Accident Sequence
On February 12, 2020, about 1527 central standard time, a Cessna 560 (Citation Encore) airplane, registration N654CE, was substantially damaged during a flight near Aledo, Texas. The aircraft was operating under Title 14 Code of Federal Regulations Part 91 as a corporate flight. The air transport pilot and the single passenger were not injured.
While climbing from flight level (FL) 225 to FL 380, the pilot reported hearing a loud crashing noise followed by a turbulent air sound similar to flying with the main landing gear extended. The airplane was on a heading of 255° at an airspeed of about 265 to 270 knots, with the autopilot engaged. The autopilot was set to lateral navigation and a 1,500-foot-per-minute vertical speed climb; engine power was set to the climb N1 setting as indicated by the N1 computer.
Pilot Actions
In response to the noise, the pilot disconnected the autopilot, notified air traffic control (ATC), slowed the airplane to 170 knots, and descended to 11,000 feet above mean sea level. No master caution or warning lights illuminated on the cockpit annunciation panel. Initially suspecting a main landing gear malfunction based on the sound, the pilot slowed further, cycled the main landing gear and flaps, and moved the flight controls. All checks appeared normal except the turbulent air sound, which worsened with rudder movement. The pilot concluded the landing gear was functioning normally and that the issue involved the empennage.
The pilot identified the nearest airport with a paved runway longer than 5,000 feet, coordinated with ATC, and landed at Mineral Wells Airport (MWL) in Mineral Wells, Texas. The landing was uneventful, and the pilot observed no abnormalities; flight controls felt fine. The airplane was shut down without further incident.
Postlanding Inspection
A postlanding inspection revealed that 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 struck the area between the leading edge of the vertical stabilizer and the top of the fuselage, causing substantial damage. The remaining pieces of the cowlings were not recovered.
The operator reported that the airplane was maintained under an approved aircraft inspection program. The last maintenance on the right engine occurred 153.6 flight hours before the accident. Pieces of the cowl doors, seal assembly, and quarter-turn fastener assemblies (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, numbered starting at the inboard position and proceeding clockwise around the engine (aft looking forward). On-scene examination showed that receptacle 29, at the forward end and inboard side of the lower cowl door, was missing its stud, snap ring, and grommet. Receptacles 26 and 28 were missing their grommets and snap rings but retained their studs. All other receptacles retained their attachment hardware.
Receptacle 29 was examined for damage or wear; the locking features on the backside of the cup were compared against an exemplar cup. No indications of damage or wear were found that would have contributed to the stud cross-pin pulling out or rotating out of the locked position. Fracture surfaces on two cowl pieces were examined for fatigue but showed none.
Maintenance and Design
The Cessna 560 manufacturer-issued preflight and postflight inspection checklists do not require the pilot to inspect engine cowlings for securement. The Cessna Model 500 Series Structural Repair Manual contains information on aerodynamic smoothness requirements, but that information is not included in the Cessna 560 Maintenance Manual.
A Federal Aviation Administration (FAA) notice of proposed rulemaking published on September 19, 1989 (54 FR 38610) would have introduced specific design requirements for engine cowling retention on transport-category airplanes. The proposal aimed to ensure cowling retention systems could withstand vibration, inertial loads, overpressure, normal air loads, and thermal conditions from an engine compartment fire after failure or improper fastening of latching devices. The FAA withdrew the proposed rule, stating that technological advances had surpassed it and that future regulatory action, harmonized with European and Transport Canada regulations, would address the issues.