Narrative:

While on visual approach at approximately 3000 ft MSL 3-4 miles from runway, cabin door blew open. I declared an emergency and landed the aircraft without further incident. It should be noted that during flight we reached a cruise altitude of 16000 ft MSL and pressurized normally to a maximum differential of 5.1 psid. Also, the preflight inspection of the cabin door annunciator checked out ok. When the door blew open the annunciator failed to illuminate. Upon further inspection on the ground the cabin light would illuminate when tested. Preliminary findings suggested corrosion on one of the microswitches that illuminates the cabin door annunciator, as well as the J hook may not have centered completely. In my experience the door may get soaked by rain, turned upside down to close and moisture may run into the door mechanism causing the corrosion. As for the J hook, it appeared locked, but obviously wasn't. I would recommend a more definitive lock check or mechanism. Callback conversation with reporter revealed the following information: the reporter stated when the door opened, no door warning was indicated and the system had been tested prior to takeoff. The reporter said maintenance has determined the lock mechanism and 'J' hook may not have centered completely. The reporter stated the door warning microswitches were both found to be corroded, as well as the locking mechanism. The reporter said the door hinges at the top, and when open for entry in the rain, the water runs down inside the door, soaking the components in and around the door. The reporter stated this is a pressurized airplane and shortly before landing was at 16000 ft with cabin pressure at 5.1 psi.

Related reports:

Google
 

Original NASA ASRS Text

Title: A BEECH 1900D ON VISUAL APPROACH AT 3000 FT DECLARED AN EMERGENCY DUE TO THE FORWARD PASSENGER DOOR OPENING IN FLIGHT CAUSED BY A FAILED OVER CENTER LOCK MECHANISM.

Narrative: WHILE ON VISUAL APPROACH AT APPROXIMATELY 3000 FT MSL 3-4 MILES FROM RWY, CABIN DOOR BLEW OPEN. I DECLARED AN EMERGENCY AND LANDED THE ACFT WITHOUT FURTHER INCIDENT. IT SHOULD BE NOTED THAT DURING FLIGHT WE REACHED A CRUISE ALTITUDE OF 16000 FT MSL AND PRESSURIZED NORMALLY TO A MAX DIFFERENTIAL OF 5.1 PSID. ALSO, THE PREFLIGHT INSPECTION OF THE CABIN DOOR ANNUNCIATOR CHECKED OUT OK. WHEN THE DOOR BLEW OPEN THE ANNUNCIATOR FAILED TO ILLUMINATE. UPON FURTHER INSPECTION ON THE GROUND THE CABIN LIGHT WOULD ILLUMINATE WHEN TESTED. PRELIMINARY FINDINGS SUGGESTED CORROSION ON ONE OF THE MICROSWITCHES THAT ILLUMINATES THE CABIN DOOR ANNUNCIATOR, AS WELL AS THE J HOOK MAY NOT HAVE CENTERED COMPLETELY. IN MY EXPERIENCE THE DOOR MAY GET SOAKED BY RAIN, TURNED UPSIDE DOWN TO CLOSE AND MOISTURE MAY RUN INTO THE DOOR MECHANISM CAUSING THE CORROSION. AS FOR THE J HOOK, IT APPEARED LOCKED, BUT OBVIOUSLY WASN'T. I WOULD RECOMMEND A MORE DEFINITIVE LOCK CHECK OR MECHANISM. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WHEN THE DOOR OPENED, NO DOOR WARNING WAS INDICATED AND THE SYSTEM HAD BEEN TESTED PRIOR TO TAKEOFF. THE RPTR SAID MAINTENANCE HAS DETERMINED THE LOCK MECHANISM AND 'J' HOOK MAY NOT HAVE CENTERED COMPLETELY. THE RPTR STATED THE DOOR WARNING MICROSWITCHES WERE BOTH FOUND TO BE CORRODED, AS WELL AS THE LOCKING MECHANISM. THE RPTR SAID THE DOOR HINGES AT THE TOP, AND WHEN OPEN FOR ENTRY IN THE RAIN, THE WATER RUNS DOWN INSIDE THE DOOR, SOAKING THE COMPONENTS IN AND AROUND THE DOOR. THE RPTR STATED THIS IS A PRESSURIZED AIRPLANE AND SHORTLY BEFORE LANDING WAS AT 16000 FT WITH CABIN PRESSURE AT 5.1 PSI.

Data retrieved from NASA's ASRS site as of July 2007 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.