Cecil mentioned D. B. Cooper’s famous exit from a Boeing 727 during his hijacking. A number of years ago I toured the United Airlines maintenance facility at SFO. The tour guide pointed out the devise used to keep the rear hatch closed while the plane is flying. They call the devise a ‘Coppervain’ in honor of D. B. The Coopervain is just a flap that hands down when the plane is not moving but once the plane is moving the flap pushes against the hatch and prevents it from opening. The faster the plane moves, the more force the Coopervain exerts on the hatch. Pretty nifty.
I don’t recall the exactly which way the emergency exit door on airliners open but on almost all small planes (and cars for that matter) the door hinge is located on the forward part of the vehicle. This means that the further the door is opened, the more wind resistance is created. At airplane speeds the door is almost impossible to open especially if the door has a large surface area.
I was once in a small plane when the door became unlatched. There was no way I could latch the door because the force to open it was just too strong at 100mph.