Narrative:

We have a procedure at sea where our tower turns aircraft to a heading of 130 degrees in a south flow. This heading is supposed to parallel the kent 4 departure off of bfi in a south flow. 2 factors occur which can cause less than lateral separation minima. One factor is wind; the other is where the aircraft starts the turn to 130 degrees. In this particular event; the E135 was flying the kent 4 off of bfi and assigned; after initial contact and radar identify; 6000 ft. Seconds later a DH8 departed sea on a pre-assigned heading of 130 degrees. The DH8 was restr to 3000 ft. I was going to stop the DH8 at 2000 ft but the aircraft was already climbing through 2100 ft while the E135 was climbing through about 2700 ft or 2800 ft in a well established climb rate. In my opinion; no meaningful separation would have been exercised had I stopped the DH8 below 3000 ft. I did not measure the lateral separation between the 2 aircraft; as I was busy with other traffic as well. I notified the watch supervisor after I was relieved from position some 15 or 20 mins later.

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Original NASA ASRS Text

Title: S46 CTLR EXPRESSED CONCERN REGARDING PROCS USED TO SEPARATE SEA AND BFI DEPS.

Narrative: WE HAVE A PROC AT SEA WHERE OUR TWR TURNS ACFT TO A HDG OF 130 DEGS IN A S FLOW. THIS HDG IS SUPPOSED TO PARALLEL THE KENT 4 DEP OFF OF BFI IN A S FLOW. 2 FACTORS OCCUR WHICH CAN CAUSE LESS THAN LATERAL SEPARATION MINIMA. ONE FACTOR IS WIND; THE OTHER IS WHERE THE ACFT STARTS THE TURN TO 130 DEGS. IN THIS PARTICULAR EVENT; THE E135 WAS FLYING THE KENT 4 OFF OF BFI AND ASSIGNED; AFTER INITIAL CONTACT AND RADAR IDENT; 6000 FT. SECONDS LATER A DH8 DEPARTED SEA ON A PRE-ASSIGNED HDG OF 130 DEGS. THE DH8 WAS RESTR TO 3000 FT. I WAS GOING TO STOP THE DH8 AT 2000 FT BUT THE ACFT WAS ALREADY CLBING THROUGH 2100 FT WHILE THE E135 WAS CLBING THROUGH ABOUT 2700 FT OR 2800 FT IN A WELL ESTABLISHED CLB RATE. IN MY OPINION; NO MEANINGFUL SEPARATION WOULD HAVE BEEN EXERCISED HAD I STOPPED THE DH8 BELOW 3000 FT. I DID NOT MEASURE THE LATERAL SEPARATION BTWN THE 2 ACFT; AS I WAS BUSY WITH OTHER TFC AS WELL. I NOTIFIED THE WATCH SUPVR AFTER I WAS RELIEVED FROM POS SOME 15 OR 20 MINS LATER.

Data retrieved from NASA's ASRS site as of January 2009 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.