Narrative:

We were descending, having been cleared to descend to cross pmd at FL230 on the ziggy 3 arrival. The ATC controller called us as we were just about over pmd and approaching FL230 and, with a very tense voice, told us to 'immediately turn 40 degrees left and expedite to FL210.' we were watching traffic coming toward us, both visually and on TCASII. The approaching aircraft's altitude was being reported as 2000 ft below us as we started the turn off course. At some point in these exchanges the controller cleared us to cross hitop at 13000 ft. The turn was accelerated as was the rate of descent. The approaching aircraft's symbol passed well to the west and we continued our descent below his altitude. We maintained visual contact with the aircraft until he passed out of our view, behind us. The controller confirmed we were passing FL210. The traffic was well to our right and behind us by this time. We told the controller we were turning to course, if that was ok with him. He confirmed our clearance to return to course. From the sound of his voice, this occurrence was more exciting for the controller than for us. We continued our descent, landing at ont without incident. There was some conversation with the controller, with him saying the other aircraft was off course. However, from my perspective, no hazardous condition existed at any time. We were watching the aircraft symbol on the TCASII. It had turned yellow and was below our altitude. The initial instruction from the controller was to expedite the descent. It was followed by turn 40 degrees left. I'm not too sure I agreed with the controller at that point. The target aircraft was obliviously climbing to our position and we were descending to his. It was my inclination to stop the descent and turn. That is what the TCASII was indicating. The instruction to expedite the descent was contrary to what the TCASII was indicating. While our fom does not directly address the issue, '...must immediately respond to TCASII information by using the TCASII guidelines.' further, under the caution, 'safe separation could be compromised if current vertical speed is changed.' in this case, stopping the descent was what the TCASII was indicating. This fom guidance is primarily directed at RA's. 3 things bother me about this occurrence: 1) if I had just been left on my own, I would have stopped the descent. 2) following the descent instructions of the controller seemed to aggravate the situation. 3) the fom does not address this type of TA situation. However, let me share with you some feelings I have concerning the controller, TCASII, pilot triangle. When flying, we develop a relationship with the controller, however brief. I was compelled to follow his instructions. I knew from the TCASII that I should have stopped the descent. I didn't because the controller instructed me to expedite the descent. Later, in my hotel room, I was thinking of the crew that had a collision with the foreign charter flight. That crew operated under the same 'relationship' with the controller, the same mode of behavior that I was under. Fortunately, a much different outcome.

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Original NASA ASRS Text

Title: SYS ERROR DURING A POTENTIAL CONFLICT WITH A LATE TURN BY ATC AND A DSCNT THROUGH OCCUPIED ALT WITH TCASII ON YELLOW, NO TA-RA NEAR PMD, CA.

Narrative: WE WERE DSNDING, HAVING BEEN CLRED TO DSND TO CROSS PMD AT FL230 ON THE ZIGGY 3 ARR. THE ATC CTLR CALLED US AS WE WERE JUST ABOUT OVER PMD AND APCHING FL230 AND, WITH A VERY TENSE VOICE, TOLD US TO 'IMMEDIATELY TURN 40 DEGS L AND EXPEDITE TO FL210.' WE WERE WATCHING TFC COMING TOWARD US, BOTH VISUALLY AND ON TCASII. THE APCHING ACFT'S ALT WAS BEING RPTED AS 2000 FT BELOW US AS WE STARTED THE TURN OFF COURSE. AT SOME POINT IN THESE EXCHANGES THE CTLR CLRED US TO CROSS HITOP AT 13000 FT. THE TURN WAS ACCELERATED AS WAS THE RATE OF DSCNT. THE APCHING ACFT'S SYMBOL PASSED WELL TO THE W AND WE CONTINUED OUR DSCNT BELOW HIS ALT. WE MAINTAINED VISUAL CONTACT WITH THE ACFT UNTIL HE PASSED OUT OF OUR VIEW, BEHIND US. THE CTLR CONFIRMED WE WERE PASSING FL210. THE TFC WAS WELL TO OUR R AND BEHIND US BY THIS TIME. WE TOLD THE CTLR WE WERE TURNING TO COURSE, IF THAT WAS OK WITH HIM. HE CONFIRMED OUR CLRNC TO RETURN TO COURSE. FROM THE SOUND OF HIS VOICE, THIS OCCURRENCE WAS MORE EXCITING FOR THE CTLR THAN FOR US. WE CONTINUED OUR DSCNT, LNDG AT ONT WITHOUT INCIDENT. THERE WAS SOME CONVERSATION WITH THE CTLR, WITH HIM SAYING THE OTHER ACFT WAS OFF COURSE. HOWEVER, FROM MY PERSPECTIVE, NO HAZARDOUS CONDITION EXISTED AT ANY TIME. WE WERE WATCHING THE ACFT SYMBOL ON THE TCASII. IT HAD TURNED YELLOW AND WAS BELOW OUR ALT. THE INITIAL INSTRUCTION FROM THE CTLR WAS TO EXPEDITE THE DSCNT. IT WAS FOLLOWED BY TURN 40 DEGS L. I'M NOT TOO SURE I AGREED WITH THE CTLR AT THAT POINT. THE TARGET ACFT WAS OBLIVIOUSLY CLBING TO OUR POS AND WE WERE DSNDING TO HIS. IT WAS MY INCLINATION TO STOP THE DSCNT AND TURN. THAT IS WHAT THE TCASII WAS INDICATING. THE INSTRUCTION TO EXPEDITE THE DSCNT WAS CONTRARY TO WHAT THE TCASII WAS INDICATING. WHILE OUR FOM DOES NOT DIRECTLY ADDRESS THE ISSUE, '...MUST IMMEDIATELY RESPOND TO TCASII INFO BY USING THE TCASII GUIDELINES.' FURTHER, UNDER THE CAUTION, 'SAFE SEPARATION COULD BE COMPROMISED IF CURRENT VERT SPD IS CHANGED.' IN THIS CASE, STOPPING THE DSCNT WAS WHAT THE TCASII WAS INDICATING. THIS FOM GUIDANCE IS PRIMARILY DIRECTED AT RA'S. 3 THINGS BOTHER ME ABOUT THIS OCCURRENCE: 1) IF I HAD JUST BEEN LEFT ON MY OWN, I WOULD HAVE STOPPED THE DSCNT. 2) FOLLOWING THE DSCNT INSTRUCTIONS OF THE CTLR SEEMED TO AGGRAVATE THE SIT. 3) THE FOM DOES NOT ADDRESS THIS TYPE OF TA SIT. HOWEVER, LET ME SHARE WITH YOU SOME FEELINGS I HAVE CONCERNING THE CTLR, TCASII, PLT TRIANGLE. WHEN FLYING, WE DEVELOP A RELATIONSHIP WITH THE CTLR, HOWEVER BRIEF. I WAS COMPELLED TO FOLLOW HIS INSTRUCTIONS. I KNEW FROM THE TCASII THAT I SHOULD HAVE STOPPED THE DSCNT. I DIDN'T BECAUSE THE CTLR INSTRUCTED ME TO EXPEDITE THE DSCNT. LATER, IN MY HOTEL ROOM, I WAS THINKING OF THE CREW THAT HAD A COLLISION WITH THE FOREIGN CHARTER FLT. THAT CREW OPERATED UNDER THE SAME 'RELATIONSHIP' WITH THE CTLR, THE SAME MODE OF BEHAVIOR THAT I WAS UNDER. FORTUNATELY, A MUCH DIFFERENT OUTCOME.

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.