On December 2, 2013, about 1610 eastern standard time (EST), a Cessna 560 Encore, N810QS, was substantially damaged when an engine cowling departed the airplane inflight over Walton, Indiana. The two flight crewmembers and one passenger on-board were not injured. The airplane was registered to NetJets Sales, Inc. and operated by NetJets Aviation, Inc. under the provisions of 14 Code of Federal Regulations Part 135 as an on-demand air taxi flight. Visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules flight plan. The flight originated from Southwest Florida International Airport (KRSW), Fort Myers, Florida, about 1330 EST and was enroute to Chicago Midway International Airport (MDW), Chicago, Illinois.
The airplane was on initial descent, passing through 17,000 feet mean sea level (MSL) when the flight crew reported hearing a "bang" and experienced a slight yawing of the airplane. They initially believed a gear door had departed the airplane and reported the event to air traffic control. There were no indications or alerts of any anomalies received in the flight deck. The flight crew disconnected the auto-pilot and the airplane "flew normally." The flight continued to the planned destination where the airplane landed without incident. A post flight inspection revealed the right engine upper and lower cowlings had departed the airplane and collided with the right horizontal stabilizer.
Three fragments of the lower cowling door and one fragment of the upper cowling door were recovered and examined at the manufacturer under the supervision of the NTSB investigator-in-charge (IIC). The cowling doors are normally affixed to the engine nacelle with quick-release fasteners. The quick-release fasteners consist of a stud with pin on the cowling door side that interface with a receptacle on the engine nacelle. In normal operation, the stud with pin is inserted into the receptacle and rotated one-quarter turn to lock the two together by engaging the pin into the receptacle. As designed, there are a total of 26 fasteners on the upper cowling and 41 fasteners on the lower cowling. After the in-flight cowling departure, the accident airplane had 11 quick-release fasteners on the lower cowling and 5 fasteners from the upper cowling missing, with deformations and tearing of the surrounding door material. The tearing appeared consistent with overload of the sheet metal of the cowling door.
Separation of the pin from the stud would not allow for engagement into the receptacle. An examination of the remaining studs with pins from the accident airplane showed that several pins were loose allowing some side to side movement. However, none of the pins examined were loose enough to allow a pin to slide completely out of the stud.
A detailed examination of the aircraft nacelle door was not possible. However, photos provided by the operator showed no stud/grommet presence and no significant distortion of the supporting structure of the lower nacelle door's inboard corner or the second stud/grommet position. This, along with the lack of studs at these two fastener locations, was consistent with a previous loss of the respective cross pins, which allowed the door to become partially unfastened at the inboard corner of the lower nacelle door. All other damaged door fastener holes showed evidence of aft-direction displacement of the doors consistent with the doors departing the aircraft.
The operator removed 20 quick-release stud samples from the upper and lower cowling doors of 5 separate C560 airplanes from their fleet. These fasteners were examined to assess the condition of the fasteners after varying times in service with specific attention on the cross-pins. Of the 20 studs examined, cross-pins of 15 studs were held tightly in place and were centered on the stud. One stud out of the 20 samples exhibited a loose cross-pin (but still captured on the stud), three studs exhibited off-center cross-pins, and one stud had no cross-pin at all. One stud sample exhibited slight bending of the cross-pin, however none of the studs showed excessive wear or deformation. All of the stud cross-pins met the minimum push-out force requirement as designed.
A review of the manufacturers service difficulty reporting system revealed in-flight cowling departures had occurred 4 other times in addition to the accident. These events occurred between March 2010 and March 2013. The reported cowling departures were either full or partial loss of the upper and/or lower cowlings. Of the 5 total cowling departures, 3 events lead to pieces of the cowling striking a tail surface after departure from the engine nacelle. Additionally, the National Transportation Safety Board (NTSB) was made aware of a 6th cowling departure event that occurred in the United Kingdom (UK) in November 2015, this event was investigated by the Air Accidents Investigations Branch (AAIB) of the UK. The AAIB concluded the cause of the event was due to a number of fasteners that had not been securely fastened during maintenance.
Although on different airplanes, the operator reported having two events within one year. Following the accident, the operator modified their C560 Normal Preflight Inspection Checklist to include an inspection of the engine cowlings of both engines.
Due to the high probability that routine maintenance completed on an engine could contribute to the fasteners not being properly engaged and subsequently loosening during flight, the NTSB asked Textron Aviation to consider releasing a safety bulletin or safety video to maintenance personnel on the importance of proper installation and engagement of the quick-release fasteners. However, on November 30, 2016, Textron Aviation notified the NTSB IIC that "while missing fasteners typically indicate either improper engagement or complete lack of engagement, the incomplete nature of the recovered parts does not allow for a definitive conclusion as to the cause to be reached." As such, Textron Aviation declined to release any safety-related information to maintenance personnel on the above-mentioned and the importance of proper engagement of the fasteners.
The flight crew of the twin-engine jet heard a “bang,” and experienced a slight yawing of the airplane during the initial descent for landing. The flight continued to a normal landing, and a post-flight inspection revealed that the right engine upper and lower cowlings had separated from the engine and struck the right horizontal stabilizer. Recovered sections of the departed cowling doors were examined, but the reason for the separation could not be determined because the engine cowling quick-release fasteners were not recovered.