You're right wing fences were added to wings to stop span wise flow. This is somewhat of a low speed problem.
Many modern airliners with swept wings have engines mounted on pylons protruding forward. This helps to generate a vortex over the top of the wing. The vortex acts like an aerodynamic wing fence.
It is also possible to have a notch somewhere along the leading edge. This also does the same as above.
I'm not sure what the likes of an MD11 have though.
The pylon protruding forward of the wing helps to balance the wing during high speed flight and prevent twisting.
Last edited by wobblyprop; 10th Oct 2002 at 14:23.