Narrative:

During FMC preflight programming; we noticed an altitude constraint at roppr message which would not allow us to activate route. We deleted altitude constraint at roppr and activated route and then put the altitude restr back in at roppr and all seemed fine. We did verify that the cruise altitude was showing FL350 and not 3500 ft. We verified the correct departure with the first fix of rbell and the second fix of roppr; etc. Prior to taking runway 25R for takeoff; the first officer updated the runway and we made sure navigation switches were down. At 400 ft on departure; the first officer selected LNAV and we immediately noticed a CDI deflection to the left of 1 DOT. The first officer was trying to correct the CDI deflection and flying a heading of approximately 230 degrees and he asked me if this looked right. I stated no because we should be on approximately a 255 degree heading. At that time; las tower asked if we were ok because we were 1 mi left of course and I told him we would be unable to continue on the RNAV departure. He gave us a heading towards roppr and I asked him if this roppr was 12 O'clock position and 5 mi as it appeared on the legs page. He got the exact distance and it appeared we were showing about a 1 mi deviation. I do not believe that there was any traffic conflict and las tower made no reference to any problem caused by the deviation. Departure continued vectoring us towards cowby and it appeared that by that time the FMC had updated itself and was giving accurate data. The rest of the flight was uneventful. We don't know what caused the deviation and without a moving map to depict exact layout of the route; it is extremely hard to tell what caused this error.

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Original NASA ASRS Text

Title: B737 FLT CREW REPORTS FMC PROGRAMMING DIFFICULTIES DURING PREFLIGHT; AND A COURSE DEVIATION DURING THE INITIAL SEGMENT OF THE COWBY 2 RNAV DEPARTURE FROM LAS.

Narrative: DURING FMC PREFLT PROGRAMMING; WE NOTICED AN ALT CONSTRAINT AT ROPPR MESSAGE WHICH WOULD NOT ALLOW US TO ACTIVATE RTE. WE DELETED ALT CONSTRAINT AT ROPPR AND ACTIVATED RTE AND THEN PUT THE ALT RESTR BACK IN AT ROPPR AND ALL SEEMED FINE. WE DID VERIFY THAT THE CRUISE ALT WAS SHOWING FL350 AND NOT 3500 FT. WE VERIFIED THE CORRECT DEP WITH THE FIRST FIX OF RBELL AND THE SECOND FIX OF ROPPR; ETC. PRIOR TO TAKING RWY 25R FOR TKOF; THE FO UPDATED THE RWY AND WE MADE SURE NAV SWITCHES WERE DOWN. AT 400 FT ON DEP; THE FO SELECTED LNAV AND WE IMMEDIATELY NOTICED A CDI DEFLECTION TO THE L OF 1 DOT. THE FO WAS TRYING TO CORRECT THE CDI DEFLECTION AND FLYING A HDG OF APPROX 230 DEGS AND HE ASKED ME IF THIS LOOKED RIGHT. I STATED NO BECAUSE WE SHOULD BE ON APPROX A 255 DEG HDG. AT THAT TIME; LAS TWR ASKED IF WE WERE OK BECAUSE WE WERE 1 MI L OF COURSE AND I TOLD HIM WE WOULD BE UNABLE TO CONTINUE ON THE RNAV DEP. HE GAVE US A HDG TOWARDS ROPPR AND I ASKED HIM IF THIS ROPPR WAS 12 O'CLOCK POS AND 5 MI AS IT APPEARED ON THE LEGS PAGE. HE GOT THE EXACT DISTANCE AND IT APPEARED WE WERE SHOWING ABOUT A 1 MI DEV. I DO NOT BELIEVE THAT THERE WAS ANY TFC CONFLICT AND LAS TWR MADE NO REF TO ANY PROB CAUSED BY THE DEV. DEP CONTINUED VECTORING US TOWARDS COWBY AND IT APPEARED THAT BY THAT TIME THE FMC HAD UPDATED ITSELF AND WAS GIVING ACCURATE DATA. THE REST OF THE FLT WAS UNEVENTFUL. WE DON'T KNOW WHAT CAUSED THE DEV AND WITHOUT A MOVING MAP TO DEPICT EXACT LAYOUT OF THE RTE; IT IS EXTREMELY HARD TO TELL WHAT CAUSED THIS ERROR.

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.