to be closed manually. On 30 May, McDonnell Douglas issued Service Bulletin 52–27, DC-10 SC 612, which called for the upgrading of the electrical wiring that drove the latches because "Three operators have reported failure of the electrical latch actuators to latch/unlatch the cargo doors. Latch actuator failure is attributed to an excessive voltage drop reducing the output torque to the actuator. This condition may prevent electrical latching/unlatching of the hooks." The modification was not compulsory, however, and had not been carried out on N103AA, the plane involved in the accident. Investigators interviewed the ground crew in Detroit and learned that the cargo loader who operated the rear door had found it extremely difficult to close. He stated that he closed the door electrically, and waited for the sound of the actuator motors to stop. When they did, he attempted to operate the locking handle but found it very difficult to close. He managed to get the latch to lock only by applying force with his knee, but he noticed that the vent plug (see below) was not entirely closed. He brought this to the attention of a mechanic, who cleared the flight. The flight engineer reported that the "door ajar" warning light on his panel was not lit at any time during the taxi out or flight. Examination of the aircraft and the cargo door, which was recovered largely intact in Windsor, demonstrated that the latches had never rotated to their locked position. In their locked position, pressure on the door presses the latches further shut, and no force is transmitted into the actuator system that closes and opens them. With the latches only partially closed, forces on the door were transmitted back into the actuator, eventually overwhelming it at about 6,600 lbf. The rapid depressurization when the door broke off caused the floor above it to partially cave in, which pulled the rudder cable to its extension limit and severed several other operating cables. Cause of door failure Passenger doors on the DC-10 are of the plug variety, which prevents the doors from opening while the aircraft is pressurized. The cargo door, however, is not. Due to its large area, the cargo door on the DC-10 could not be swung inside the fuselage without taking up a considerable amount of valuable cargo space. Instead, the door swung outward, allowing cargo to be stored directly behind it. The outward-opening