Cessna 402 pilot reported the upper clamshell door half opened after takeoff.

Attributes
ACN	1748508
Time
Date	202007
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 402/402C/B379 Businessliner/Utiliner
Operating Under FAR Part	Part 135
Flight Phase	Takeoff
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Exterior Pax/Crew Door
Person 1
Function	Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Published Material / Policy

Narrative:

Top clam shell of cabin door opened on takeoff.cause - cabin door not secured properly? Did not notice if door warning.

Original NASA ASRS Text

Title: Cessna 402 pilot reported the upper clamshell door half opened after takeoff.

Narrative: Top Clam shell of cabin door opened on takeoff.Cause - Cabin door not secured properly? Did not notice if door warning.

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.