Pilot reported that a malfunction caused a door to open in flight.

Attributes
ACN	1710720
Time
Date	201912
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Cessna Aircraft Undifferentiated or Other Model
Operating Under FAR Part	Part 135
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Door
Person 1
Function	Captain Pilot Flying
Events
Anomaly	Aircraft Equipment Problem Critical

Narrative:

During climb out at approximately 1;500 ft. AGL the door warning annunciator actuated. No obvious door abnormalities were observed by the flight crew and the flight continued to ZZZ. During descent into ZZZ the top clam shell hatch on the main cabin door opened. PIC was entering a left downwind to runway xx when the door opened. Passengers were instructed to remain calm and do not touch the door. PIC [advised ATC] to ensure priority to the runway. Landed without incident.

Original NASA ASRS Text

Title: Pilot reported that a malfunction caused a door to open in flight.

Narrative: During climb out at approximately 1;500 ft. AGL the door warning annunciator actuated. No obvious door abnormalities were observed by the flight crew and the flight continued to ZZZ. During descent into ZZZ the top clam shell hatch on the main cabin door opened. PIC was entering a left downwind to Runway XX when the door opened. Passengers were instructed to remain calm and do not touch the door. PIC [advised ATC] to ensure priority to the runway. Landed without incident.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.