Incident Description
The first officer reported that during the first flight of the day, he performed the cabin and cargo door checks, which appeared normal. The airstair door operated normally on three previous flights. On the incident flight, no abnormalities were noted with the forward airstair door operation or locking. The cabin door annunciator light extinguished as normal after closing and locking. Both pilots verified that the doors were closed and annunciator lights were out.
Approximately five minutes into the flight, while passing 8,000 feet mean sea level, the cabin door warning light illuminated. The crew executed the checklist, which directed passengers to fasten seat belts. The captain instructed the first officer to inform approach control that they were returning to the airport; no emergency was declared at that time.
About two minutes after the warning light illuminated, the captain initiated a 20- to 25-degree bank left turn to return to the airport. During the turn, a pop was heard, and the cabin door opened, causing damage to the door, doorframe, and left propeller. The crew then declared an emergency and landed with the cabin door open.
Examination Findings
An examination of the cabin airstair door revealed no anomalies with the locking mechanism or circuitry. However, examination of the door sensor for the forward cabin door warning light showed that the door handle could be placed in a position where the door's pressure lock plunger was just engaging the latch plate but had not dropped into the "keyhole," yet the warning light would extinguish. When the handle was moved through the closed position to the point where it could no longer move forward, the pressure lock plunger would fully engage in its seated position, and the warning light would go out.