We have 10 738 in our fleet and and problem same with all.The slight left bank occur on autopilot A and B either manual flight when the put 40 flap.I dont have classic 737 experience and this is my second jet after MD83.I heard
both wing dihedral angle are not same.Very small different between both dihedral angle and it makes small bank when use of 40 flap.Did u hear anything abaout it before?