Cessna 525A accident near Scotts Bluff, Nebraska, December 18, 2019

The NTSB's narrative final · quoted from the NTSB record

The airplane was in cruise flight at FL250 when it experienced a separation of the right engine cowl, which struck and substantially damaged the right horizontal stabilizer and elevator. The airplane landed at the destination airport without further incident. Postaccident examination of the airplane found that two fasteners located at the forward inboard corner of the right engine lower cowl and a third fastener (which joined the right engine upper and lower cowls) were missing. The cowling and three fasteners were not recovered. Examination of the remaining fasteners used to secure the separated right engine cowl revealed that two were -4 studs, which were 0.03 inch longer than the other (-3) fasteners. The aircraft manufacturer’s drawings and the Illustrated Parts Catalog (IPC) for the engine lower cowl door indicated only -3 studs. Longer studs would allow for increased grip length, which would induce protrusion and misalignment of the engine cowl with adjacent structure. In addition, Skybolt grommets were used in the fastener assembly to secure the right engine lower cowl on the accident airplane; Skybolt grommets have a greater height than those cited in the IPC and the airplane manufacturer’s drawing, and, per these sources, Skybolt was not an approved vendor for the fastener assembly. The use of taller grommets would have also caused the cowl to protrude and be out of alignment with adjacent structure. Postaccident photographs of the left engine lower cowl showed that it exhibited protrusion/mismatch (or misalignment) that exceeded aerodynamic smoothness requirements cited in the airplane structural repair manual. The left engine lower cowl had at least two -3 studs that were recessed below Skybolt grommets, which were located in the same area as that of the right engine lower cowl that had the three missing studs. Without recovery of the right engine cowl, its aerodynamic smoothness could not be verified; however, using the observed postaccident condition of left engine cowl as an exemplar, available evidence suggests that the use of incorrect mounting hardware on the right engine cowl would have similarly affected its aerodynamic smoothness. Therefore, the right engine cowl likely separated from the airplane due to the use of incorrect mounting hardware.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.