WDB FLT CREW MISTOOK TRAFFIC CALL AS ALT ASSIGNMENT AND DESCENDED TO WITHIN 500' AND 2 MILES OF AN MLG. CTLR VECTORED FOR SEPARATION. FA IN COCKPIT WAS DISTR.

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THE COPLT WAS FLYING AND WE WERE CLRED FROM FL370 TO FL330; THEN FL310. ACKNOWLEDGED BY THE CAPT. THERE WERE TSTMS IN THE AREA. I WAS DISTRACTED BY SOMETHING AND WHEN I LOOKED UP I NOTICED WE WERE COMING DOWN THROUGH FL300 AT A GOOD RATE; BUT THAT FL290 WAS SET IN THE ALT ARM WINDOW. I CALLED LEVELING AT FL290 TO THE CENTER AND WAS GIVEN AN IMMEDIATE RIGHT TURN TO 180 DEGS AND TO CLB TO FL310; WHICH WE DID. I DO NOT REMEMBER SETTING THE LAST 2 ALTS IN THE WINDOW OR BEING CLRED TO FL290. AN F/A WAS IN THE COCKPIT GIVING THE S/O THE LOG WRITE UPS. THE LOWEST WE GOT WAS 29400' AND AN ACR MLG Y WENT UNDER US. EVIDENTLY WE WERE TOLD OF THE MLG Y CLBING TO FL290 AND THE COPLT SET FL290 IN THE WINDOW AND I DID NOT NOTICE IT. IT NOW SEEMS THAT WE WERE THE ONES AT FAULT. I HAVE 3 SUGGESTIONS TO MAKE THAT MIGHT HELP PREVENT THIS FROM HAPPENING IN THE FUTURE; 2 OF WHICH ARE BEING LOOKED AT BY COMPANY FOR INCORPORATION INTO OUR PROCS. LOGBOOK ITEMS WILL NOT BE BROUGHT TO THE FLT DECK DURING DSCNT BY THE F/A'S. ONLY THE PNF WILL SET THE ALT INTO THE ALT WARNING. I HAVE ALREADY PUT THIS INTO MY PROCS. GET THE FAA TO STUDY NOT GIVING ALT RPTS OF OTHER ACFT. THIS ONLY TENDS TO CLUTTER UP THE VERY LIMITED RADIO SPECTRUM AND I CAN SEE VERY LIMITED USE OF SAID RPTS. I AM ENCLOSING WITH THIS FOLLOW UP RPT A XEROX OF MY CAPT'S RPT TO THE COMPANY PLUS THE F/O AND S/O RPTS. IN THE COMPANY HEARING; IT CAME OUT THAT THE COPLT WAS FLYING AND WAS ALMOST SURE THAT HE SET THE 29000 APPARENTLY FROM SOME FL290 TFC GIVEN US. THE CAPT WAS ON THE RADAR AND DID NOT CATCH THE 29000 SETTING. THE ENGINEER WAS BUSY WITH LOG WRITE UPS AND CHKLIST. I HOPE THAT THIS WILL HELP YOU SET UP PROCS WHICH WILL TEND TO PREVENT THIS KIND OF THING FROM HAPPENING AGAIN. SUPPLEMENTAL INFO FROM ACN 89617: AFTER LNDG WE CALLED THE ATC CENTER. THEY SAY THE TAPE SHOWS THAT WE WERE ADVISED OF TFC AT FL290 AND TO EXPECT PROFILE DSCNT. IT IS PROBABLE THAT I MISSED PART OF THE XMISSION AND INTERPRETED THIS AS 'CLRED TO FL290; EXPECT PROFILE DSCNT.' AN F/A WAS IN THE COCKPIT AT THE TIME GIVING CABIN WRITE UPS. THEN ATC MADE THE XMISSIONS WHICH I MISINTERPRETED. IT WENT SOMETHING LIKE THIS: 'TFC 11-12 MI; 15 SW; CLB(ING) 290; EXPECT LOWER IN 2 MI.' THERE WAS A SLIGHT BREAK BTWN 'SW' AND 'CLB.' ALSO; THE 'ING' SYLLABLE OF CLBING DID NOT COME THROUGH ON THE RADIO. THE XMISSION SOUNDED LIKE A COMMAND 'CLB TO FL290; EXPECT LOWER IN 2 MI.' OF COURSE A DSCNT TO FL290 WOULD HAVE BEEN REQUIRED; BUT I EVIDENTLY HEARD WHAT I WANTED AND EXPECTED TO HEAR.