Incident Description
On July 17, 2008, at 1655 central daylight time, an Eclipse Aviation Corporation EA500, N875NA, operated by North American Jet Charter group, LLC, sustained minor damage during an in-flight separation of the left hand wing to body fairing near Rockford, Illinois. Visual meteorological conditions prevailed. The positioning flight was operating under 14 CFR Part 91 on an instrument flight rules flight plan. The two pilots were uninjured. The flight departed Pinedale, Wyoming, and was en route to Chicago Executive Airport in Illinois, where it landed without further incident.
Flight Crew Report
The captain and first officer reported the airplane was in cruise flight at 5,000 feet mean sea level and 250 knot equivalent airspeed. The first officer reduced engine power to enter an approaching cloud deck at 220-230 KEAS. Upon entering clouds, the airplane buffeted "slightly." Immediately after, the crew heard a "loud bang and short rumble" from the left rear. The airplane landed without further incident.
Post-Inspection Findings
Inspection revealed the left wing to body fairing (part number 12238005) had separated except for an outboard curved piece. The fairing was carbon fiber with foam core. The right fairing was attached with all screws; it displayed a 1/4-inch gap along its bottom inboard edge. Later serial number EA500s in the operator's fleet showed progressively smaller gaps. The left engine had no foreign object damage from the fairing separation.
All left fairing screws were attached except the forward three, which were missing. Their nuts were in place and reused. The underlying structure had four screw holes but the fairing used only three. Other EA500s in the operator's fleet also used either three or four screw holes. The forward outboard curved piece was retained by one screw; all but one screw had pulled through the fairing. The recovered pieces were sent to the NTSB Materials Laboratory.
Laboratory Examination
The fairing is secured with grommets designed to conform to countersunk recesses. Grommets are specified to be glued in during manufacturing. Exemplar screws and grommets measured 0.273 inch and 0.311 inch, respectively. Some panels used grommets with a large flange. Significant differences in screw hole damage were observed: the flat piece interior was not deformed; the curved piece interior was raised. Delamination of fiberglass layers from reinforcement was observed in the flat piece. The curved piece's layers remained adhered. Delamination indicator color change was seen around screw holes on both pieces.
Screw holes on the flat piece did not contain grommets. Radial cracking of finished coat was observed around two or three of the four holes. The airplane and fairing had 261 hours total time in service. The fairing had been removed several times for fuel system maintenance. It was last removed on July 16, 2008, at 255 hours for autopilot maintenance. The operator reported that metal grommets loosen when the fairing is repetitively removed. The operator had not submitted FAA Service Difficulty Reporting System reports regarding grommet issues.