Hawkins and Powers Aviation Hercules C130A plane crash
Walker, California, USA
The airplane was making a fire retardant drop on a 10,000 acre forest fire near Walker, California, when the wings
separated from the fuselage. A videotape of the accident sequence showed the airplane as it flew down the valley and
proceeded to make a fire retardant drop. When the drop was almost completed, the airplane's nose began moving up, and the
airplane started to arrest its descent and level out. The nose of the airplane continued to rise, and the airplane's wings
folded upward until they detached from the fuselage at the center wing box beam-to-fuselage attachment location.
Close examination of the video revealed that the right wing folded upward first, followed by the left wing about 1 second later. Metallurgical examination of the center wing box lower skin revealed a 12-inch long fatigue crack on the lower surface of the right wing beneath the forward doubler, with two separate fatigue crack initiation sites at stringer attachment rivet holes (which join the external doubler and the internal stringers to the lower skin panel). The cracks from both initiation sites eventually linked up to create a single crack. The portion of the wing skin containing the fatigue crack was covered by a manufacturer-installed doubler, which would have hidden the crack from view and, therefore, prevented detection of the crack from a visual inspection of the exterior of the airplane.
The investigation found that the airplane was probably operated within the maximum takeoff gross weight limits specified in the airplane flight manual.
The NTSB concluded that the probable cause of the accident is the in-flight failure of the right wing due to fatigue cracking in the center wing lower skin and underlying structural members. A factor contributing to the accident was inadequate maintenance procedures to detect fatigue cracking.