Narrative:

We were flying the 'mountain five' departure out of seattle tacoma international airport, taking off on runway 34R. The departure is to 'intercept and proceed via sea R-341, cross D8 sea minimum 4000 ft then turn right to a 070 heading for radar vectors to assigned route.' we programmed the FMS with a route from sea to the sea 341/8, then a right turn to 070. We took off in LNAV and engaged the autoplt after flap retraction. As the autoplt started a right turn to 070, the departure controller called and said '...you turned too early, you're supposed to wait until 8 DME to start the turn...sometimes your computer thinks you have to be on track by 8DME and turns you early.' this is in fact, what is was doing. It always leads turns to roll out on track. Next time I won't program the turn to 070 degrees into the FMS, so the aircraft will keep tracking out the radial. Then I'll 'heading select' the turn, right at 8 DME.

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Original NASA ASRS Text

Title: B757 FLC ON SID ALLOWS FMS TO LEAD A TURN TO THE OUTBOUND HEADING PRIOR TO REACHING A PUBLISHED CROSSING FIX.

Narrative: WE WERE FLYING THE 'MOUNTAIN FIVE' DEP OUT OF SEATTLE TACOMA INTL ARPT, TAKING OFF ON RWY 34R. THE DEP IS TO 'INTERCEPT AND PROCEED VIA SEA R-341, CROSS D8 SEA MINIMUM 4000 FT THEN TURN R TO A 070 HEADING FOR RADAR VECTORS TO ASSIGNED RTE.' WE PROGRAMMED THE FMS WITH A RTE FROM SEA TO THE SEA 341/8, THEN A R TURN TO 070. WE TOOK OFF IN LNAV AND ENGAGED THE AUTOPLT AFTER FLAP RETRACTION. AS THE AUTOPLT STARTED A R TURN TO 070, THE DEP CTLR CALLED AND SAID '...YOU TURNED TOO EARLY, YOU'RE SUPPOSED TO WAIT UNTIL 8 DME TO START THE TURN...SOMETIMES YOUR COMPUTER THINKS YOU HAVE TO BE ON TRACK BY 8DME AND TURNS YOU EARLY.' THIS IS IN FACT, WHAT IS WAS DOING. IT ALWAYS LEADS TURNS TO ROLL OUT ON TRACK. NEXT TIME I WON'T PROGRAM THE TURN TO 070 DEGS INTO THE FMS, SO THE ACFT WILL KEEP TRACKING OUT THE RADIAL. THEN I'LL 'HEADING SELECT' THE TURN, R AT 8 DME.

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.