Narrative:

During north departure from sea; aircraft committed to early turn approximately 1.5 NM prior to designated fix. ATC reported numerous aircraft had executed similar maneuver that morning and asked why the early turn. During preflight of FMS; it was noted that in spite of a runway entry of runway 34R; the mount 5 departure would display a southerly departure to nichy with an east vector over nichy. The correct departure should have displayed a northerly departure over nezbug and then an east vector. To correct the error manually required entering sea to nezbug on the legs page and deleting the southerly routing. The 'gotcha' is that the easterly turn at nezbug to a heading of 070 degrees; if displayed on the FMC; results in an anticipatory lead turn to the easterly heading and does not allow for an 'overfly' of nezbug. Since this is not an RNAV departure and had to be entered manually; the error wasn't recognized until the aircraft had already started the easterly turn. The turn was shallowed but apparently it had already been observed by ATC. After having confirmed the software error by re-entering the departure in route #2 of the FMC; dispatch was advised and it was recommended to disseminate the information as soon as possible. Further discussion with ATC via landline also suggested that no action was pending and that they appreciated the explanation.

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Original NASA ASRS Text

Title: A B757-200 DEPARTING SEA ON THE MOUNT 5 DEP DEVIATED FROM THE PUBLISHED TRACK WHEN THE FMC COMMANDED AN EARLY TURN OVER A FIX.

Narrative: DURING N DEP FROM SEA; ACFT COMMITTED TO EARLY TURN APPROX 1.5 NM PRIOR TO DESIGNATED FIX. ATC RPTED NUMEROUS ACFT HAD EXECUTED SIMILAR MANEUVER THAT MORNING AND ASKED WHY THE EARLY TURN. DURING PREFLT OF FMS; IT WAS NOTED THAT IN SPITE OF A RWY ENTRY OF RWY 34R; THE MOUNT 5 DEP WOULD DISPLAY A SOUTHERLY DEP TO NICHY WITH AN E VECTOR OVER NICHY. THE CORRECT DEP SHOULD HAVE DISPLAYED A NORTHERLY DEP OVER NEZBUG AND THEN AN E VECTOR. TO CORRECT THE ERROR MANUALLY REQUIRED ENTERING SEA TO NEZBUG ON THE LEGS PAGE AND DELETING THE SOUTHERLY ROUTING. THE 'GOTCHA' IS THAT THE EASTERLY TURN AT NEZBUG TO A HDG OF 070 DEGS; IF DISPLAYED ON THE FMC; RESULTS IN AN ANTICIPATORY LEAD TURN TO THE EASTERLY HDG AND DOES NOT ALLOW FOR AN 'OVERFLY' OF NEZBUG. SINCE THIS IS NOT AN RNAV DEP AND HAD TO BE ENTERED MANUALLY; THE ERROR WASN'T RECOGNIZED UNTIL THE ACFT HAD ALREADY STARTED THE EASTERLY TURN. THE TURN WAS SHALLOWED BUT APPARENTLY IT HAD ALREADY BEEN OBSERVED BY ATC. AFTER HAVING CONFIRMED THE SOFTWARE ERROR BY RE-ENTERING THE DEP IN RTE #2 OF THE FMC; DISPATCH WAS ADVISED AND IT WAS RECOMMENDED TO DISSEMINATE THE INFO ASAP. FURTHER DISCUSSION WITH ATC VIA LANDLINE ALSO SUGGESTED THAT NO ACTION WAS PENDING AND THAT THEY APPRECIATED THE EXPLANATION.

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.