B737 crew deviates from cleared track in bos airspace.

Attributes
ACN	413428
Time
Date	199809
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : bos
State Reference	MA
Altitude	agl bound lower : 0 agl bound upper : 1000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : bos tracon : cvg
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	climbout : initial climbout : takeoff ground other : taxi
Route In Use	departure other departure sid : sid enroute other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : cfi
Experience	flight time last 90 days : 170 flight time total : 6000 flight time type : 510
ASRS Report	413428
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	non adherence : published procedure other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Bos clearance inquired if we could accept wylyy 1 departure instead of logan 2 as cleared on pre departure clearance. We consulted procedure. We verified that we were 'slant F.' discussed note about updating navigation system to runway 27 latitude/longitude prior to departure. This aircraft does not have takeoff shift feature. We agreed to do a quick alignment on runway and accepted departure. During taxi out, FMS caused a route discontinuity from the runway heading fix to garve waypoint. Repaired discontinuity 2 times. Took position on runway and did a quick IRS alignment. Flew departure in LNAV. After 1000 ft AGL, engaged autoplt. Aircraft immediately started a large right turn that appeared excessive. Downgraded autoplt to control wheel steering and re-engaged LNAV. Course bar began to swing back and forth 180 degrees every 10 seconds. Waypoint indicator appeared to be pointing to next fix, wylyy, but course information was erratic and unusable. Requested vectors from ATC. ATC queried reason for vectors. We advised them of unreliable navigation data. Rest of departure uneventful.

Original NASA ASRS Text

Title: B737 CREW DEVIATES FROM CLRED TRACK IN BOS AIRSPACE.

Narrative: BOS CLRNC INQUIRED IF WE COULD ACCEPT WYLYY 1 DEP INSTEAD OF LOGAN 2 AS CLRED ON PDC. WE CONSULTED PROC. WE VERIFIED THAT WE WERE 'SLANT F.' DISCUSSED NOTE ABOUT UPDATING NAV SYS TO RWY 27 LATITUDE/LONGITUDE PRIOR TO DEP. THIS ACFT DOES NOT HAVE TKOF SHIFT FEATURE. WE AGREED TO DO A QUICK ALIGNMENT ON RWY AND ACCEPTED DEP. DURING TAXI OUT, FMS CAUSED A RTE DISCONTINUITY FROM THE RWY HDG FIX TO GARVE WAYPOINT. REPAIRED DISCONTINUITY 2 TIMES. TOOK POS ON RWY AND DID A QUICK IRS ALIGNMENT. FLEW DEP IN LNAV. AFTER 1000 FT AGL, ENGAGED AUTOPLT. ACFT IMMEDIATELY STARTED A LARGE R TURN THAT APPEARED EXCESSIVE. DOWNGRADED AUTOPLT TO CTL WHEEL STEERING AND RE-ENGAGED LNAV. COURSE BAR BEGAN TO SWING BACK AND FORTH 180 DEGS EVERY 10 SECONDS. WAYPOINT INDICATOR APPEARED TO BE POINTING TO NEXT FIX, WYLYY, BUT COURSE INFO WAS ERRATIC AND UNUSABLE. REQUESTED VECTORS FROM ATC. ATC QUERIED REASON FOR VECTORS. WE ADVISED THEM OF UNRELIABLE NAV DATA. REST OF DEP UNEVENTFUL.

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.