B737-300 fails to track initial course on cadit RNAV SID from runway 26L at atl.

Attributes
ACN	696678
Time
Date	200605
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : atl.airport
State Reference	GA
Altitude	msl single value : 1000
Aircraft 1
Controlling Facilities	tracon : a80.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	climbout : initial
Route In Use	departure sid : cadit
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 195 flight time total : 12169 flight time type : 2225
ASRS Report	696678
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : critical non adherence : published procedure other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory
Supplementary
Problem Areas	Flight Crew Human Performance Chart Or Publication Aircraft
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation

Narrative:

Flying CADIT4 on runway 26L. Confirmed with tower on taxi; first fix snufy. Runway update accomplished per SOP. Both navigation radios in automatic. LNAV engaged at 400 ft. I began following LNAV track which started the aircraft in a slight left turn. Myself; captain; and ATC all seemed to see the problem at about the same time. Atl departure or tower; cannot remember; gave new assigned heading to fly. Complied with at once. Eventually ended up slightly north of track on ATC heading. Then reclred direct snufy. We were very close to snufy at time of clearance. I proceeded direct snufy and then attempted to maintain LNAV track for CADIT4. Our angle and closeness to snufy made maintaining desired track virtually impossible. Departure then assigned new northeast heading and then reclred us to a new fix on the departure; which I cannot remember. There must have been some type of map shift shortly after takeoff. ATC assigned headings to correct our path and all new headings/instructions were complied with.

Original NASA ASRS Text

Title: B737-300 FAILS TO TRACK INITIAL COURSE ON CADIT RNAV SID FROM RWY 26L AT ATL.

Narrative: FLYING CADIT4 ON RWY 26L. CONFIRMED WITH TWR ON TAXI; FIRST FIX SNUFY. RWY UPDATE ACCOMPLISHED PER SOP. BOTH NAV RADIOS IN AUTO. LNAV ENGAGED AT 400 FT. I BEGAN FOLLOWING LNAV TRACK WHICH STARTED THE ACFT IN A SLIGHT L TURN. MYSELF; CAPT; AND ATC ALL SEEMED TO SEE THE PROB AT ABOUT THE SAME TIME. ATL DEP OR TWR; CANNOT REMEMBER; GAVE NEW ASSIGNED HDG TO FLY. COMPLIED WITH AT ONCE. EVENTUALLY ENDED UP SLIGHTLY N OF TRACK ON ATC HDG. THEN RECLRED DIRECT SNUFY. WE WERE VERY CLOSE TO SNUFY AT TIME OF CLRNC. I PROCEEDED DIRECT SNUFY AND THEN ATTEMPTED TO MAINTAIN LNAV TRACK FOR CADIT4. OUR ANGLE AND CLOSENESS TO SNUFY MADE MAINTAINING DESIRED TRACK VIRTUALLY IMPOSSIBLE. DEP THEN ASSIGNED NEW NE HDG AND THEN RECLRED US TO A NEW FIX ON THE DEP; WHICH I CANNOT REMEMBER. THERE MUST HAVE BEEN SOME TYPE OF MAP SHIFT SHORTLY AFTER TKOF. ATC ASSIGNED HDGS TO CORRECT OUR PATH AND ALL NEW HDGS/INSTRUCTIONS WERE COMPLIED WITH.

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.