Narrative:

We were assigned the cowboy departure, expecting runway 25L. When we were cleared to taxi, the runway assignment changed to runway 19L. We completely reprogrammed the runway and departure, verifying the fixes on departure. At 400 ft AGL, LNAV was selected, and 'legs page.' at 1000 ft AGL, I called for 'flaps 1 degree, climb thrust' and selected the 'a' autoplt. Even though the 'legs page' showed the turn in 4 mi, the aircraft started an immediate right turn. I deselected autoplt and reselected heading select. We continued the departure flying route manually with information from the legs page. Overhead 'fixix' we turned toward roppr (from 190 degrees toward 235 degree heading). Las approach asked if we were in turn toward roppr and we responded 'yes.' he then assigned us a heading of 210 degrees. We were passing 200 degree heading when this was assigned. The deviation was minor, if any, and the controller stated there was 'no problem.' in conclusion, the aircraft began a turn about 4 mi early. We reviewed our setup and process. Every fix was listed and in the appropriate order, off the appropriate runway. The pilot monitoring was quick to verbalize the error of early turn and the PF simply hand flew the course.

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Original NASA ASRS Text

Title: HAVING DIFFICULTY WITH LNAV AND AUTOPLT ON THE LAS COWBY ONE RNAV DEP A B737-300 CREW TURNS OVER THE 'FIXIX' FIX CAUSING A MINOR DEV NOTED BY AND CORRECTED WITH A VECTOR BY ATC.

Narrative: WE WERE ASSIGNED THE COWBOY DEP, EXPECTING RWY 25L. WHEN WE WERE CLRED TO TAXI, THE RWY ASSIGNMENT CHANGED TO RWY 19L. WE COMPLETELY REPROGRAMMED THE RWY AND DEP, VERIFYING THE FIXES ON DEP. AT 400 FT AGL, LNAV WAS SELECTED, AND 'LEGS PAGE.' AT 1000 FT AGL, I CALLED FOR 'FLAPS 1 DEG, CLB THRUST' AND SELECTED THE 'A' AUTOPLT. EVEN THOUGH THE 'LEGS PAGE' SHOWED THE TURN IN 4 MI, THE ACFT STARTED AN IMMEDIATE R TURN. I DESELECTED AUTOPLT AND RESELECTED HDG SELECT. WE CONTINUED THE DEP FLYING RTE MANUALLY WITH INFO FROM THE LEGS PAGE. OVERHEAD 'FIXIX' WE TURNED TOWARD ROPPR (FROM 190 DEGS TOWARD 235 DEG HDG). LAS APCH ASKED IF WE WERE IN TURN TOWARD ROPPR AND WE RESPONDED 'YES.' HE THEN ASSIGNED US A HDG OF 210 DEGS. WE WERE PASSING 200 DEG HDG WHEN THIS WAS ASSIGNED. THE DEV WAS MINOR, IF ANY, AND THE CTLR STATED THERE WAS 'NO PROB.' IN CONCLUSION, THE ACFT BEGAN A TURN ABOUT 4 MI EARLY. WE REVIEWED OUR SETUP AND PROCESS. EVERY FIX WAS LISTED AND IN THE APPROPRIATE ORDER, OFF THE APPROPRIATE RWY. THE PLT MONITORING WAS QUICK TO VERBALIZE THE ERROR OF EARLY TURN AND THE PF SIMPLY HAND FLEW THE COURSE.

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.