Victorville, CA, USA
N820TJ
BOEING 737
After departure, the air carrier airplane was climbing through an altitude of about 8,000 ft, at which time the flight crew heard a loud bang. Because the flight instruments and gauges showed nothing abnormal, the flight crew elected to continue the flight to the destination airport. The airplane landed uneventfully. Postflight inspection of the airplane found that the dorsal fin and three panels on the lower left side of the vertical stabilizer were missing, and that the left horizontal stabilizer was substantially damaged. Most of the dorsal fin, the vertical stabilizer panels, and 7 of the 14 bolts attaching the dorsal fin to the fuselage were not recovered. Of the seven bolts that were recovered, four remined installed, and three were found loose. Three of the four installed bolts remained attached to pieces of the dorsal fin structure. Examination of the photographs showed that the bolts installed in three positions on the left side of the dorsal fin were the correct part number. The bolt installed in a position on the right side of the dorsal fin was not the correct part number, and the bolt was longer than required. The photographs also showed remnants of old sealant at each of the bolt locations but no evidence that sealant was applied during maintenance of the dorsal fin in February 2019 (less than 3 months before the accident). The seven missing bolts for the dorsal fin structure became loose, had fractured, or were not properly installed.
HISTORY OF FLIGHTOn May 19, 2020, about 1516 Pacific daylight time, Swift Air flight 3518, a Boeing 737-800, N820TJ, sustained substantial damage while climbing after departure from Victorville Airport (VCV), Victorville, California. None of the 95 airplane occupants were injured. The flight was operated as a Title 14 Code of Federal Regulations Part 121 scheduled passenger flight from VCV to San Diego International Airport (SAN), San Diego, California. While the airplane was climbing through an altitude of about 8,000 ft, the flight crew heard a loud bang. Because the flight instruments and gauges showed that the airplane was operating normally, the flight crew elected to continue to SAN. The airplane landed uneventfully. Postflight inspection of the airplane found that the dorsal fin and three panels on the lower left side of the vertical stabilizer were missing (see figure 1). This accident occurred during the Covid-19 pandemic, and National Transportation Safety Board staff did not travel to assess the accident airplane. The operator provided photographs of the damage to the airplane and the recovered components. Figure 1. Location of missing dorsal fin and vertical stabilizer panels (Source: FAA). AIRCRAFT INFORMATIONThe dorsal fin is attached to the upper fuselage at 14 locations. Figure 2 shows the attach points on the accident airplane with the holes numbered sequentially in red and the required fastener (bolt) part numbers in black. Figure 2. Dorsal fin attach points and bolt part numbers (Source: Swift Air and Boeing Company). On February 26, 2019 (less than 3 months before the accident), the fuselage skin under the dorsal fin was inspected at iAero Repair Station, Miami, Florida. This inspection was done every 24,000 flight cycles to identify and remove corrosion. The maintenance instructions for the dorsal fin indicated that the mechanic should note the type and location of the bolts removed to ensure that each bolt would be returned to the same location from where it was removed. (The dorsal fin had different bolts with varying grip lengths.) Once the bolts were reinstalled, sealant was applied to the bolt heads. AIRPORT INFORMATIONThe dorsal fin is attached to the upper fuselage at 14 locations. Figure 2 shows the attach points on the accident airplane with the holes numbered sequentially in red and the required fastener (bolt) part numbers in black. Figure 2. Dorsal fin attach points and bolt part numbers (Source: Swift Air and Boeing Company). On February 26, 2019 (less than 3 months before the accident), the fuselage skin under the dorsal fin was inspected at iAero Repair Station, Miami, Florida. This inspection was done every 24,000 flight cycles to identify and remove corrosion. The maintenance instructions for the dorsal fin indicated that the mechanic should note the type and location of the bolts removed to ensure that each bolt would be returned to the same location from where it was removed. (The dorsal fin had different bolts with varying grip lengths.) Once the bolts were reinstalled, sealant was applied to the bolt heads. WRECKAGE AND IMPACT INFORMATIONThe dorsal fin and the missing vertical stabilizer panels were not recovered. The fuselage crown skin, left horizontal stabilizer, and vertical stabilizer sustained substantial damage. The upper fuselage crown skin was dented, punctured, and gouged. The left horizontal stabilizer leading edge, lower skin, and upper skin had multiple dents, gouges, and punctures. The left horizontal stabilizer front spar upper chord was fractured, the lower chord was deformed, and the web was gouged in two locations. The panel attach structure on the lower left side of the vertical stabilizer was fractured and deformed in several places. No bolts were found in holes 1-6, 8, 10, 12, and 13. Bolts remained in holes 7, 9, 11, and 14. Pieces of the dorsal fin rib structures remained attached to the bolts in holes 9, 11, and 14. Three loose bolts were recovered on the interior ceiling panels beneath the dorsal fin location. The airplane operator provided photographs of the four installed bolts (7, 9, 11, and 14; see figure 3). The bolt installed in hole 7 was marked NAS6705-9, and the bolts in holes 9, 11, and 14 were each marked NAS6705-12. One of the three loose bolts was marked NAS6705-9, one was marked NAS6705-12, and the marking on the other loose bolt could not be fully discerned but contained an 8, a 3, a 0, the number and letter 1M, and an S (see figure 4). Hole 7 Hole 9 Hole 11 Hole 14 Figure 3. Bolts that remained in place (Source: Swift Air). Figure 4. One of the three bolts found on the interior ceiling panels (Source: Swift Air). ADDITIONAL INFORMATIONSwift Air revised its aircraft maintenance manual on October 15, 2020, to clarify the removal and installation procedures for the dorsal fin. Swift Air also sent a letter to iAero Repair Station, requesting that it review procedures and oversight by quality control inspectors. iAero implemented changes on July 22, 2020. According to iAero Repair Station, it provided additional training on the maintenance procedures for removing and installing the dorsal fin, focusing on the installation procedure. The training emphasized that quality control inspectors should ensure that the associated tasks “are comprehensively examined” and inspected. Further, on February 15, 2022, Boeing updated the dorsal fin removal and installation instructions in its 737-600/700/800/900 Aircraft Maintenance Manual. The note in the manual to install the bolts in the same location from where they were removed was changed to a caution to emphasis the importance of matching the correct bolt and grip length. The caution states the following: MAKE SURE THAT YOU MONITOR WHERE YOU REMOVE EACH FASTENER AT EACH LOCATION. THERE ARE DIFFERENT SIZES FOR THE FASTENERS. IF YOU INSTALL THE FASTENERS AT THE INCORRECT LOCATIONS, DAMAGE TO THE AIRPLANE CAN OCCUR. Similarly, the following caution was added to the information about reinstalling the bolts that secure the dorsal fin: MAKE SURE THAT YOU INSTALL EACH FASTENER AT THE CORRECT LOCATION. THERE ARE DIFFERENT DIMENSIONS FOR THE FASTENERS. IF YOU DO NOT INSTALL THE FASTENERS AT THE CORRECT LOCATIONS, DAMAGE TO THE AIRPLANE CAN OCCUR. In addition, the part number in illustrated parts catalog 55-32-11-01, figure 1, item 15, was corrected for the proper grip length.
Improper installation of dorsal fin attach bolts which caused the dorsal fin to separate during flight, substantially damaging the left horizontal stabilizer.
Source: NTSB Aviation Accident Database
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