After an uneventful take-off a Cessna 560 Citation Encore was climbing toward its initial cruise flight level, on this day in aviation history in 2020. Onboard the aircraft the pilot, and one passenger. (the flight was operated as a Part 91 flight, General aviation - Executive/Corporate)
The damaged empennage (© FAA) As the aircraft climbed through FL 225, for FL 380, the occupants heard a loud crashing noise which was followed by the sound of turbulent air (similar to flying with the landing gear extended)
At the moment the noise occurred, the aircraft was at a heading of 255º with an airspeed of approximately 265 knots while the autopilot was engaged with the lateral navigation mode engaged, and the vertical speed set to +1500 ft/min. The engines were controlled by the N1 computer system, with the N1 set to the climb setting.
The pilot immediately disconnected the autopilot, reduced speed to 170 knots, and initiated a descent to 11.000 feet, after alerting ATC of the situation. A scan of the instruments did not reveal the reason for the abnormal sound, there were no caution or warning lights. Based on the sound observed the pilot suspected a landing gear malfunction, to ascertain if it was a landing gear-related issue he reduced the speed of the aircraft further and cycled the landing gear and flaps. No abnormalities were apparent. Aileron and elevator operations were carefully checked, again without abnormalities being observed. However, when moving the rudder the sound of turbulent air around the aircraft increased. This led to the conclusion by the pilot that there was an issue with the empennage.
#2 engine with its cowling missing and damage to the empennage (© Aircraft operator)
The nearest suitable airport (with a runway longer than 5000 ft) was Mineral Wells Airport in Texas. A diversion was initiated and coordinated with ATC. The following approach and landing were uneventful, except for the noise of air rushing around the aircraft. No abnormalities were felt with the aircraft during the approach and landing. After arriving at the appointed parking position the aircraft was shut down and an external inspection of the aircraft was initiated.
During the inspection, the upper and lower cowlings of the righthand (#2) engine were found to be missing, while part of the cowling was wrapped around the (inboard) part of the horizontal stabiliser. Another part of the separated cowlings impacted
the area between the leading edge of the vertical stabilizer and the top of the fuselage causing extensive damage.
Remnants of the cowlings and fasteners were sent to the National Transportation
Safety Board (NTSB) Materials Laboratory for examination. 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. No indications of damage, wear, or material fatigue was found.
The fractured and partly missing upper engine cowling (© Aircraft operator) In their report (which served as a source for this blog) the NTSB stated that the most likely cause of the cowl door departure is that either the stud at receptacle 29 was left
unlocked when the cowl door was attached, or an oversized stud was used at receptacle 29 that was able to rotate from the locked to the unlocked position. Without the stud to examine and additional on-wing testing, it cannot be determined whether the stud at that location was a longer grip-length stud or if spontaneous unlocking of a longer grip-length stud is an actual possibility.
The NTSB concluded that the possible cause for the in-flight separation of the upper and lower #2 engine cowlings could not be determined based on the available evidence. Click on the .pdf file below to get access to the NTSB Aviation Investigation Final Report;