On February 12, 2020, about 1527 central standard time, a Cessna 560 (Citation Encore) airplane, N654CE, was substantially damaged when it was involved in an accident near Aledo, Texas. The air transport pilot and the one passenger were not injured. The airplane was operated under the provisions of Title 14 Code of Federal Regulations Part 91 as a corporate flight. The airplane was climbing through flight level (FL) 225 for FL 380, when the pilot heard a "loud crashing noise" followed by a "turbulent air sound," he described as being similar to flying with the main landing gear extended. The pilot reported the airplane was on a heading of 255°, the airspeed was about 265 to 270 kts, and the autopilot system was engaged. The autopilot system was set to lateral navigation and the vertical speed was set for a 1,500 ft-per-minute climb. The power was set to the climb N1 setting as given by the N1 computer system. The pilot reacted to the initial noise by disconnecting the autopilot system, announcing the issue to air traffic control (ATC), slowing the airplane to 170 kts, and descending to 11,000 ft above mean sea level. There were no master caution or warning lights illuminated on the cockpit annunciation panel. The pilot initially thought the airplane had a main landing gear malfunction based of the sound of the airflow around the cabin. The pilot slowed the airplane further, cycled the main landing gear, cycled the flaps, and moved the flight controls. He reported everything checked out normal except the "turbulent air sound," which was exacerbated with rudder movement. The pilot decided the main landing gear was functioning normally and there was a problem with the empennage. The pilot located the closest airport with a paved runway longer than a 5,000 ft, contacted ATC, and proceeded to land at Mineral Wells Airport (MWL), Mineral Wells, Texas. The pilot executed an uneventful landing and shut down the airplane without further incident. He reported that during the landing, he did not notice any abnormalities with the airplane, and the flight controls felt "fine." A postlanding inspection revealed the upper and lower cowlings for the No. 2 (right) engine were missing. A portion of the separated cowlings was found wrapped around the inboard section of the right horizontal stabilizer. Another portion of the separated cowlings impacted the area between the leading edge of the vertical stabilizer and the top of the fuselage, resulting in substantial damage. The remaining pieces of the separated cowlings have not been recovered. According to the operator, the airplane was maintained through an approved aircraft inspection program and the last maintenance work performed on the right engine occurred 153.6 flight hours prior to the accident. Pieces of the upper cowl door, lower cowl door, seal assembly, and quarter-turn fastener assemblies (also known as camlocks), used to secure the forward ends of the right-side engine cowl doors, were sent to the National Transportation Safety Board Materials Laboratory for examination. There were 32 fasteners at the forward end, and for documentation purposes, they were numbered starting at the inboard position, proceeding clockwise around the engine (aft looking forward). On-scene examination revealed that receptacle 29, at the forward end and inboard side of the lower cowl door, was missing its stud, snap ring, and grommet and that receptacles 26 and 28 were missing their grommets and snap rings but had retained their studs. All other receptacles had retained their attachment hardware. Receptacle 29 was visually examined for indications of damage or wear and the locking features on the backside of the cup were quantitatively compared against an exemplar cup. There were no apparent indications of damage or wear that would have contributed to the stud cross-pin pulling out or rotating out of the locked position. The fracture surfaces on the two cowl pieces were examined for indications of fatigue, but none were found. The Cessna 560 manufacturer-issued preflight and postflight inspection checklists do not require the pilot to inspect the engine cowlings for securement. The Cessna Model 500 Series Structural Repair Manual contains information on the topic of aerodynamic smoothness requirements; however, information on that topic is not listed in the Cessna 560 Maintenance Manual. A search found a Federal Aviation Administration (FAA) notice of proposed rulemaking (54 FR 38610, September 19, 1989) that would have provided improved engine cowling retention for transport-category airplanes by adding specific design requirements for cowling retention systems. The proposed rule would have promoted design and construction of cowling retention systems to withstand vibration, inertial loads, overpressure, normal air loads, and thermal conditions of an engine compartment fire after failure or improper fastening of latching devices. The FAA reported the proposed rule was withdrawn and states: We are withdrawing the proposed rule because the proposal has been surpassed by technological advances. The issues will be addressed by future regulatory action based on recommendations from the Aviation Rulemaking Advisory Committee and will be harmonized with similar regulations in Europe and Transport Canada.