Event
On December 2, 2013, about 1610 eastern standard time, a Cessna 560 Encore, registration N810QS, sustained substantial damage when an engine cowling departed the airplane in flight. The aircraft was operating under 14 Code of Federal Regulations Part 135 as an on-demand air taxi flight, registered to NetJets Sales, Inc. and operated by NetJets Aviation, Inc. The flight originated from Southwest Florida International Airport (KRSW) in Fort Myers, Florida, about 1330 EST, en route to Chicago Midway International Airport (MDW) in Chicago, Illinois. Visual meteorological conditions prevailed, and the flight was on an instrument flight rules flight plan.
During initial descent, passing through 17,000 feet mean sea level, the flight crew reported hearing a "bang" and experienced a slight yaw. Initially believing a gear door had separated, they notified air traffic control. No cockpit indications or alerts of anomalies were observed. The flight crew disconnected the autopilot and found that the airplane "flew normally." The flight continued to its destination and landed without incident. A postflight inspection revealed that the right engine upper and lower cowlings had departed and collided with the right horizontal stabilizer.
Investigation
Three fragments of the lower cowling door and one fragment of the upper cowling door were recovered and examined at the manufacturer under supervision of the National Transportation Safety Board (NTSB) investigator-in-charge. The cowling doors are normally secured to the engine nacelle with quick-release fasteners, consisting of a stud with a pin on the door side that interfaces with a receptacle on the nacelle. A one-quarter turn locks the pin into the receptacle. The design includes 26 fasteners on the upper cowling and 41 on the lower. After the departure, 11 fasteners were missing from the lower cowling and 5 from the upper, with deformations and tearing of the surrounding material consistent with overload.
Examination of remaining studs showed several pins with some side-to-side movement, but none loose enough to allow the pin to slide completely out of the stud. Detailed examination of the aircraft nacelle door was not possible, but photographs provided by the operator showed no stud or grommet at the lower nacelle door's inboard corner and the second stud position, with no significant distortion of the supporting structure. This, combined with the missing studs at those two locations, was consistent with a previous loss of the respective cross pins, allowing the door to become partially unfastened at the inboard corner. Other damaged fastener holes showed evidence of aft-direction displacement, consistent with the doors departing the aircraft.
Previous Occurrences
A review of the manufacturer's service difficulty reporting system revealed that in-flight cowling departures had occurred four other times in addition to this accident, between March 2010 and March 2013. These events involved full or partial loss of the upper and/or lower cowlings. In three of the five total cowling departures (including this one), pieces of the cowling struck a tail surface after departure. Additionally, the NTSB was made aware of a sixth cowling departure in the United Kingdom in November 2015, investigated by the Air Accidents Investigation Branch, which concluded that a number of fasteners had not been securely fastened during maintenance.
Maintenance and Operational Responses
The operator removed 20 quick-release stud samples from the upper and lower cowling doors of five separate C560 airplanes from their fleet. Examination of these studs showed that 15 had cross-pins held tightly and centered. Among the other five: one stud had a loose but captured cross-pin, three had off-center cross-pins, one had no cross-pin, and one had a slightly bent cross-pin. None showed excessive wear or deformation, and all met the minimum push-out force requirement. Following the accident, the operator modified their C560 Normal Preflight Inspection Checklist to include an inspection of both engine cowlings.
The NTSB asked Textron Aviation to consider releasing a safety bulletin or video for maintenance personnel on the importance of proper installation and engagement of the quick-release fasteners. On November 30, 2016, Textron Aviation notified the NTSB 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." Textron Aviation declined to release any safety-related information on the matter.