A B732 CREW; CLBING THROUGH FL317 TO MAINTAIN FL350; WAS TOLD BY ATC TO MAINTAIN FL310; THEIR PREVIOUSLY AMENDED ALT.

Narrative

WE WERE CLBING TO FL350 (HAVING BEEN PREVIOUSLY CLRED) ON AN ASSIGNED 215 DEG HDG (DIRECT OKC WHEN ABLE). PASSING FL317; ZKC CTLR TOLD US TO DSND TO FL310. CTLR THEN SAID 'YOUR ALT WAS AMENDED TO FL310.' CTLR THEN DIRECTED US TO TURN TO A 180 DEG HDG AND (EXPEDITE TO FL310). THE CONFLICTING TFC WAS AN ACR Y FLT THAT MUST HAVE BEEN EBOUND AT FL330. I THINK HE WAS GIVEN A CLB TO FL350. AS I EXECUTED THE TURNING DSCNT; I CHKED THE TCASII AND NOTICED A TARGET AT THE 2 O'CLOCK POS; APPROX 7 MI; AND AT A RELATIVE ALT OF +1800 FT. THE ACR Y FLT CALLED US IN SIGHT VISUALLY. I NEVER SEARCHED FOR NOR ACQUIRED HIM VISUALLY AS I WAS CONCENTRATING ON EXECUTING THE CTR INSTRUCTIONS IN AN EXPEDITIOUS MANNER. WE NEVER RECEIVED A TCASII ALERT. I THINK THE PRIMARY FACTOR IN THIS EVENT WAS EXTREME CONGESTION ON THE FREQ. THERE WAS ALMOST NON-STOP CHATTER AND MANY XMISSIONS WERE BLOCKED. INDEED; A PREVIOUS XMISSION FROM THIS CTLR TO US HAD TO BE REPEATED BECAUSE THE CALL SIGN WAS BLOCKED. SHE WAS VERY BUSY AND MY IMPRESSION WAS THAT CTLR WAS HAVING DIFFICULTY MAINTAINING 'SITUATIONAL AWARENESS.' MY TAKE: CTLR EITHER NEVER GAVE US THE INSTRUCTION TO MAINTAIN FL310; OR SHE DID AND THE XMISSION WAS BLOCKED. I KNOW THAT WE NEVER RECEIVED OR ACKNOWLEDGED ANY INSTRUCTION TO MAINTAIN FL310 AS A FINAL ALT. AS A PERSONAL TECHNIQUE; I IMMEDIATELY SET THE PRESSURIZATION CTLR TO THE AMENDED ALT IN A SIT LIKE THAT. OUR CTLR WAS STILL SET TO MAINTAIN FL350. PREVENTATIVE ACTION IN THIS CASE IS OBVIOUS. ALL OF US; PLTS AND ATC; MUST ENSURE THAT ALL CLRNCS ARE RECEIVED AND UNDERSTOOD/ACKNOWLEDGED.