Narrative:

The flight was at cruise altitude of FL350 en route to cmh. We had been cleared direct hnn approximately 2 mins earlier. I was working radios, captain was flying. I was in the process of locating cmh approach plates in my manual when we were assigned a heading of 330 degrees for traffic. I acknowledged the heading, set my heading bug, and resumed flipping through my charts. (Previous heading for direct hnn was 325 degrees.) at this time, lead flight attendant entered cockpit. The next thing I knew, I looked up from my charts and saw we were at FL340 and descending. I checked altitude window and saw it was set at 33000 ft. I was completely startled by this and knew we had not been given lower altitude. I questioned captain. He said we had been cleared to 330. I knew our traffic was at FL330 and TCASII verified this. I told captain the clearance was for heading 330 degrees not altitude 330. He then agreed and climb was immediately initiated back to FL350. TCASII did issue a TA for approach 1 second just as climb was initiated. ATC was notified of deviation immediately. This event shook me up considerably. I don't remember the ATC facility because of this and also due to several frequency changes in a short time span. I believe it was ind, if not it was atl. As for separation, I believe the other aircraft had just barely broken the 10 DME arc of our TCASII display and vertical separation never was less than 1000 ft. As for what could have prevented this occurrence, I'm not really sure. Miscom is a constant threat, one that I try to avoid with great vigilance. The captain's failure to utilize proper altitude awareness procedures was probably the biggest contributing factor.

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Original NASA ASRS Text

Title: CAPT OF AN MLG ACR ACFT MISTOOK ATC CLRNC OF A HDG CHANGE FOR AN ALT CHANGE RESULTING IN A DEV FROM ASSIGNED ALT AND TRACK.

Narrative: THE FLT WAS AT CRUISE ALT OF FL350 ENRTE TO CMH. WE HAD BEEN CLRED DIRECT HNN APPROX 2 MINS EARLIER. I WAS WORKING RADIOS, CAPT WAS FLYING. I WAS IN THE PROCESS OF LOCATING CMH APCH PLATES IN MY MANUAL WHEN WE WERE ASSIGNED A HDG OF 330 DEGS FOR TFC. I ACKNOWLEDGED THE HDG, SET MY HDG BUG, AND RESUMED FLIPPING THROUGH MY CHARTS. (PREVIOUS HDG FOR DIRECT HNN WAS 325 DEGS.) AT THIS TIME, LEAD FLT ATTENDANT ENTERED COCKPIT. THE NEXT THING I KNEW, I LOOKED UP FROM MY CHARTS AND SAW WE WERE AT FL340 AND DSNDING. I CHKED ALT WINDOW AND SAW IT WAS SET AT 33000 FT. I WAS COMPLETELY STARTLED BY THIS AND KNEW WE HAD NOT BEEN GIVEN LOWER ALT. I QUESTIONED CAPT. HE SAID WE HAD BEEN CLRED TO 330. I KNEW OUR TFC WAS AT FL330 AND TCASII VERIFIED THIS. I TOLD CAPT THE CLRNC WAS FOR HDG 330 DEGS NOT ALT 330. HE THEN AGREED AND CLB WAS IMMEDIATELY INITIATED BACK TO FL350. TCASII DID ISSUE A TA FOR APCH 1 SECOND JUST AS CLB WAS INITIATED. ATC WAS NOTIFIED OF DEV IMMEDIATELY. THIS EVENT SHOOK ME UP CONSIDERABLY. I DON'T REMEMBER THE ATC FACILITY BECAUSE OF THIS AND ALSO DUE TO SEVERAL FREQ CHANGES IN A SHORT TIME SPAN. I BELIEVE IT WAS IND, IF NOT IT WAS ATL. AS FOR SEPARATION, I BELIEVE THE OTHER ACFT HAD JUST BARELY BROKEN THE 10 DME ARC OF OUR TCASII DISPLAY AND VERT SEPARATION NEVER WAS LESS THAN 1000 FT. AS FOR WHAT COULD HAVE PREVENTED THIS OCCURRENCE, I'M NOT REALLY SURE. MISCOM IS A CONSTANT THREAT, ONE THAT I TRY TO AVOID WITH GREAT VIGILANCE. THE CAPT'S FAILURE TO UTILIZE PROPER ALT AWARENESS PROCS WAS PROBABLY THE BIGGEST CONTRIBUTING FACTOR.

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.