Narrative:

While being vectored for a visual approach initially to sea runway 34L we were sequenced between a number of aircraft arriving sea from the southeast and southwest. We were asked to report sighting a dash-8 over seatac mall as we were on a base leg arriving from the east. After reporting that aircraft in sight, we were cleared for a visual approach to runway 34R and to maintain visual separation from the dash-8. We had slowed to 170 KTS and were approximately 3 mi or more behind and to the right of the dash-8. I do not recall any instructions to the dash-8 to maintain a faster approach speed. As we continued to configure and slow (our adjusted reference speed was approximately 150 KTS) for a B737-900 landing at approximately 141000 pounds, our closure rate on the dash-8 rapidly increased. Tower recognized this and pointed us out to the dash-8 before we passed him. We eventually went by the dash-8 at our adjusted reference speed. It was nighttime and visual closure rates at night are difficult at best to judge. We had adjusted our final approach course intercept heading to reduce the angle offset initially given by approach control in order to avoid any chance of a course overshoot. My recommendation is not to accept visual separation clrncs at night with less than 5 mi initial separation when following a turboprop.

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Original NASA ASRS Text

Title: B737 CREW LNDG SEA OVERTOOK A DH8A WHILE CONDUCTING A VISUAL SIDE BY APCH TO THE ARPT.

Narrative: WHILE BEING VECTORED FOR A VISUAL APCH INITIALLY TO SEA RWY 34L WE WERE SEQUENCED BTWN A NUMBER OF ACFT ARRIVING SEA FROM THE SE AND SW. WE WERE ASKED TO RPT SIGHTING A DASH-8 OVER SEATAC MALL AS WE WERE ON A BASE LEG ARRIVING FROM THE E. AFTER RPTING THAT ACFT IN SIGHT, WE WERE CLRED FOR A VISUAL APCH TO RWY 34R AND TO MAINTAIN VISUAL SEPARATION FROM THE DASH-8. WE HAD SLOWED TO 170 KTS AND WERE APPROX 3 MI OR MORE BEHIND AND TO THE R OF THE DASH-8. I DO NOT RECALL ANY INSTRUCTIONS TO THE DASH-8 TO MAINTAIN A FASTER APCH SPD. AS WE CONTINUED TO CONFIGURE AND SLOW (OUR ADJUSTED REF SPD WAS APPROX 150 KTS) FOR A B737-900 LNDG AT APPROX 141000 LBS, OUR CLOSURE RATE ON THE DASH-8 RAPIDLY INCREASED. TWR RECOGNIZED THIS AND POINTED US OUT TO THE DASH-8 BEFORE WE PASSED HIM. WE EVENTUALLY WENT BY THE DASH-8 AT OUR ADJUSTED REF SPD. IT WAS NIGHTTIME AND VISUAL CLOSURE RATES AT NIGHT ARE DIFFICULT AT BEST TO JUDGE. WE HAD ADJUSTED OUR FINAL APCH COURSE INTERCEPT HEADING TO REDUCE THE ANGLE OFFSET INITIALLY GIVEN BY APCH CTL IN ORDER TO AVOID ANY CHANCE OF A COURSE OVERSHOOT. MY RECOMMENDATION IS NOT TO ACCEPT VISUAL SEPARATION CLRNCS AT NIGHT WITH LESS THAN 5 MI INITIAL SEPARATION WHEN FOLLOWING A TURBOPROP.

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.