American Flight 96 had departed Detroit-Metropolitan Airport and was climbing through 11,750 feet at 260 kt IAS when the flightcrew felt a “thud”. Simultaneously dust and dirt flew up into their faces, the rudder pedals moved to the full left-rudder position, the three thrust levers moved back to the near flight-idle position and the airplane yawed to the right. The crew managed to regain control of the plane, although a.o. elevator response was sluggish, and rudder control was not available. An emergency was declared and the crew returned to Detroit. A safe runway 03 long final approach and landing were carried out. It appeared that a cargo door had separated, causing a rapid decompression, which, in turn, caused failure of the cabin floor over the bulk cargo compartment. The separated door caused minor damage to the fuselage above the door and substantial damage to the leading edge and upper surface of the left horizontal stabilizer. PROBABLE CAUSE: “The improper engagement of the latching mechanism for the aft bulk cargo compartment door during the preparation of the airplane for flight. The design characteristics of the door latching mechanism permitted the door to be apparently closed when, in fact, the latches were not fully engaged, and the latch lockpins were not in place.”