What happened
Shortly after takeoff, at an altitude of approximately 800 feet AGL, the main passenger cabin door of the aircraft type opened. Following the incident, the airplane returned to the airport for landing. During the final approach, the door separated from the aircraft.
The flight attendant reported hearing a very loud "humming noise" emanating from the door immediately before it opened. There were no injuries reported during the event.
The investigation
Investigators determined that all indications suggested the door had been closed and locked prior to takeoff. However, the lack of structural damage to the aircraft indicated that the door was actually unlocked or unlatched at the moment it opened.
The aircraft was equipped with a newly designed handrail/door handle that reversed the direction of motion required to lock and unlock the door. This specific design had been installed on fourteen airplanes within the fleet. The flight attendant involved had operated this new handle on 16 of her 60 total flights since being hired. On the day of the incident, the crew member had been on duty for approximately 14.5 hours and had only received five hours of sleep the previous night due to her flight schedule.