When you reach the plane go to the tail section.

Jump onto the seat on the right. Face the back of the plane and repeatedly press the jump button.

If you are standing in the right position, you are able to jump through the tear in the plane and onto the roof.

Wait for everyone to get up there and then proceed on. You don't have to open the plane door so the horde will not come.

If you are playing single player, the bots will not follow you, but they will come when you reach the first building.