AN MD80 FLC PREMATURELY LEAVES THEIR ASSIGNED ALT ON THE OLM 2 ARR WHEN THE FO; PF; MISUNDERSTANDS THE PARAMETERS OF THE CLRNC TERM FOR RESUMING THE ARR PROC INTO SEA; WA.

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ON DSCNT INTO SEATAC ARPT; WE HAD BEEN CLRED TO DSND VIA THE OLM 2 ARR. BECAUSE OF HVY TFC INTO SEATTLE; WE WERE GIVEN DELAY VECTORS AND TOLD TO MAINTAIN 17000 FT INSTEAD OF DSNDING VIA THE OLM 2 ARR. DUE TO OUR FLT BEING LATE INTO SEATTLE; THERE WERE SEVERAL PAX SVC REQUESTS THAT WERE BEING DEALT WITH BY THE PNF. APCH CTL GAVE THE PF A CLRNC TO PROCEED 'DIRECT TO OLM VOR; RESUME THE OLM 2 ARR.' THE PF THOUGHT THAT MEANT TO DSND VIA THE OLM 2 ARR AND SET THE ALT ALERTER TO 16000 FT AND STARTED DOWN OUT OF 17000 FT; WHILE THE PNF WAS LOOKING AT SOMETHING ELSE WHILE DOING THE PAX SVC REQUESTS. THE PNF LOOKED UP AND SAW THEY WERE DSNDING OUT OF 16700 FT AND THAT 16000 FT WAS SET IN THE ALT ALERTER AND SAID TO THE PF THAT THEY WERE NOT CLRED OUT OF 17000 FT BUT ONLY THE OLM 2 ARR FOR ROUTING. PF STARTED A CLB BACK TO 17000 FT AND PNF SAID HE WOULD GET CONFIRMATION AS THERE WAS SOME CONFUSION ON THE PF SIDE. ATC CONFIRMED THAT THEY WERE NOT CLRED TO DSND OUT OF 17000 FT BUT THAT NOW THEY COULD DSND VIA THE OLM 2 ARR. WHAT REALLY CAUSED THE PROB WAS THE CONFUSION IN THE CLRNC THAT ATC ISSUED THAT ONE PLT UNDERSTOOD BUT THE OTHER PLT DIDN'T. A CONTRIBUTING FACTOR WAS ONE PLT BEING HEADS DOWN DEALING WITH THE PAX SVC REQUESTS AND NOT TOTALLY WATCHING THE PROCS BEING FLOWN. TO CORRECT PROBS LIKE THIS; WHEN AN ACFT IS DSNDING VIA AN ARR; THEN TOLD TO STOP FOR VECTORS OR WHATEVER; THEN CLRED VIA THE ARR; THE ALT TO MAINTAIN SHOULD BE RESTATED OR THE DSND VIA CLRNC SHOULD BE RESTATED; WHICHEVER IS APPROPRIATE. ALSO; MAYBE THE STERILE COCKPIT PROCS FOR TURBOJETS SHOULD BE RAISED FROM 10000 FT TO FL180 TO AVOID THESE PROBS AND KEEP BOTH PLTS TOTALLY HEADS UP BELOW FL180.