B737 captain reports track deviation departing sea runway 34R on the SUMMA7 departure. Crew was hand...

Narrative:

Departing sea runway 34R SUMMA7 departure; at 1000' MSL engaged LNAV. Using HUD guidance commanded a left turn to intercept the 341 radial from sea VORTAC (the magenta line). We began to follow the guidance; however; realized the guidance was too excessive. I shifted my scan to the flight guidance bars which appeared too excessive. I switched guidance to VOR and display to full VOR which confirmed that we were left [west] of course. I began my correction back to course and continued to clean up. Departure controller asked us what departure we were on. We informed him the summa but that we were experiencing some GPS navigational problems. We received a couple of heading changes and was given direct to summa waypoint. We continued the flight in LNAV/VNAV; staying on our filed flight plan; but monitored our flight with VOR cross check. We noticed no other problems during the flight. Both pilots noted during the navigational problem that the magenta line was visible to the nezug waypoint on the summa departure and there was no magenta line by which the aircraft was turning to.

Original NASA ASRS Text

Title: B737 Captain reports track deviation departing SEA Runway 34R on the SUMMA7 departure. Crew was hand flying using LNAV and a HUD which indicated a left turn away from the desired track.

Narrative: Departing SEA Runway 34R SUMMA7 Departure; at 1000' MSL engaged LNAV. Using HUD guidance commanded a left turn to intercept the 341 radial from SEA VORTAC (the magenta line). We began to follow the guidance; however; realized the guidance was too excessive. I Shifted my scan to the flight guidance bars which appeared too excessive. I switched guidance to VOR and display to full VOR which confirmed that we were left [west] of course. I began my correction back to course and continued to clean up. Departure Controller asked us what departure we were on. We informed him the SUMMA but that we were experiencing some GPS navigational problems. We received a couple of heading changes and was given direct to SUMMA waypoint. We continued the flight in LNAV/VNAV; staying ON our filed flight plan; but monitored our flight with VOR cross check. We noticed no other problems during the flight. Both pilots noted during the navigational problem that the magenta line was visible to the NEZUG waypoint on the SUMMA departure and there was no magenta line by which the aircraft was turning to.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	883365
Time
Date	201004
Local Time Of Day	0001-0600
Place
Locale Reference	SEA.Airport
State Reference	WA
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Takeoff
Route In Use	SID SUMMA7
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Track / Heading All Types

B737 captain reports track deviation departing sea runway 34R on the SUMMA7 departure. Crew was hand flying using LNAV and a HUD which indicated a left turn away from the desired track.