Narrative:

I (first officer) was flying the aircraft (md-80) with the autoplt. The captain was working the radio. We were cleared to cross the olm VOR at 12000 ft MSL. At 250 KTS as we were approaching the olm VOR seattle approach cleared us to 10000 ft MSL and to keep our speed up. Our altitude at this time was 12400 MSL and I started to turn the aircraft with the autoplt to intercept the outbound radial on the olm 1 arrival. The captain repeated the clearance to 10000 and speed up. Back to approach control and set 10000 ft in the altitude window and pushed the vertical speed button to keep the aircraft descending while put 280 KTS in the speed window and continued to turn the aircraft with the heading knob. At 11000 ft MSL we noticed a target on TCASII 1000 ft below us at 12 O'clock approximately 2 miles. I started to level at 11000 ft when TCASII called 'traffic'. I stayed level at 11000 while the captain called approach control about our clearance. Seattle approach said that we should be at 11000 ft and that we had traffic at 12 O'clock at 10000 ft that we were overtaking for the approach. We were going to be #1 landing to the north at seattle. The controller asked if we were at 11000 ft and we affirmed that we were level at 11000 ft. We passed over the twin engine commuter airplane and landed. The captain and I both heard approach control's clearance to 10000 ft and the captain repeated that clearance back to approach control and approach made comment about the 'read-back'. There was no TCASII RA or any evasive action. Thank you TCASII for giving us a picture. I feel that the controller should have listened more carefully to our 'read-back' to 10000 ft. Supplemental information from acn 294274: at approximately 5 mi south of the olm VOR recieved ATC clearance to cross the VOR at 12000 ft then dscnd to 10000 ft, plus keep our speed up. At approximately 10 mi northeast of the olm VOR on the 022 degree radial at 10800 ft a TCASII TA indicated traffic at 12 O'clock and approximately 4 mi, 800 ft below our altitude. We stopped our descent and leveled off at 11000 ft. I called approach control about the traffic. ATC asked if we were at 11000 ft, I said yes, then he gave us a traffic report that was at 10000 ft. The problem is I was also cleared to 10000 ft and if not for the TCASII we would have descended into the traffic. The error was probably caused by the approach controller's work load as he was very busy with visual approachs into both seattle and boeing field.

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Original NASA ASRS Text

Title: ACR X DSCNT TO OCCUPIED ALT. TCASII TA WITH Y STIPPED DSCNT. LTSS AVOIDED.

Narrative: I (FO) WAS FLYING THE ACFT (MD-80) WITH THE AUTOPLT. THE CAPT WAS WORKING THE RADIO. WE WERE CLRED TO CROSS THE OLM VOR AT 12000 FT MSL. AT 250 KTS AS WE WERE APCHING THE OLM VOR SEATTLE APCH CLRED US TO 10000 FT MSL AND TO KEEP OUR SPD UP. OUR ALT AT THIS TIME WAS 12400 MSL AND I STARTED TO TURN THE ACFT WITH THE AUTOPLT TO INTERCEPT THE OUTBOUND RADIAL ON THE OLM 1 ARR. THE CAPT REPEATED THE CLRNC TO 10000 AND SPD UP. BACK TO APCH CTL AND SET 10000 FT IN THE ALT WINDOW AND PUSHED THE VERT SPD BUTTON TO KEEP THE ACFT DSNDING WHILE PUT 280 KTS IN THE SPD WINDOW AND CONTINUED TO TURN THE ACFT WITH THE HDG KNOB. AT 11000 FT MSL WE NOTICED A TARGET ON TCASII 1000 FT BELOW US AT 12 O'CLOCK APPROX 2 MILES. I STARTED TO LEVEL AT 11000 FT WHEN TCASII CALLED 'TFC'. I STAYED LEVEL AT 11000 WHILE THE CAPT CALLED APCH CTL ABOUT OUR CLRNC. SEATTLE APCH SAID THAT WE SHOULD BE AT 11000 FT AND THAT WE HAD TFC AT 12 O'CLOCK AT 10000 FT THAT WE WERE OVERTAKING FOR THE APCH. WE WERE GOING TO BE #1 LNDG TO THE N AT SEATTLE. THE CTLR ASKED IF WE WERE AT 11000 FT AND WE AFFIRMED THAT WE WERE LEVEL AT 11000 FT. WE PASSED OVER THE TWIN ENG COMMUTER AIRPLANE AND LANDED. THE CAPT AND I BOTH HEARD APCH CTL'S CLRNC TO 10000 FT AND THE CAPT REPEATED THAT CLRNC BACK TO APCH CTL AND APCH MADE COMMENT ABOUT THE 'READ-BACK'. THERE WAS NO TCASII RA OR ANY EVASIVE ACTION. THANK YOU TCASII FOR GIVING US A PICTURE. I FEEL THAT THE CTLR SHOULD HAVE LISTENED MORE CAREFULLY TO OUR 'READ-BACK' TO 10000 FT. SUPPLEMENTAL INFO FROM ACN 294274: AT APPROX 5 MI S OF THE OLM VOR RECIEVED ATC CLRNC TO CROSS THE VOR AT 12000 FT THEN DSCND TO 10000 FT, PLUS KEEP OUR SPD UP. AT APPROX 10 MI NE OF THE OLM VOR ON THE 022 DEG RADIAL AT 10800 FT A TCASII TA INDICATED TFC AT 12 O'CLOCK AND APPROX 4 MI, 800 FT BELOW OUR ALT. WE STOPPED OUR DSCNT AND LEVELED OFF AT 11000 FT. I CALLED APCH CTL ABOUT THE TFC. ATC ASKED IF WE WERE AT 11000 FT, I SAID YES, THEN HE GAVE US A TFC RPT THAT WAS AT 10000 FT. THE PROB IS I WAS ALSO CLRED TO 10000 FT AND IF NOT FOR THE TCASII WE WOULD HAVE DSNDED INTO THE TFC. THE ERROR WAS PROBABLY CAUSED BY THE APCH CTLR'S WORK LOAD AS HE WAS VERY BUSY WITH VISUAL APCHS INTO BOTH SEATTLE AND BOEING FIELD.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.