What was found
DURING CRUISE FLIGHT, A LOUD BANG WAS HEARD AND SUBSEQUENT LOUD WIND NOISES. UPPER DOOR HINGE HAD BROKEN ALLOWING DOOR TO TILT OUTWARD SINCE IT WAS NOW ONLY HELD BY LOWER HINGE AND REAR LATCH (UPPER LATCH RELEASED WHEN HINGE BROKE). DOOR OPENED APPROXIMATELY 6 INCHES AT TOP AND ACTED AS LARGE AIRSLOOP. PLANE WAS IMMEDIATELY SLOWED DOWN AND A SAFE LANDING WAS EXPECTED. TTAF 3,510 HOURS. EXAMINATION SHOWED FAILURE WAS DUE TO INTERGRANULAR CORROSION AND IS ALMOST IMPOSSIBLE TO INSPECT.