B737-300 flight crew departing Runway 18L at CLT via the MERIL 6 RNAV experiences a track deviation when the autopilot is engaged at 1;000 FT AGL. The flying First Officer aggressively returns the aircraft to the desired track as ATC is noting the deviation.
Synopsis
B737-300 flight crew departing Runway 18L at CLT via the MERIL 6 RNAV experiences a track deviation when the autopilot is engaged at 1;000 FT AGL. The flying First Officer aggressively returns the aircraft to the desired track as ATC is noting the deviation.
Narrative
Runway 18L Meril 6 RNAV departure to HISOR; First Officer was flying. Departure was briefed at gate; we both verified heading; altitude; departure procedure. VNAV was called for and engaged at 400 FT. Autopilot was engaged at 1;000 FT. First Officer called for flaps 1. Just as I selected flaps 1 I noticed the control wheel roll to the right. I was surprised as it is basically a straight out departure. As I looked at the compass; the Autopilot press to reset light started flashing in front of me and I noticed that our heading was about 200-205. The First Officer had overridden the autopilot and was initiating a turn back to the departure course. At the same time the Controller was telling us to turn left 20 degrees; that we had traffic to our right departing off of 18C. We both saw the other aircraft slightly ahead at about our altitude. We never got a warning from the TCAS. We then recoupled the autopilot and resumed the departure with no further problems. I have no idea as to why this happened. Everything was set up correctly and we followed all of the RNAV departure procedures. As I first noticed the wheel rolling to the right I thought that it might be correcting for the strong westerly winds that were programmed into the FMC for the flight; but it way over-corrected if that was the case. As I don't know why this happened I can't really suggest anything. The First Officer did an excellent job of catching the problem as quickly as he did and over-riding the autopilot.
Second reporter narrative
As the pilot flying; I programmed the FMC at the gate. All the departure waypoints and routing was verified by the Captain. Taking the runway I accomplished the runway update procedure and we were cleared for takeoff; RNAV to HISOR. At 400 FT I called for L NAV which was engaged by the Captain. At 1;000 FT I called for 'climb thrust; set 210 and flaps one'. At the same time I engaged the auto pilot B channel. For a second or two the auto pilot kept wings level; but then it started an aggressive right turn. I immediately reached for the yoke and using the CWS mode of the auto pilot; I aggressively started correcting back to the left towards our course. By the time the aircraft reversed rolling from right to left we were about 20 degrees off course; about on a 200 heading. I was already in the left turn correcting; when Tower gave us a 20 degree left turn. We saw departing traffic on 18C; slightly ahead of us as we were turning away from it. We never got a TCAS warning. I then reengaged the auto pilot and continued the departure without any further problems. I cannot give a logical explanation as to why the autopilot turned right; especially since all heading pointers and course indicators were checked and verified by the both of us at the gate and lining up on the runway.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.