Narrative:

We were descending into ZZZ out of roughly 14;000 feet for landing when we heard and felt a very loud pop! I instinctively dropped the passenger oxygen masks and enabled the flow. Looking back into the cabin; I saw one of the right side passenger side windows had blown out. Two seconds later I realized that a portion of the right side engine cowling (door) had departed the aircraft and struck the window causing the blow out. I disconnected the autopilot immediately to ensure control integrity. So; with all systems functioning normally; I proceeded with the IFR approach to an uneventful landing at our intended destination eight minutes later. Upon examination of the right engine it was determined that the six fail-safe screws (three on each side) had either backed out during flight or were not correctly installed after that cowl was last closed. On the day before; we had a mechanic come out to the hangar to troubleshoot a generator issue we were having. I arrived before him and opened the cowling for him to inspect. He said lets run it; so I closed the cowl and latched it securely. After the run up; we adjusted the generator control unit (gcu) inside the cabin which fixed the issue. We closed all the panels that were opened up and cleaned the cabin. My assumption was that he reinstalled the backup screws...while he obviously assumed I did. Preflight for the flight the next morning (today) was normal and while I remember specifically checking all four latches; I honestly don't remember seeing the screws.....or lack thereof; properly installed.

Related reports:

Google
 

Original NASA ASRS Text

Title: A pilot reports hearing and feeling a very loud pop during descent in a Beechcraft King Air-300 aircraft. He noticed a right side passenger window had blown out from impact when the Right Engine door cowling had separated from the aircraft.

Narrative: We were descending into ZZZ out of roughly 14;000 feet for landing when we heard and felt a very loud pop! I instinctively dropped the passenger oxygen masks and enabled the flow. Looking back into the cabin; I saw one of the right side passenger side windows had blown out. Two seconds later I realized that a portion of the right side engine cowling (door) had departed the aircraft and struck the window causing the blow out. I disconnected the autopilot immediately to ensure control integrity. So; with all systems functioning normally; I proceeded with the IFR approach to an uneventful landing at our intended destination eight minutes later. Upon examination of the right engine it was determined that the six fail-safe screws (three on each side) had either backed out during flight or were not correctly installed after that cowl was last closed. On the day before; we had a mechanic come out to the hangar to troubleshoot a generator issue we were having. I arrived before him and opened the cowling for him to inspect. He said lets run it; so I closed the cowl and latched it securely. After the run up; we adjusted the Generator Control Unit (GCU) inside the cabin which fixed the issue. We closed all the panels that were opened up and cleaned the cabin. My assumption was that he reinstalled the backup screws...while he obviously assumed I did. Preflight for the flight the next morning (today) was normal and while I remember specifically checking all four latches; I honestly don't remember seeing the screws.....or lack thereof; properly installed.

Data retrieved from NASA's ASRS site 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.