What happened
The incident involved a twin-engine turboprop airplane that was operating at an altitude of 20,000 feet mean sea level (MSL). During the descent phase to 7,000 feet MSL, the upper accessory case cowl located on the aft section of the left engine detached from its mounting points. The separated cowl assembly subsequently impacted the left side of the fuselage, as well as both the vertical and horizontal stabilizers.
The impact with the fuselage resulted in a rapid decompression of the cabin. Despite this structural failure and the loss of pressurization, the crew managed to maintain control of the aircraft and executed a landing without further incident or damage that would prevent safe operation on the ground.
The investigation
The primary mechanical finding was the uncommanded separation of the left engine's upper accessory case cowl. This component failure initiated a chain of events leading to secondary impacts with critical aerodynamic surfaces, specifically the empennage and fuselage side.