Narrative:

On IFR flight plan to las I was flying at 10000 ft MSL and sometime before dag VOR I was asked if I could accept 11000 ft MSL for terrain clearance and I said yes. Mins later I received an amended clearance to fly direct dag (I think) and cleared to las via the creso 3 arrival, maintain 11000 ft. I climbed to 11000 ft, and after reaching dag VOR I began descent to 10000 ft as published in creso 3 arrival. I heard interaction between controller and commercial jet (?) whose TCASII was triggered. I don't recall if other aircraft had to change course. Controller then asked me if I was at 11000 ft, and I said no, I was following published arrival procedure which states 10000 ft and controller said, 'I told you to maintain 11000 ft.' I had decreased in altitude to 10300 ft and immediately climbed back to 11000 ft within 40 seconds. I assumed my altitude restr of 11000 ft was valid until I entered the published arrival procedure, and then I assumed I was supposed to follow that procedure. The controller wanted to have me follow the procedure but stay at 11000 ft. Contributing to the miscom were the questions presented to me earlier about being able to climb to 11000 ft for terrain clearance, not traffic avoidance.

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Original NASA ASRS Text

Title: A PVT PLT IN A PA28-235 PERFORMS AN ALTDEV EXCURSION FROM ASSIGNED. NON ADHERENCE TO ATC CLRNC. POTENTIAL CONFLICT. DSCNT TO OCCUPIED ALT.

Narrative: ON IFR FLT PLAN TO LAS I WAS FLYING AT 10000 FT MSL AND SOMETIME BEFORE DAG VOR I WAS ASKED IF I COULD ACCEPT 11000 FT MSL FOR TERRAIN CLRNC AND I SAID YES. MINS LATER I RECEIVED AN AMENDED CLRNC TO FLY DIRECT DAG (I THINK) AND CLRED TO LAS VIA THE CRESO 3 ARR, MAINTAIN 11000 FT. I CLBED TO 11000 FT, AND AFTER REACHING DAG VOR I BEGAN DSCNT TO 10000 FT AS PUBLISHED IN CRESO 3 ARR. I HEARD INTERACTION BTWN CTLR AND COMMERCIAL JET (?) WHOSE TCASII WAS TRIGGERED. I DON'T RECALL IF OTHER ACFT HAD TO CHANGE COURSE. CTLR THEN ASKED ME IF I WAS AT 11000 FT, AND I SAID NO, I WAS FOLLOWING PUBLISHED ARR PROC WHICH STATES 10000 FT AND CTLR SAID, 'I TOLD YOU TO MAINTAIN 11000 FT.' I HAD DECREASED IN ALT TO 10300 FT AND IMMEDIATELY CLBED BACK TO 11000 FT WITHIN 40 SECONDS. I ASSUMED MY ALT RESTR OF 11000 FT WAS VALID UNTIL I ENTERED THE PUBLISHED ARR PROC, AND THEN I ASSUMED I WAS SUPPOSED TO FOLLOW THAT PROC. THE CTLR WANTED TO HAVE ME FOLLOW THE PROC BUT STAY AT 11000 FT. CONTRIBUTING TO THE MISCOM WERE THE QUESTIONS PRESENTED TO ME EARLIER ABOUT BEING ABLE TO CLB TO 11000 FT FOR TERRAIN CLRNC, NOT TFC AVOIDANCE.

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.