What happened
On December 2, 2013, a Cessna 560 Encore, registration N810QS, was performing an on-demand air taxi flight from Fort Myers, Florida, to Chicago, Illinois. While descending through 17,000 feet MSL near Walton, Indiana, the flight crew reported hearing a loud bang and experiencing a slight yaw in the aircraft. The crew initially suspected a gear door had departed the airplane.
Despite the event, the aircraft continued to Chicago Midway International Airport and landed without further incident. A post-flight inspection revealed that the upper and lower cowlings of the right engine had separated from the engine and struck the right horizontal stabilizer. There were no injuries to the two crewmembers or the single passenger on board.
The investigation
Investigators recovered fragments of the lower and upper cowling doors. The engine cowlings are secured to the nacelle using quick-release fasteners consisting of a stud and a pin. Upon inspection of the aircraft, several fasteners were found to be missing from both the upper and lower cowlings, with the surrounding sheet metal showing evidence of tearing consistent with an overload.
While some remaining studs showed loose pins that allowed side-to-side movement, none were loose enough to allow the pins to slide out of the studs entirely. Examination of the nacelle door suggested that the loss of cross-pins at two specific locations may have allowed the lower nacelle door to become partially unfastened at the inboard corner.
To assess potential fleet-wide issues, the operator examined 20 quick-release stud samples from five different aircraft in their fleet. The examination found that most pins were secure, though one stud had no pin and others showed off-center or loose pins. None of the samples showed excessive wear or deformation.
Records indicated that four other cowling departure events had been reported via the manufacturer's service difficulty reporting system between March 2010 and March 2013. A sixth event was noted in the United Kingdom in November 2015.