Date: 201707
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Super King Air 200
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Exterior Pax/Crew Door
Component problem: Improperly Operated
Reporter organization: Corporate
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Training / Qualification
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Passenger; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Startup and taxi to runway was uneventful. All checklists were complied with. We were cleared for takeoff on runway XX. At approx 600 ft the whole cabin became very loud with the sound of air. I looked back in the cabin and the passengers were pointing towards the door. They were all safely buckled in their seats. We told the Tower we needed to return for landing. They asked why and the PNF (Pilot Not Flying) told them for a door issue. We made left traffic; ran the before landing checklist and landed. After taxi in and shut down we inspected the door. All seemed fine; with the door operating normally. We contacted our mechanic and also the service center we use for the aircraft. The door closed normally; with the door warning light extinguished when the door was closed and locked. The PNF then stated he suspects he may have closed but not fully locked the door. When he closed it; it came up faster and with more force than he anticipated. This subsequently disrupted his routine locking of the door. Then; on initial climb out when the aircraft started to pressurize the door came open. After further conferring with our mechanic and the service center; we decided to continue our flight unpressurized just to be on the safe side. The flight was uneventful. We then brought the aircraft to the service center for them to inspect the door and the warning light and rigging. They could not find any issues. Going forward; I am determined to always double check the door and visually confirm the green bars that show when the door is locked.


Second narrative: I was the person who closed the main cabin door after boarding. The door did close faster than I had anticipated and strongly slammed shut which took me by surprise. Normally; the door handle must be partially turned to fully retract the door latches prior to moving the door flush with the fuselage and fully engaging the latches. The force of the door closing made this unnecessary and in my shock I mentally registered the door as latched and went to the cockpit I believe without subsequently moving the door handle to the closed position. This allowed the door to open in flight after the aircraft began to pressurize. In the future I will confirm the proper position of the door handle and green indicators on the door latches.


Synopsis: King Air 200 flight crew reported the main cabin door opening during climb out. The crew returned to the departure airport.