had filed the cargo door's locking pins down to less than a quarter of an inch (), when they experienced difficulty closing the door. Subsequent investigative tests proved the door yielded to approximately of pressure, in contrast to the that it had been designed to withstand. Cause The passenger doors on the DC-10 are inward-opening plug doors, designed to prevent opening while the aircraft is pressurized. However, due to its large radius, a cargo hatch on the DC-10 could not open inside the fuselage without taking up valuable cargo space, so the hatch was designed to open outward, allowing cargo to be stored directly behind it. The outward-opening design presents the risk of the hatch being blown open by the pressure inside the cargo area if the latch were to fail during flight. To prevent this from happening, the DC-10 uses a latching system whose main security principle is an "over-center concept": four C-shaped latches mounted on a common torque shaft are rotated over latching pins ("spools") fixed to the aircraft fuselage. The rotating movement of the torque shaft is brought about by an electric actuator, through a linkage that includes a crankshaft that ensures the "over-center" position of the whole system. Due to the shape of the latches and the over-center design, when the latches are in the correct position, internal pressure on the hatch not only produces insufficient torque to open the hatch, but it also makes the whole system safer because the over-center safety principle is increased. The system has a hand crank provided as a backup. To ensure this rotation was complete and the latches were in the proper position, the DC-10 cargo hatch design included a separate locking mechanism that consisted of small locking pins that slid behind flanges on the lock torque tube (which transferred the actuator force to the latch hooks through a linkage). When the locking pins were in place, any rotation of the latches would cause the torque tube flanges to contact the locking pins, making further rotation impossible. The pins were pushed into place by an operating handle on the outside of the hatch. If the latches were not properly closed, the pins would strike the torque tube flanges and the handle would remain open, visually indicating a problem. Additionally, the handle moved a metal plug into a vent cut in the outer hatch panel. If the vent was not