One of 4 groups on the tour, listening to a flight engineer describing the operation and performance of the sliding door on the roof of the plane (bulge in fuselage behind wing). The door slides from port side across the top and over to starboard side. Significant effort was expended designing the bulge and door so that it does not affect the aerodynamics of the plane while in flight. Several tests were performed landing the plane while the door was open, which prepared for some later episodes where the door malfunctioned mid-flight, requiring an 'open-door' landing.