Airliner 99
Flight Crew Commercial; Flight Crew Multiengine
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Returned To Departure Airport
Aircraft
Upon loading the aircraft for the pm run; pilot noted that the Cabin Door annunciator wasn't illuminated during Before Start flow. After shutting off power; the pilot re-opened and closed the door; after which the annunciator extinguished. During normal startup; taxi; and takeoff; the annunciator remained off. The pilot initiated a left turn southbound during initial climb to a heading of 130; at which point there was a loud bang and rushing wind noise heard from the cockpit. Looking back; the pilot could see the air-stair portion of the cabin door was open. The pilot immediately reduced power to slow the aircraft and prevent further potential damage to the aircraft. He entered a left pattern for runway XX and proceeded to land; extending flaps and gear cautiously to prevent shifts in airflow changing the existing flight characteristics. The plane landed without further incident. During flair the pilot landed slightly flat to prevent the door from striking the ground. Upon shutting down; no visible damage was noted to the airframe.During climb out; a loud bang and wind noise in the cockpit alerted me to the issue.Faulty door sensor incorrectly indicating whether the door was locked or not.Executed an immediate shortened pattern and slowed the aircraft to land on the departure runway.Check the aircraft door inside and outside if an issue with the annunciator is detected. Confirm position of interior and exterior handles; lock buttons; and check internal door latch indicator.
Pilot flying C99 aircraft encountered passenger door opening inflight.