Boeing 727
The 727 was meant for small airports. This meant that it would have to do without some things like stairs. Boeing built airstairs into the 727. At first, the airstairs could be opened while the plane was flying. However, D. B. Cooper, who was a hijacker, jumped out of a 727. This happened while it was flying over the Pacific Northwest. Boeing then changed the design so that the airstairs could not be opened while the plane was flying.  The 727 also had an auxiliary power unit (APU). This meant that the 727 could run its systems without any power from the ground. The APU is near the undercarriage. This is quite unusual.