Almost all narrowbody jetliners, and many widebodies as well, use overwing exits to augment their evacuation capabilities; to escape the aircraft, a passenger or crewmember in the exit row pops the hatch next to their seat, and then everyone else1 chases them out onto the wing.
At this point, depending on the aircraft, they may simply slide down the flaps to safety, or a slide might inflate if the wings are too high above the ground. Either way, though, they’ll be going over the trailing edge of the wings if it’s a land evacuation;2 even if the aircraft has multiple overwing exits per side (common among the larger jetliners), or if there’s a tail-mounted engine waiting to gobble up trailing-edge evacuees (like with the Fokker 70), everyone going out on the wings still evacuates off the trailing edge.
I know of only a few airliners with overwing exits that specifically direct evacuees over the leading edge of the wings, and two of these are very special cases:
- The Concorde’s wings extend for most of the length of its fuselage, so it would be essentially impossible to fit enough exits in without having some going over the leading edge.
- The Il-62 has a set of engines so close behind the wings that even the emergency-exit designers realised it would be a bad idea to send evacuees over the trailing edge (also, its overwing exits don’t have slides - instead, you have to slide down a rope over the leading edge of the wing).
- The DC-10/MD-11 family has overwing evacuation slides that go over the leading edge of the wing, as do some A310s, making these, so far as I am aware, the only subsonic airliners without engines right behind the wings to use leading-edge overwing exits.
For low-slung narrowbodies, where you slide down the flaps to get to the ground, the predominance of trailing-edge overwing exits is excusable, as the flaps are on the trailing edge of the wing; however, even for large narrowbodies and widebodies with inflatable overwing evacuation slides, the slides still overwhelmingly open rearwards, over the trailing edge.
Why are overwing exit slides leading off the trailing edge of the wings so much more common than ones that go out over the leading edge?
1: I exaggerate slightly; some of the passengers will likely lock themselves in the lavatories or run up and down the aisle(s) screaming like chickens with their heads cut off, and one or two might even use the exit doors at the front and/or rear of the plane.
2: Ditchings work somewhat differently, and some aircraft do call for occupants to fall off the leading, rather than trailing, edge in this case.