B757 flight crew on SID allows FMS to lead a turn to the outbound heading prior to reaching a publis...

Attributes
ACN	543218
Time
Date	200204
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : sea.vortac
State Reference	WA
Altitude	msl single value : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : s46.tracon
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	climbout : intermediate altitude
Route In Use	departure sid : 388
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : private pilot : flight engineer pilot : instrument pilot : multi engine pilot : atp
Experience	flight time last 90 days : 180 flight time total : 9000 flight time type : 1000
ASRS Report	543218
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	non adherence : published procedure non adherence : clearance other spatial deviation
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	controller : issued advisory none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

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.

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.

B757 flight crew on SID allows FMS to lead a turn to the outbound heading prior to reaching a published crossing fix.