On December 18, 2019, about 0900 mountain standard time, a Cessna 525A airplane, Canada registration CGOJG, was substantially damaged when it was involved in an accident near Scottsbluff, Nebraska. The pilot and four passengers were uninjured. The airplane was operated as a non-U.S./non-commercial corporate/executive flight.
The pilot stated that while the airplane was at cruise altitude of flight level 250 at about 10-15 kts below the never exceed speed (Vne), in smooth air, there was a loud “bang.” The airplane pitched down into negative-G flight, and the overspeed warning sounded. He reduced engine power to idle and began to pull back on the control yoke. He said that he experienced resistance in control yoke movement. To alleviate control yoke forces, he used manual trim since trim changes were more rapid than using electric trim.
The pilot said he performed a gradual pull up, and once the airplane was level, he applied engine power. He said the recovery altitude was about 21,400 ft and the indicated airspeed was about 180 kts. He said there was some control column buffet, so he reduced engine power again to reduce the indicated airspeed to about 161 kts. He said the roll axis control was normal, and the pitch axis control was very stiff. The control column did not have a significant range of motion, and the rudder pedals felt “weird and were in unequal positions.” In checking airplane controllability, he selected flaps 15°, and there was no change in controllability. He retracted flaps and flew to the destination airport where he landed the airplane without further incident using an indicated airspeed of about 145 kts and flaps at 0°.
Postaccident examination of the airplane revealed the right engine lower cowl separated from the airplane, struck the right engine upper cowl, and damaged about a third of the outboard side of the right horizontal stabilizer and elevator, resulting in substantial damage. The cowl fastener assemblies were all quarter-turn fasteners. Two consecutive fasteners located at the forward inboard corner of the right engine lower cowl (labeled receptacles 10 and 11 in figure 1) were missing. A third fastener, which joined the right engine upper and lower cowls through an angle/tab, was missing (see figure 1). The receptacles associated with these fasteners did not display tearing or fractures. All the remaining right engine lower cowl fasteners were in place.
Figure 1: Missing fasteners at receptacles 10 and 11 and at the angle/tab on the right engine lower cowl.
The separated cowling and missing fasteners were not located. The remaining fastener assemblies were removed by the repair facility that recovered the airplane and sent to the National Transportation Safety Board Materials Laboratory for examination.
The fastener assemblies were examined for part numbers or other identifying marks. None were found on the receptacles. The stud heads were labelled with dash numbers and all studs were “-3” except for the studs at receptacle location 4 on both the forward and aft attachment flanges, which were “-4” studs (see figure 2).
Figure 2: Illustrated Parts Catalog detail of engine lower cowl and locations of number 4, 10, and 11 receptacles.
The Illustrated Parts Catalog (IPC) for the airplane listed the engine lower cowl fastener studs as part number S3412-103, which had a Phillips head and the same stud length as part number S4312-3, which had a straight-slot head. Each subsequent increase in dash number increased fastener length by about 0.03 inch. The IPC did not list any -4 studs for the upper or lower engine cowl. A -4 stud would correspond to part number S3412-104; no S3412-104 fasteners are specified on the Textron Aviation drawing for the lower cowling door.
The fastener assembly grommets that were on the accident airplane right engine lower cowl were all labelled as “SKYBOLT HS.” According to Skybolt product information, HS-type grommets are nominally 0.197 inch in height. Measured with a micrometer, the height of one of the grommets from the accident airplane was 0.204 inch. Figure 3 shows a top-down view of an HS grommet from the accident airplane and a profile view, comparing the height with that of a S2319-63 (from the IPC). Skybolt was not an approved vendor for these assemblies per the Textron Aviation drawing and the IPC.
Figure 3: Profile view of the Skybolt HS grommet from the accident airplane and the grommet listed in the IPC (S2319-63) for the airplane.
According to the repair facility that performed postaccident repairs to the airplane, the left engine lower cowl was intact and secure with all its fasteners. Photos provided by the repair facility showed several of the retained fastener heads were recessed and not flush with the cowl surface, as specified in the manufacturer structural repair manual. The repair facility reported that the fasteners that were not recessed were too long. Further, the cowl surface visibly protruded about an 1/8 inch and was not flush with the adjacent structure (see figure 4). The photos of the fasteners shown in figure 4, showed the grommet labelled as “SKYBOLT HS” and “-3” studs. The 1/8-inch protrusion exceeded aerodynamic smoothness requirements cited in the aircraft structural repair manual (SRM). The aircraft maintenance/service manual did not cite aerodynamic smoothness requirements.
Figure 4: Shows the left engine inboard lower cowl mismatch and screw heads recessed beneath their grommets at the same relative receptacle location numbers 10 and 11. The grommets were labeled “HS SKYBOLT” and the studs were labeled “-3.”
The postaccident repair of the airplane included replacement of fasteners with shorter fasteners in the left engine cowl, and through approval by a designated engineering representative, additional fastener(s) were installed in the engine cowlings.
The SRM had the following values for maximum allowable gap and mismatch for zones 1, 2, and 3, which were defined as: ZONE 1 - LEADING EDGES OF WINGS, STABILIZERS AND NACELLES, ZONE 2 - FORWARD FUSELAGE, WINGS AND DORSAL FIN, and ZONE 3 - CABIN, TAILCONE, NACELLES, STABILIZERS, RUDDER AND ELEVATORS.
Zone Maximum gap Maximum mismatch perpendicular to airstream Maximum mismatch parallel to airstream 1 0.060 0.020 0.040 2 0.060 0.030 (NOTE 2) 0.040 (NOTE 3) 3 0.060 0.040 (NOTE 2) 0.040 (NOTE 3)
According to the FAA, a fault hazard analysis (FHA) is a deductive method of analysis, in reference to safety issues, that can be used exclusively as a qualitative analysis or, if desired, expanded to a quantitative one. The FHA requires a detailed investigation of the subsystems to determine component hazard modes, causes of these hazards, and resultant effects to the subsystem and its operation. Cessna Aircraft representative(s) stated that an FHA/safety analysis for departure of an engine cowl for the make and model of airplane was not performed.
The airplane was certified under Part 23, and an FHA was not required by certification regulations during the initial certification of the airplane. Section 23.1193 had similar certification requirements to Section 25.1193, Cowling and Nacelle Skin. Section 25.1193 addresses the design of engine cowlings but does not address the single failure of a latch or hinge, or an improperly fastened latch.
On September 19, 1989, the FAA published Notice of Proposed Rulemaking (NPRM) 89–25 (54 FR 38610) to propose an amendment to 14 CFR Part 25 and invited public comment on the issue of engine cowling retention. The NPRM was prompted by several in-flight incidents of engine cowling separation that resulted in damage to airplanes and property on the ground and highlighted the need to re-evaluate the design and maintenance requirements applicable to engine cowlings. NPRM 89–25 proposed to specify standards for failsafe criteria in the design of engine cowling retention systems, which would enable the systems to withstand the loss of a single latch and easily detect unlocked or improperly closed latches.
The comment period for the NPRM closed March 19, 1990, and it was withdrawn in 2002 to allow the Federal Aviation Administration “to consider harmonization concerns and address the issues more completely in future regulatory actions in consideration of recommendations developed within the [Aviation Rulemaking Committee].”