A Chartright Air Cessna 560 Encore was conducting flight HRT705 from Toronto/Lester B. Pearson Intl. (CYYZ), ON to Fort Lauderdale Exec Airport (KFXE), FL. At approximately 12:11, while the aircraft was in climb over Cayuga, Ontario, at an altitude of approximately 23000 feet above sea level (ASL), the whole upper cowl door and most of the lower cowl door on the left engine partially detached from the nacelle, before becoming completely detached, resulting in a sudden increase in parasite drag on the left side of the aircraft. The aircraft suddenly yawed and rolled to the left, and the flight crew then lost control of the aircraft as it rapidly lost altitude. The flight crew subsequently declared an emergency by transmitting a Mayday call, and at approximately 3200 feet ASL, they regained control of the aircraft and stopped the descent. The flight crew requested a diversion to Buffalo Niagara International Airport (KBUF), New York, U.S. They subsequently burned off some of the aircraft?s fuel and followed air traffic control (ATC) guidance. Although the aircraft remained difficult to control, the crew landed on Runway 23 at KBUF at approximately 12:41, with aircraft rescue and firefighting (ARFF) emergency vehicles standing by. While the aircraft was taxiing to the ramp, a remaining portion of the lower cowl door fell off the horizontal stabilizer onto the taxiway. After the aircraft came to a stop on the apron, ARFF informed the flight crew that the remaining portion of the cowl had detached from the aircraft. In the days before the accident maintenance work was carried out at the company?s maintenance hangar. The work performed addressed 2 reported defects, requiring the removal and reinstallation of the lower cowl doors on both the left and right engines. The accident flight was the first flight after maintenance. Findings as to causes and contributing factors 1. During the reinstallation of the lower cowl doors, the non-sequential order in which the fasteners were tightened likely led to a slip of attention that combined with an unclear expectation on the part of both aircraft maintenance engineers about who would be securing the remaining fasteners. As a result, 6 consecutive fasteners were left unsecured. 2. The reinstallation of the left-engine cowl doors and the daily inspection were conducted and certified during the same shift and by the same aircraft maintenance engineer. Given that the cowl door reinstallation was completed as part of the maintenance work at the very end of the work shift, the daily inspection item for checking the condition and security of the engine cowl doors was not performed as a specific, separate operation. As a result, the improperly secured condition of the cowl door went unnoticed. 3. The location of the unsecured fasteners on the cowl door is not easily visible and was not typically checked during pre-flight walkarounds, and no specific guidance on how to look for unsecured fasteners on this aircraft was available to company flight crews. As a result, the unsecured fasteners went undetected by the flight crew despite their multiple walkaround inspections before the occurrence flight. 4. Because some of the fasteners were not secured, there was likely a gap between the cowl door and the nacelle that allowed air to flow inside the nacelle. The cowl doors then partially detached from the nacelle, resulting in the loss of control and rapid descent. Following this, a portion of cowl door became lodged on the horizontal stabilizer and resulted in continued difficulty to control the aircraft.