Narrative:

We were assigned the triss 1 RNAV departure as the first part of the our clearance. During preflight; I had noted that all the SID fixes depicted on the chart agreed with what was in the FMS and that prchdg displayed on the pfd. I had also noted that Q values were good. During taxi; it was re-verified that the departure page showed the correct runway and first fix and everything looked correct on the mfd. After takeoff; I called for navigation to be selected and began following the course indication on my pfd. Almost immediately I noticed that it seemed like the turn toward navie started early; but the urge to follow course guidance is pervasive; so I followed. Pretty quickly; the first officer asked where I was going. I immediately glanced at his pfd and saw it indicated I was off course to the left. I quickly determined; partly by my location visually; that his pfd was giving the correct course guidance and I turned toward it and selected cross side data of his navigation on my pfd. I can't say if we were still on tower frequency when this error was discovered or if we had already gone to departure; but nobody from ATC ever said anything to us about being off course. Perhaps we were not off course enough for them to mention it. I did happen to notice that the off course distance stated on the first officer's mfd when I first looked at it showed .60 of a mile. Perhaps ATC felt it was not enough to mention as we were off course to the east and therefore away from the opposite side departure corridor. Perhaps nobody at ATC noticed. Perhaps the error was noted and something will be said later. In any case we returned to the correct course as quickly as possible. It would appear that this event happened because; although I thought I had done a complete preflight of the FMS; in retrospect I found I had missed one thing. Of course we wanted to know why we were getting bad data from the #1 FMS and we began to check everything out. Quickly; it was observed that on the navigation 2/2 page; while the Q still showed okay at Q=4; the navigation mode was not GPS; it was DME. I had failed to observe this when I checked the Q during the preflight. I am now pretty sure that it must have been this way during the preflight as when I went to data 2/4 I found that both GPS inputs had been deselected. I have no idea who might have done this or when it might have been done; but after I reselected the GPS inputs they stayed selected for the rest of the time I had the aircraft and GPS data stayed accurate and/or agreed with the other side's data. The only other thing I am not sure about is while this seemed to be the problem and the unit appeared to give very accurate data after this problem was corrected; according to aom volume 1 pre-flight; I should have gotten good guidance even with one FMS in DME mode. It says; 'if one FMS does not indicate 'GPS;' it will automatically use the GPS signal from the other FMS and either FMS can be used to fly the RNAV SID.' this does not appear to have been correct in this case. My solution for avoiding a recurrence of this event is to make sure I notice the navigation mode and not just the Q value during my preflight preparation.

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Original NASA ASRS Text

Title: An Embraer departed DFW with the flying pilot's FMS not utilizing GPS and had a track deviation immediately after takeoff; but corrected quickly enough that ATC did not comment.

Narrative: We were assigned the TRISS 1 RNAV departure as the first part of the our clearance. During preflight; I had noted that all the SID fixes depicted on the chart agreed with what was in the FMS and that PRCHDG displayed on the PFD. I had also noted that Q values were good. During taxi; it was re-verified that the departure page showed the correct runway and first fix and everything looked correct on the MFD. After takeoff; I called for NAV to be selected and began following the course indication on my PFD. Almost immediately I noticed that it seemed like the turn toward NAVIE started early; but the urge to follow course guidance is pervasive; so I followed. Pretty quickly; the First Officer asked where I was going. I immediately glanced at his PFD and saw it indicated I was off course to the left. I quickly determined; partly by my location visually; that his PFD was giving the correct course guidance and I turned toward it and selected cross side data of his NAV on my PFD. I can't say if we were still on Tower frequency when this error was discovered or if we had already gone to Departure; but nobody from ATC ever said anything to us about being off course. Perhaps we were not off course enough for them to mention it. I did happen to notice that the off course distance stated on the First Officer's MFD when I first looked at it showed .60 of a mile. Perhaps ATC felt it was not enough to mention as we were off course to the east and therefore away from the opposite side departure corridor. Perhaps nobody at ATC noticed. Perhaps the error was noted and something will be said later. In any case we returned to the correct course as quickly as possible. It would appear that this event happened because; although I thought I had done a complete preflight of the FMS; in retrospect I found I had missed one thing. Of course we wanted to know why we were getting bad data from the #1 FMS and we began to check everything out. Quickly; it was observed that on the NAV 2/2 page; while the Q still showed okay at Q=4; the NAV mode was not GPS; it was DME. I had failed to observe this when I checked the Q during the preflight. I am now pretty sure that it must have been this way during the preflight as when I went to DATA 2/4 I found that both GPS inputs had been deselected. I have no idea who might have done this or when it might have been done; but after I reselected the GPS inputs they stayed selected for the rest of the time I had the aircraft and GPS data stayed accurate and/or agreed with the other side's data. The only other thing I am not sure about is while this seemed to be the problem and the unit appeared to give very accurate data after this problem was corrected; according to AOM volume 1 PRE-FLIGHT; I should have gotten good guidance even with one FMS in DME mode. It says; 'If one FMS does not indicate 'GPS;' it will automatically use the GPS signal from the other FMS and either FMS can be used to fly the RNAV SID.' This does not appear to have been correct in this case. My solution for avoiding a recurrence of this event is to make sure I notice the NAV mode and not just the Q value during my preflight preparation.

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.