On April 6, 1997, approximately 1348 Pacific daylight time, a Beech 1900D, N75ZV, owned by Mesa Airlines and operating as Air Shuttle flight 7793 (a United Express flight), was substantially damaged when the aft cargo door opened on takeoff from Portland International Airport, Portland, Oregon. The 14 CFR 121 scheduled domestic passenger/cargo flight, with a planned destination of Yakima, Washington, returned to Portland and landed without further incident. There were no injuries to the airline transport pilot-in-command, first officer, or 12 passengers on board. Visual meteorological conditions existed at the time, and the flight was on an instrument flight rules (IFR) flight plan. The Mesa ramp agent who handled the departing flight from Portland reported that as the last passenger was boarding the aircraft, additional baggage was brought to the aircraft which necessitated reopening the aft cargo door, which had already been closed for departure. The ramp agent stated that after loading the bags, she took the baggage slip to the captain while the first officer closed the aft cargo door. She stated that "as the aft of the plane passed by me upon my releasing them to taxi I did not notice anything different, all doors were closed." Another Mesa ramp agent who saw the aircraft taxi out reported that as the aircraft passed by her, she "seemed to notice that the aft lock looked unlocked but I was bringing in a plane at the same time and was concerned with that plane." The captain reported that as the gear was brought up on takeoff, the flight crew heard a "loud wind noise." He stated that they had no indications of anything wrong on the annunciator panel (which contains a red CARGO DOOR warning light to indicate that the cargo door is open or not secure.) He reported that they then looked in the back of the aircraft, and the passengers told them that the cargo door had come open. The crew then returned to Portland and landed. An inventory of the cargo compartment contents after landing revealed that no baggage had been lost. The FAA inspector assigned to the accident reported that upon post-accident examination of the aircraft, the aft cargo door locking handle was observed in the unlocked position as the door was swung down, and that visual inspection of the aft cargo door locks, switches, and annunciator cargo door warning systems found no discrepancies with the aft cargo door or its indicating/warning system. The Mesa station operations manual assigns responsibility to the ramp agent handling a flight to check that cargo doors are locked during a pre-departure walkaround. The aircraft's flight crew checklist also instructs the flight crew to check that cargo and cabin doors are secured, and to check the warning annunciator lights, before engine start.