Flying a 737-200 on a 1000 mile leg over the South Pacific. Blue skies. Visited the cabin and got talking, leaving the experienced co-pilot up front. On return to cockpit I noticed the copilot reading a magazine which is why he had not noticed we were gently going off track. I checked the cross track reading on the INS and saw it read 25 miles off track. Noticed the FD 108 artificial horizon showing about 6 degrees from level flight and aircraft heading 30 degrees from flight plan heading.
What happened was slight turbulence had occurred during the time I was down the back but was corrected by the autopilot. The bank angle corrections showed on the ADI. When the turbulence stopped the AP was still in the process of correcting back to level flight with the ADI showing 6 degrees angle of bank. Inside 6 degrees angle of bank the autopilot will automatically go wings level but when the turbulence stopped the angle of bank was close to 6.5 degrees and stayed there. The prolonged angle of bank of 6.5 went unnoticed by the co-pilot and gyro drift started. We manually returned to track while the ADI continued to display slight bank angle for the rest of the flight.
Last edited by Centaurus; 14th Mar 2018 at 10:32.