ZSE RADAR CTLR FAILS TO HEAR DH8 CREW READBACK RESULTING IN AN OPERROR.

Narrative

ACFT X CHKED ON MY FREQ; SAYING; '11000 FOR 7'. ZSE SHOULD HAVE ASSIGNED HIM 10000 FT. I WAS WORKING WITH A GA ACFT Y THAT HAD FILED AN IFR FLT FROM AN ARPT IN OUR AIRSPACE. HE FILED HIS FLT PLAN USING LAT/LONGITUDES FOR THE DEP ARPT. THIS CAUSED THE ARTS TO NOT ACCEPT THE ACFT AS A DEP. I WAS TRYING TO FORCE THE DATA BLOCK TO THE ARTS COMPUTER (SO I WOULD HAVE A DATA TAG) WHEN ACFT X CHKED ON. I VERIFIED HIS ALT AND THE CORRECT ATIS INFO BUT FAILED TO HEAR 'DSNDING TO 70.' WHY SHOULD HE BE? THE CTR IS SUPPOSED TO ASSIGN 10000 FT. I FIRST NOTICED THE PROB WHEN I SAW THE ALT READOUT GO FROM 100 TO 097. AT THAT POINT I ASKED THE ACFT WHAT ALT THE CTR HAD ASSIGNED HIM. HE REPLIED; 'SEVEN THOUSAND.' I IMMEDIATELY TOLD ACFT TO MAINTAIN 10000 FT. WHEN I SAW HIS DSCNT RATE (IT WAS FAST) I THEN TOLD THE ACFT TO EXPEDITE HIS CLB TO 10000 FT. HE CLBED BACK UP TO 10000 FT AND THE REMAINDER OF HIS FLT WAS ROUTINE. ACFT X LOWEST ALT I SAW ON THE HELNS ARR WAS 90; 28 MI N OF BTG. SHOULD BE 10000 FT UNTIL 22 MI N BTG; THEN 7000 FT IS OK. SUPPLEMENTAL INFO FROM ACN 563145: THE POSSIBLE ALTDEV IN QUESTION OCCURRED ON OCT/SUN/02; ON A FLT FROM SEATTLE-TACOMA TO PORTLAND INTL ARPT. I WAS THE PF WHEN THE PNF RESPONDED TO ZSE INSTRUCTIONS TO DSND AT PLT'S DISCRETION FROM 11000 FT TO CROSS KRATR INTXN AT 7000 FT ON THE HLNS FOUR ARR. THE PNF DIALED 7000 FT IN THE ALT ALERTER AND VERIFIED WITH ME IF THAT WAS CORRECT. I CONCURRED. I POSTPONED MY DSCNT UNTIL I WAS CLOSER TO KRATR INTXN. WE WERE HANDED OFF SHORTLY AFTER THE PREVIOUSLY MENTIONED ATC XMISSION (CURRENTLY A FEW MI OUTSIDE HLNS INTXN). THE PNF CONTACTED PORTLAND APCH CTL AND ADVISED THEM THAT WE HAD BEGUN OUR DSCNT FROM 11000 FT TO 7000 FT. THEN; THE PNF WENT OFF FREQ TO CALL COMPANY OPS ON COM #2; IN WHICH CASE; I WOULD MONITOR ATC ON COM #1. ONCE I HAD DSNDED THROUGH 9700 FT AT A RATE OF 1800 FPM; APCH CTL QUERIED WHAT ALT DID ZSE HAD GIVEN US. I REPLIED 7000 FT. HE; IN RETURN; INSTRUCTED ME TO CLB AND MAINTAIN 10000 FT. BY THAT TIME I STOPPED MY DSCNT AND INITIATED MY CLB TO 10000 FT; I NOTICED THE ALT OF 8900 FT. THE REMAINDER OF OUR FLT WENT UNEVENTFULLY. HOWEVER; I BELIEVE WITH THE PNF THAT WE WERE INSTRUCTED TO 7000 FT. IF WE WERE WRONG WITH THAT ALT; IT WOULD'VE BEEN CORRECTED ONCE WE HAD CHKED IN WITH PORTLAND APCH. NONETHELESS; I WILL BE MORE ASSERTIVE IN INITIATING MY CLB RATE SHOULD A SIMILAR INCIDENT OCCUR IN THE FUTURE. SUPPLEMENTAL INFO FROM ACN 562993: APCHING HELNS ON HELNS 4 ARR; ZSE DIRECTED US TO CROSS KRATR AT 7000 FT. AT LEAST THAT IS WHAT WE THOUGHT WE HEARD. AS PNF; I RESPONDED TO THEIR INSTRUCTION AND SET 7000 FT IN OUR ALT ALERTER/PRE-SELECTOR. I PAUSED AT THAT TIME BECAUSE WE USUALLY RECEIVE INSTRUCTIONS TO CROSS KRATR AT 10000 FT. I QUERIED THE FO (PNF) 'WAS THAT 7 OR 10000?' HE RESPONDED 'SEVEN' WITHOUT HESITATION. ABOUT THAT TIME CTR SWITCHED US TO PDX APCH. I CHKED IN '...11000 FT DSNDING TO 7000 FT WITH (ATIS).' APCH ACKNOWLEDGED WITHOUT COMMENT. I THEN TURNED THE ATC RADIO OVER TO THE FO SO I COULD TALK TO COMPANY ON THE OTHER COM IN ORDER TO RPT IN-RANGE. WHILE I WAS TALKING TO COMPANY; I HEARD ATC ASK WHAT ZSE CTR ASSIGNED US. WE WERE DSNDING THROUGH ABOUT 9500 FT AT 1800 FPM AT THIS TIME. THE FO RESPONDED '7000 FT.' ATC THEN TOLD US TO CLB BACK TO 10000 FT. THE FO DID THIS. THE LOWEST ALT I OBSERVED WAS 8900 FT. IF INDEED WE WERE ASSIGNED 10000 FT BY CTR (NOT PROVEN AT THIS POINT BUT TO BE INVESTIGATED) IT WAS MY FAULT FOR NOT QUERYING ATC ABOUT WHAT I KNEW TO BE AN UNUSUAL INSTRUCTION. FAILURE OF PDX APCH TO CATCH THE ERROR WHEN I CHKED ON WAS A CONTRIBUTOR.