Narrative:

No traffic conflicts. We were cleared for the mountain 6 departure. After nichy we expected to get vectors to join J90 to mwh (moses lake VOR).we departed RW16L with the navigation mode selected so the aircraft would follow the depicted track- which it did until we passed nichy. As the pilot flying (hand-flying); I expected the aircraft to turn to heading 070 with navigation selected. Our aircraft was very light and the temperature was cool; so aircraft performance was much greater than usual. This added to the error because instead of directing the proper amount of attention to navigation situational awareness; I spent more time focusing on aircraft control. After correcting a minor airspeed deviation; I rechecked our position and noticed that we weren't turning as I expected. I noticed that the lateral mode had reverted to manual heading. Confused as to why it reverted to heading; I reselected navigation- the only problem being the way that the FMS builds the departure and leaves a flight pln discontinuity since that is where the SID terminates leaving the pilot to have to select heading manually. Anyway; I reconfirmed the 070 heading off the SID page and initiated the left turn. We were climbing out of around 10;000 ft and flew past nichy on a 161 heading (approximate) by maybe 1 or 2NM. ATC queried us just as I initiated the left turn and gave us a heading on course followed by a clearance to proceed to normy then flight plan route.human factors- I briefed the departure but since the mountain 6 departure is a 'navable' SID; I orally reviewed the SID and did a quick comparison of the text versus what the FMS showed but I missed the fact that the heading didn't show up on the FMS. I relied on the fact that the FMS would take fly the SID exactly as published. The captain said that he should have caught my mistake and that may be true; but missing something that basic is embarrassing. So; I rushed through the briefing; didn't compare the FMS to the text of the SID; and then I hand-flew a very high performance aircraft(climbing at >6000 FPM). I wish I would have given more consideration to aircraft performance and how that might complicate our departure. If I had selected the ap on; I would have caught the lateral mode reverting to manual heading. The early departure may have contributed but I felt rested. The captain seemed to be well-rested too. In the future; I will use a more disciplined and methodical approach to the accomplishment of my duties.

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Original NASA ASRS Text

Title: An air carrier pilot programmed his aircraft's FMC for the SEA Mountain 6 Departure but forgot to close the route discontinuity where the SID ends and cleared route begins. Therefore the aircraft reverted to HEADING HOLD and continued on course until the pilot turned and selected an active way point.

Narrative: No traffic conflicts. We were cleared for the Mountain 6 departure. After NICHY we expected to get vectors to join J90 to MWH (Moses Lake VOR).We departed RW16L with the NAV mode selected so the aircraft would follow the depicted track- which it did until we passed NICHY. As the pilot flying (hand-flying); I expected the aircraft to turn to heading 070 with NAV selected. Our aircraft was very light and the temperature was cool; so aircraft performance was much greater than usual. This added to the error because instead of directing the proper amount of attention to navigation situational awareness; I spent more time focusing on aircraft control. After correcting a minor airspeed deviation; I rechecked our position and noticed that we weren't turning as I expected. I noticed that the lateral mode had reverted to manual heading. Confused as to why it reverted to heading; I reselected NAV- the only problem being the way that the FMS builds the departure and leaves a FLT PLN discontinuity since that is where the SID terminates leaving the pilot to have to select heading manually. Anyway; I reconfirmed the 070 heading off the SID page and initiated the left turn. We were climbing out of around 10;000 FT and flew past NICHY on a 161 heading (approximate) by maybe 1 or 2NM. ATC queried us just as I initiated the left turn and gave us a heading on course followed by a clearance to proceed to Normy then flight plan route.Human Factors- I briefed the departure but since the Mountain 6 departure is a 'NAVable' SID; I orally reviewed the SID and did a quick comparison of the text versus what the FMS showed but I missed the fact that the heading didn't show up on the FMS. I relied on the fact that the FMS would take fly the SID exactly as published. The captain said that he should have caught my mistake and that may be true; but missing something that basic is embarrassing. So; I rushed through the briefing; didn't compare the FMS to the text of the SID; and then I hand-flew a very high performance aircraft(climbing at >6000 FPM). I wish I would have given more consideration to aircraft performance and how that might complicate our departure. If I had selected the AP on; I would have caught the lateral mode reverting to manual heading. The early departure may have contributed but I felt rested. The captain seemed to be well-rested too. In the future; I will use a more disciplined and methodical approach to the accomplishment of my duties.

Data retrieved from NASA's ASRS site as of April 2012 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.