Date: 201412
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Weather elements / visibility: 3
Light: Daylight
Aircraft: Super King Air 300
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Cowling
Component problem: Improperly Operated
Reporter organization: Corporate
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Commercial; Flight Crew Flight Instructor
Human factors: Time Pressure; Situational Awareness; Workload; Distraction; Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Object
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Aircraft Damaged; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Incorrect / Not Installed / Unavailable Part; Procedure
Primary problem: Human Factors

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.

Callback: Reporter stated the day prior to the Right Engine door cowling departing and striking the passenger window; both he and a Mechanic were rush; rush; rushing to get the King Air-300 ready for the next day's flight. After correcting a GSU paralleling issue; he closed up the Right Engine cowling latches; but continued to check engine oils and worked a scavenge pump issue. Reporter stated their King Air already had the Stainless Steel Kit accomplished where all the screw fasteners were replaced with stainless steel type fasteners. But the Stainless Steel Kit was only a band-aid approach applied to the King Air-300 engine door cowlings due to the type of vertical latch pins that would unlock under lifting loads placed on the aircraft during flight. The Back-up [Safety] screws were crucial and the only fix by Beechcraft other than a latch design change. Horizontal operating latches were later used on other models; but not the -300 aircraft.Reporter stated that both doors; the inboard and outboard cowling sections on each engine are hinged at the top; are non-structural and provide access to the engine accessory area that include pumps; belts and other components. They were fortunate the high horizontal T-Tail and elevators on the King Air-300 were not damaged as the # 2 Engine cowling doors peeled-off from outboard to inboard striking the passenger window.



Synopsis: 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.