What happened
During a mission to distribute fire retardant, the aircraft type experienced a catastrophic structural failure. While performing maneuvers, the left wing detached from the wing root. This separation caused the crew to lose all control of the aircraft, which subsequently entered a 45-degree dive with a counterclockwise roll before impacting mountainous terrain. Ground observers captured photographic evidence of the wing separating from the fuselage during the descent.
The aircraft, which had been converted for civilian firefighting use in 1958 after serving in the military from 1945 to 1956, was operating in a capacity for which it was not originally designed. There were 0 fatalities reported among the crew, though the impact was total.
Findings
Post-accident inspections of the wreckage identified significant preexisting fatigue within the lower spar cap of the left wing's forward spar, as well as damage to the adjacent spar web and lower wing skin. The specific area where the fatigue cracks developed was hidden from view by the aircraft's internal retardant tanks, making it impossible to identify the damage through standard external visual inspections.
Further investigation of two similar air tankers revealed that the structural area prone to such failure was obscured by the fuselage construction. While the operator had established maintenance and inspection protocols, these procedures were insufficient for detecting critical fatigue. The inspection guidelines were derived from outdated 1948 U.S. Navy structural repair manuals, which failed to provide adequate instructions for locating cracks in these specific, obscured locations.