A B737-300 flying an RNAV departure with no moving map; no GPS and a single FMC had a track deviation at 1;000 FT but the navigation systems indicated normal tracking to the first waypoint while ATC saw it turn early.
Synopsis
A B737-300 flying an RNAV departure with no moving map; no GPS and a single FMC had a track deviation at 1;000 FT but the navigation systems indicated normal tracking to the first waypoint while ATC saw it turn early.
Narrative
[We were] assigned an RNAV Departure and a standard transition. Departure was loaded into FMC and verified by both pilots at the gate. Runway 25R was also verified as the correct departure runway. Runway update was complete at the end of 25R and both NAV switches with placed in the down position. LNAV was selected at 400 FT AGL. At 1;000 FT AGL; LEGS page was selected on the single FMC. Our first waypoint; verified on LEGS page1; was 'magenta' and at the top left position. Command bars began to show a left turn for the waypoint. The wind at the airport was southerly in direction. The FMC showed a 235 heading to the waypoint. I immediately assumed a strong south wind and engaged the autopilot shortly after 1;000 FT. Departure ATC immediately said we made our left turn to early. All Flight Operations Manual procedures were followed and verified. With no MAP and one FMC; we had no choice but to believe the FMC was correct. The First Officer and I have reviewed this over and over. We cannot figure out why this aircraft turned early.
Second reporter narrative
The wind at the time was 200/15 and I thought for a moment that the aircraft was crabbing into the wind to keep its ground track to the first waypoint. I reselected the first waypoint and brought it up to the number one spot and executed it and it still showed a heading of 235 degrees. After some discussion with Departure Control; we made a turn to the left and proceeded to the second point on the route and reconnected the autopilot and proceeded without incident.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.