI. MY VSI HAD DISPLAYED A RED ARC FROM 0 TO 1500 FPM DOWN, AND A GREEN ARC BTWN 1500-2000 FPM DOWN. BOTH OF US FELT 90 PERCENT SURE IT WAS A FALSE RA, WHICH IS WHY HE ONLY BEGAN A SHALLOW DSCNT AND WHY I DIDN'T INSIST ON ANYTHING STEEPER. THE CTLR SEEMED ONLY MILDLY IRRITATED, PERHAPS HE HEARD THE RA'S IN THE BACKGND OF MY XMISSION. WE WERE VMC AND I HAD BEEN SCANNING FOR TFC PRIOR TO THE TA'S AND RA'S. THE TCASII UNIT WAS WRITTEN UP.

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.