Narrative:

I (aircraft X) was on IFR flight plan on jun/xa/99 from gtf to ukf. On my descent to ukf from FL410, we were stepped down through different flight levels then received a clearance to 15000 ft then clearance to 11000 ft. There was a broken layer of clouds at approximately 10000 ft MSL. At approximately 14000 ft, I noticed an airplane on TCASII. As I looked out the window, a plane (aircraft Y) appeared to be at my altitude. It was dark, so it was hard to be sure. The TCASII gave a warning so I started to take corrective action, then I realized we would miss. I was surprised that the controller never called out traffic. I contacted the controller to report to him the traffic. He asked me what my assigned altitude was. I told him 11000 ft. He said, 'no, you were cleared to 15000 ft.' I reported to the controller that my clearance was 11000 ft and repeated 11000 ft back to him. We were still descending. I asked the controller if he wanted me at 11000 ft or to go back to 15000 ft. He told me to continue my descent to 11000 ft. I don't know who was at fault. Communication will always be a major cause for aviation accidents, whether pilot or controller error. No matter how hard we listen or concentrate there still seems to be failures in communication.

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Original NASA ASRS Text

Title: C550 FLC INITIATES TCASII EVASIVE MANEUVER WHEN ENCOUNTERING XING TFC WHILE IN DSCNT.

Narrative: I (ACFT X) WAS ON IFR FLT PLAN ON JUN/XA/99 FROM GTF TO UKF. ON MY DSCNT TO UKF FROM FL410, WE WERE STEPPED DOWN THROUGH DIFFERENT FLT LEVELS THEN RECEIVED A CLRNC TO 15000 FT THEN CLRNC TO 11000 FT. THERE WAS A BROKEN LAYER OF CLOUDS AT APPROX 10000 FT MSL. AT APPROX 14000 FT, I NOTICED AN AIRPLANE ON TCASII. AS I LOOKED OUT THE WINDOW, A PLANE (ACFT Y) APPEARED TO BE AT MY ALT. IT WAS DARK, SO IT WAS HARD TO BE SURE. THE TCASII GAVE A WARNING SO I STARTED TO TAKE CORRECTIVE ACTION, THEN I REALIZED WE WOULD MISS. I WAS SURPRISED THAT THE CTLR NEVER CALLED OUT TFC. I CONTACTED THE CTLR TO RPT TO HIM THE TFC. HE ASKED ME WHAT MY ASSIGNED ALT WAS. I TOLD HIM 11000 FT. HE SAID, 'NO, YOU WERE CLRED TO 15000 FT.' I RPTED TO THE CTLR THAT MY CLRNC WAS 11000 FT AND REPEATED 11000 FT BACK TO HIM. WE WERE STILL DSNDING. I ASKED THE CTLR IF HE WANTED ME AT 11000 FT OR TO GO BACK TO 15000 FT. HE TOLD ME TO CONTINUE MY DSCNT TO 11000 FT. I DON'T KNOW WHO WAS AT FAULT. COM WILL ALWAYS BE A MAJOR CAUSE FOR AVIATION ACCIDENTS, WHETHER PLT OR CTLR ERROR. NO MATTER HOW HARD WE LISTEN OR CONCENTRATE THERE STILL SEEMS TO BE FAILURES IN COM.

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.