What happened
During final approach with full flaps selected and indicated, the left inboard trailing edge flap and vane separated from the aircraft. This separation caused the aircraft to develop a left banking tendency, requiring approximately 1/2 right aileron input to maintain control. The autopilot disconnected during the event. The landing was described as firm and characterized by reduced controllability. The contact from the flap or vane resulted in structural damage to the left side of the fuselage.
The investigation
Investigators examined the failed components and found that the four H-11 type bolts at the forward attach point of the outboard hinge of the left inboard trailing edge flap had failed due to stress corrosion cracking. Chromium was detected in the fracture origin of bolts numbers 1 through 3, though the reason for its presence was not determined. Additionally, the two H-11 type bolts at the aft attach point of the outboard hinge failed due to ductile overload. No material or processing discrepancies were identified during the examination, noting that H-11 bolts are susceptible to stress corrosion cracking.
Flight Data Recorder (FDR) analysis showed a maximum left roll of 8.09 degrees. The data also indicated that the flap extension speed was exceeded for approximately 13 seconds during landing No. 1 while the flaps were extending from 16 to 20 degrees. The aircraft lacks flap overspeed protection within that specific range.