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.