Narrative:

Cleared for the detus 1E STAR LNAV turned us appropriately on this arrival from a 359 degree heading to a 171 degree heading to intercept the localizer to runway 17. LNAV should fly you precisely into the ILS course on this arrival as published. The FMC looked approx like this: 359* detus 120B 32nm 359* D333L 9nm 171* C117 2500A 171* OM17 1500 171* RW17 missed approach.... Basically there is a 9 NM LNAV course that reverses our course from 359* to 171*. In our case; LNAV turned us properly towards the inbound course but turned us inside the localizer course. Once established on 171*heading; we were about 1 quarter mile left of the desired track. The aircraft computed the error and made a 35* left turn towards the active way point which in this case was C117. This was to intercept the ILS. (We were flaps 15 and 170 KIAS). A way to solve this would be to have a conditional waypoint installed in the database in the middle of the turn. The FMC can't keep up with the variable winds as we were descending and turning. (This similar thing happens in mexico city.) this 9 mile 180* turn is pretty simple. Maybe heading select would be an option to capture the localizer. The controller notified us that we were turning inside the desired track as the LNAV was turning us back towards the localizer. I don't believe that there was a foul here but wanted to bring this to your attention for publication. Monitoring progress page 2 x-track is a way to monitor this too. I fly this trip regularly. This is the first time I have ever seen a GPS; triple mixed 767 do this. There were no route discontinuities. We were slow and methodical. The magenta line showed a complete course to the IAF with no undershoot or overshoot.

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Original NASA ASRS Text

Title: B767 Captain reports small navigation error by the FMC when transitioning from the DETUS 1E Arrival to the LOC RWY 17 at ESMS; FMC self corrected to the LOC.

Narrative: Cleared for the DETUS 1E STAR LNAV turned us appropriately on this arrival from a 359 degree heading to a 171 degree heading to intercept the Localizer to Runway 17. LNAV should fly you precisely into the ILS course on this arrival as published. The FMC looked approx like this: 359* Detus 120B 32nm 359* D333L 9nm 171* C117 2500A 171* OM17 1500 171* RW17 Missed approach.... Basically there is a 9 NM LNAV course that reverses our course from 359* to 171*. In our case; LNAV turned us properly towards the inbound course but turned us inside the localizer course. Once established on 171*heading; we were about 1 quarter mile left of the desired track. The aircraft computed the error and made a 35* left turn towards the active way point which in this case was C117. This was to intercept the ILS. (We were flaps 15 and 170 KIAS). A way to solve this would be to have a conditional waypoint installed in the database in the middle of the turn. The FMC can't keep up with the variable winds as we were descending and turning. (This similar thing happens in Mexico City.) This 9 mile 180* turn is pretty simple. Maybe heading select would be an option to capture the localizer. The controller notified us that we were turning inside the desired track as the LNAV was turning us back towards the Localizer. I don't believe that there was a foul here but wanted to bring this to your attention for publication. Monitoring Progress Page 2 X-track is a way to monitor this too. I fly this trip regularly. This is the first time I have ever seen a GPS; triple mixed 767 do this. There were no route discontinuities. We were slow and methodical. The magenta line showed a complete course to the IAF with no undershoot or overshoot.

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.