N100FN

On September 14, 2021, about 1230 eastern daylight time, a Hawker Beechcraft Corp 400A airplane, N100FN, was substantially damaged when it was involved in an accident en route to the Charleston Executive Airport (JZI), Charleston, South Carolina. The two flight crewmembers were not injured. The airplane was operated by Aircraft Management Group as a Title 14 Code of Federal Regulations Part 91 repositioning flight.

The flight crew reported that they performed a standard preflight inspection of the airplane as it had just been released from scheduled maintenance from Winner Aviation, a Part 145 repair station at Youngstown Regional Airport (YNG), Youngstown, Ohio. The only discrepancy noted during preflight inspection was a loose screw on the nose cone, which was tightened by a mechanic before departure.

The flight was uneventful until the airplane was about 15 minutes from JZI. The flight crew reported that while descending about 1,500 feet per minute through about 21,000 ft, they heard a loud bang and felt a shudder and yaw. They disconnected the autopilot, and a noticeable yaw was felt; however, there were no anomalies noted on any flight or engine instruments. The flight crew continued and landed without further incident. During the post-flight inspection, they discovered that the left engine cowling had entirely separated.

Photographs provided by the Federal Aviation Administration (FAA) revealed that the fuselage and horizontal stabilator sustained substantial damage. Figure 1 shown below provides an overview of the left engine and cowling fastener areas at the forward and aft portion of the engine.

Figure 1: View of the left engine and cowling attachment areas. (Photos Courtesy of FAA)

Postaccident examination of the left engine found that the entire cowling had separated. Several fasteners that would normally be installed on the forward and aft attachment areas around the engine were missing. There were no abnormalities discovered with cowling fastener receptacle areas or nut plates that remained in place. The aft top attachment area, shown in the top right photo in Figure 1, showed evidence of a rearward pulling of the metal attachment area.

The right cowling remained attached. About 1 month after the accident, a large dent and rippling were discovered in the middle and top inboard cowling areas. A stripped fastener was observed on the right cowling; however, no fasteners were missing.

Investigators contacted local authorities near the presumed area of the cowling separation; however, the cowling sections were never located.

According to a representative of Winner Aviation and company work orders, on September 13, 2021, the airplane had been signed off for an A and B inspection, which required the removal and re-installation of the engine cowlings. The mechanics involved in re-installing the cowlings reported that several fasteners on the left cowling had to be loosened and tightened a second time during re-installation due to an alignment issue with the cowling panels. The mechanic who resolved the alignment issue reported that he re-tightened the fasteners and noted that “everything was secure and double checked.” The mechanic also reported that he replaced some of the fasteners as some were missing and the overall condition of others warranted replacement. The following day another mechanic looked at the airplane before the accident flight and did not notice any abnormalities. No mechanics observed any dents on either cowling.

According to the Hawker Beechcraft 400/400A Maintenance Manual, the cowling consisted of four removable sections that fully enclosed the engine. The cowling panels were secured by several quarter-turn fasteners located around the circumferences of the forward and aft engine areas.

In December 2006, Raytheon Aircraft Company issued a Service Bulletin titled Nacelles – Engine Cowling Fastener Modification. The bulletin required a modification to the engine cowling attachment hardware to provide threaded fasteners at specific locations on the engine cowling panels. The bulletin stated that three reports had been received related to cowling panel separations and that aerodynamic forces acting on unsecured cowlings could cause panel separation or damage to the engine and nacelle components. In October 2007, the FAA issued an Airworthiness Directive making the Raytheon Service Bulletin mandatory. The accident airplane was manufactured after the issuance of the original service bulletin, and the cowlings had been modified per the bulletin.

On April 29, 2022, the FAA published Special Airworthiness Information Bulletin (SAIB-AIR-22-10), Engine Cowling System, Quarter-Turn Fastener Maintenance Information. The bulletin highlighted several incidents and accidents that involved cowling separations and provided several best practices that mechanics can use to ensure cowlings are properly and securely installed. This accident was one of several events highlighted in the SAIB.