Narrative:

We were issued the following clearance and our readback was correct. 'Cleared to pdx via frees 6 departure; frees direct rbl then as filed.' the route was programmed into the FMS as cleared; which resulted in a very sharp 160 degree right turn from frees to rbl (even the turn as published to geter on the eni transition is a 133 degree turn). We departed runway 14 and turned to the 250 degree heading as published. Our speed was approximately 180-200 KTS; and as we approached to intercept the 202 degree course from sts (using the FMS course) to frees; the FMS advanced the waypoint to the next fix (rbl). (The FMS anticipates turns and defaults to a 'fly-by' waypoint mode.) in this case with such a sharp turn; the FMS will command the turn several mi from frees in order to not overshoot the course to rbl. This early turn apparently surprised the controller enough that the controller asked where we were going. I tried to explain it briefly but xmissions were blocked once or twice and effective aircraft/ATC communication was not happening. After the confusion; we were issued a heading and later steered on course with no further incident. Upon landing at pdx; I telephoned the sts tower and confirmed the clearance was correct. It appears to me that ATC should expect such early turns on waypoints when assigning departures with such sharp turns -- especially when assigning it to a turbojet aircraft with large turn radiuses and high speeds. Or perhaps this departure (the frees 6) needs to be redesigned.

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Original NASA ASRS Text

Title: BE40 CAPTAIN REPORTS FMC DEVIATION DEPARTING STS RWY 14 ON THE FREES 6 DEPARTURE.

Narrative: WE WERE ISSUED THE FOLLOWING CLRNC AND OUR READBACK WAS CORRECT. 'CLRED TO PDX VIA FREES 6 DEP; FREES DIRECT RBL THEN AS FILED.' THE RTE WAS PROGRAMMED INTO THE FMS AS CLRED; WHICH RESULTED IN A VERY SHARP 160 DEG R TURN FROM FREES TO RBL (EVEN THE TURN AS PUBLISHED TO GETER ON THE ENI TRANSITION IS A 133 DEG TURN). WE DEPARTED RWY 14 AND TURNED TO THE 250 DEG HDG AS PUBLISHED. OUR SPD WAS APPROX 180-200 KTS; AND AS WE APCHED TO INTERCEPT THE 202 DEG COURSE FROM STS (USING THE FMS COURSE) TO FREES; THE FMS ADVANCED THE WAYPOINT TO THE NEXT FIX (RBL). (THE FMS ANTICIPATES TURNS AND DEFAULTS TO A 'FLY-BY' WAYPOINT MODE.) IN THIS CASE WITH SUCH A SHARP TURN; THE FMS WILL COMMAND THE TURN SEVERAL MI FROM FREES IN ORDER TO NOT OVERSHOOT THE COURSE TO RBL. THIS EARLY TURN APPARENTLY SURPRISED THE CTLR ENOUGH THAT THE CTLR ASKED WHERE WE WERE GOING. I TRIED TO EXPLAIN IT BRIEFLY BUT XMISSIONS WERE BLOCKED ONCE OR TWICE AND EFFECTIVE ACFT/ATC COM WAS NOT HAPPENING. AFTER THE CONFUSION; WE WERE ISSUED A HDG AND LATER STEERED ON COURSE WITH NO FURTHER INCIDENT. UPON LNDG AT PDX; I TELEPHONED THE STS TWR AND CONFIRMED THE CLRNC WAS CORRECT. IT APPEARS TO ME THAT ATC SHOULD EXPECT SUCH EARLY TURNS ON WAYPOINTS WHEN ASSIGNING DEPS WITH SUCH SHARP TURNS -- ESPECIALLY WHEN ASSIGNING IT TO A TURBOJET ACFT WITH LARGE TURN RADIUSES AND HIGH SPDS. OR PERHAPS THIS DEP (THE FREES 6) NEEDS TO BE REDESIGNED.

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.