Narrative:

We were operating an air carrier flight in clear WX at 23000 ft on an IFR flight plan in ZID's airspace. The controller was very busy with aircraft on his frequency and we were expecting a climb clearance since our filed cruise altitude was 35000 ft. It was at this point that our flight received a clearance to 250. We did not hear the 'climb to' in this clearance, nor did we hear 'fly heading of' in this clearance. I was the PF and after the first officer read back the clearance 'as 250' without stating climb to or leaving 230 for 250, I started the climb. At 23600 ft ATC asked what our assigned altitude was. We restated FL250. He said that was supposed to be a heading of 250 degrees. He told us to descend to FL230, immediately. I, as PF, disconnected the autoplt and descended to FL230. As I was doing this the TCASII audio was stating traffic, descend, descend. After the conflict had passed, I asked the controller how close the traffic was and he stated no separation boundaries had been violated. In hindsight, since we did not hear the climb to in the clearance we should have asked if this indeed was a climb clearance. The first officer stated he usually did this, but this time did not. I told him we both re-learned a valuable lesson that day, always to verify that climb or descend phrases are included in the terminology of clrncs when altitude changes occur. Lesson learned again! (TCASII is our friend.) supplemental information from acn 489057: we understood the clearance to be climb to 250 and responded by only saying 250 and our call sign. The assigned heading was for traffic above us. We saw the traffic and our TCASII displayed the traffic as we returned to FL230. The controller did say that separation was not compromised. I feel that the 2 events that caused this altitude deviation was the lack of 'fly heading' in the clearance and our incomplete readback of the clearance, ie, climb to FL250 instead of just 250.

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Original NASA ASRS Text

Title: AN A320 FLC ACCEPTS AN IMPROPER CLRNC THOUGHT AS AN ALT CHANGE, AND CREATES A POTENTIAL CONFLICT WITH ANOTHER ACR FLT ABOVE THEM NEAR ROD, OH.

Narrative: WE WERE OPERATING AN ACR FLT IN CLR WX AT 23000 FT ON AN IFR FLT PLAN IN ZID'S AIRSPACE. THE CTLR WAS VERY BUSY WITH ACFT ON HIS FREQ AND WE WERE EXPECTING A CLB CLRNC SINCE OUR FILED CRUISE ALT WAS 35000 FT. IT WAS AT THIS POINT THAT OUR FLT RECEIVED A CLRNC TO 250. WE DID NOT HEAR THE 'CLB TO' IN THIS CLRNC, NOR DID WE HEAR 'FLY HEADING OF' IN THIS CLRNC. I WAS THE PF AND AFTER THE FO READ BACK THE CLRNC 'AS 250' WITHOUT STATING CLB TO OR LEAVING 230 FOR 250, I STARTED THE CLB. AT 23600 FT ATC ASKED WHAT OUR ASSIGNED ALT WAS. WE RESTATED FL250. HE SAID THAT WAS SUPPOSED TO BE A HEADING OF 250 DEGS. HE TOLD US TO DSND TO FL230, IMMEDIATELY. I, AS PF, DISCONNECTED THE AUTOPLT AND DSNDED TO FL230. AS I WAS DOING THIS THE TCASII AUDIO WAS STATING TFC, DSND, DSND. AFTER THE CONFLICT HAD PASSED, I ASKED THE CTLR HOW CLOSE THE TFC WAS AND HE STATED NO SEPARATION BOUNDARIES HAD BEEN VIOLATED. IN HINDSIGHT, SINCE WE DID NOT HEAR THE CLB TO IN THE CLRNC WE SHOULD HAVE ASKED IF THIS INDEED WAS A CLB CLRNC. THE FO STATED HE USUALLY DID THIS, BUT THIS TIME DID NOT. I TOLD HIM WE BOTH RE-LEARNED A VALUABLE LESSON THAT DAY, ALWAYS TO VERIFY THAT CLB OR DSND PHRASES ARE INCLUDED IN THE TERMINOLOGY OF CLRNCS WHEN ALT CHANGES OCCUR. LESSON LEARNED AGAIN! (TCASII IS OUR FRIEND.) SUPPLEMENTAL INFO FROM ACN 489057: WE UNDERSTOOD THE CLRNC TO BE CLB TO 250 AND RESPONDED BY ONLY SAYING 250 AND OUR CALL SIGN. THE ASSIGNED HEADING WAS FOR TFC ABOVE US. WE SAW THE TFC AND OUR TCASII DISPLAYED THE TFC AS WE RETURNED TO FL230. THE CTLR DID SAY THAT SEPARATION WAS NOT COMPROMISED. I FEEL THAT THE 2 EVENTS THAT CAUSED THIS ALTDEV WAS THE LACK OF 'FLY HEADING' IN THE CLRNC AND OUR INCOMPLETE READBACK OF THE CLRNC, IE, CLB TO FL250 INSTEAD OF JUST 250.

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.