ERA21LA379 · Hawker Beechcraft Corp 400A

The flight crew initiated a positioning flight following major maintenance, which included the removal and re-installation of both engine cowlings. The flight was uneventful until about 15 minutes from the destination. While descending, a sudden bang and yaw occurred. There were no annunciations or other warnings observed before or after the event, and the airplane remained controllable. The flight crew continued to the destination and landed without further incident. On the ground, it was discovered that the entirety of the left engine’s four-panel cowling had separated in flight and impacted the rear fuselage and horizontal stabilizer resulting in substantial damage. Further examination of the left engine area revealed that several cowling fasteners were missing from the receptacles. During maintenance immediately preceding the flight, the left cowling fasteners had to be removed a second time due to an alignment issue with the panels. The mechanic who performed the work reported that the misalignment was resolved, and the fasteners were tightened. A second mechanic visually inspected the work and did not notice any abnormalities, nor did the pilots during their preflight inspection. It is likely that several fasteners departed the left cowling during this first flight after maintenance, which resulted in a rapid departure of the cowling panels. Given that none of the cowling was recovered, the initiating factors that would lead to the separation could not be determined. Denting and rippling were discovered on the right cowling; however, the investigation was unable to determine when the damage was incurred as it was not discovered until about 1 month after the accident. The mechanics and flight crew did not recall observing any damage to either cowling before flight.

Verbatim NTSB analysis and probable cause from the NTSB dataset